Patent Publication Number: US-7914108-B2

Title: Image forming apparatus and method, and ink set

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image forming apparatus and method, and an ink set, and more particularly to image forming technology suitable for an inkjet recording apparatus which forms images of high quality on a recording medium by using ink containing a coloring material, and a treatment liquid. 
     2. Description of the Related Art 
     Japanese Patent Application Publication No. 10-287035 discloses an inkjet recording method wherein a reaction liquid includes a photopolymerization initiator, and the ink composition includes an acrylate monomer. Japanese Patent Application Publication No. 2000-135781 discloses an inkjet recording apparatus comprising an ink unit which ejects recording ink toward a recording member, and a treatment liquid unit which ejects treatment liquid toward the recording ink deposited by the ink unit, in such a manner that at least a portion of the image is formed by means of the recording ink and the treatment liquid mixing and curing on the recording member. Japanese Patent Application Publication No. 2003-12971 discloses an inkjet recording method in which the ink composition includes a polymerizable compound and a coloring material, the content of the polymerizable compound in the ink composition being 30 to 98 wt %, and a reaction liquid includes a polymerizable compound and a polymerization initiator. 
     Japanese Patent Application Publication Nos. 10-287035, 2000-135781 and 2003-12971 disclose technology for separating an ultraviolet-curable ink (so-called “UV ink”) into two-liquid phases; however, there is a problem in that the functional effects of the treatment liquid are attenuated (or reduced) due to the permeation of the treatment liquid in cases where the recording medium has high permeability and the first liquid permeates rapidly. In respect of this problem, there is no disclosure or suggestion about a method for causing the two liquids to effectively mix together. 
     Japanese Patent Application Publication No. 5-4343 discloses a recording apparatus comprising a recording head which applies a recording liquid having electrorheological properties and a device for creating an electric field on the surface to which the recording liquid is applied. Japanese Patent Application Publication No. 5-4343 discloses technology for preventing bleeding and color mixing by means of the effects of an electrorheological fluid; however, the relationship with the UV ink is not described. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of the foregoing circumstances, an object thereof being to provide an image forming apparatus and method, and an ink set, which make it possible to achieve effective mixing (reaction) of two liquids, and to form images of high quality, even when using a recording medium of high permeability. 
     In order to attain the aforementioned object, the present invention is directed to an image forming apparatus, comprising: a treatment liquid deposition device which deposits treatment liquid onto a recording medium, the treatment liquid containing a polymerization initiator and particles introducing electrorheological properties; an electric field application device which applies an electric field to the treatment liquid having been deposited on the recording medium; an ink ejection device which ejects ink toward the recording medium on which the treatment liquid has been deposited, the ink containing a coloring material and a radiation-curable polymerizable compound; and a radiation irradiation device which irradiates radiation to cure the ink having been deposited on the recording medium. 
     According to the present invention, before ink is deposited onto the recording medium, a treatment liquid (pre-treatment liquid) is deposited onto the recording medium, and an electric field is applied to the treatment liquid on the recording medium. The viscosity of the treatment liquid is increased by an electrorheological effect, thereby suppressing permeation of the treatment liquid into the recording medium (lowering the speed of permeation into the recording medium). In this way, by ejecting ink in a state where there is sufficient treatment liquid remaining on the recording medium, it is possible to achieve reliable mixing of the two liquids, and it is also possible to avoid a phenomenon in which the ink droplets combine together (unite) (known as “landing interference”). Furthermore, by promoting the curing reaction by irradiating radiation, it is possible to achieve curing and fixing of the ink in a short period of time, as well as forming images of high quality and achieving high-speed printing. Examples of the radiation are: electromagnetic waves, such as visible light, ultraviolet light, and X rays, an electron beam, and the like. 
     Furthermore, when applying an electric field, a desirable mode is one in which control is performed in order to apply the minimum level of electric field required in order to suppress the permeation of the treatment liquid. Accordingly, it is possible to prevent increase in the viscosity of the treatment liquid in the head when using a liquid ejection head as a treatment liquid deposition device. 
     In one example of an electric field application device for achieving an electrorheological effect, there is a mode having a structure in which an electrode pair comprising a first electrode and a second electrode are arranged, and a prescribed electric field intensity is generated, in the region peripheral to the electrode pair, when a relative potential difference is applied between the first electrode and the second electrode (in other words, when a voltage is applied to same). In this mode, in the electrode pair comprising the first and second electrodes, naturally, one electrode is a positive electrode and the other electrode is a negative electrode, and either of the electrodes may be used as the positive or negative electrode. 
     The electric field application device may also be combined with an electrostatic attraction device which holds the recording medium stably by means of electrostatic attraction. In other words, it is possible to apply an electric field to the treatment liquid on the recording medium by using the electric field created by the electrostatic attraction device which holds the recording medium (on a belt or roller, for example). 
     Other modes of the electric field application device include modes, such as: (a) a mode where the treatment liquid is interposed between plate-shaped electrodes having a high potential difference; (b) a combination of charging of the recording medium and the treatment liquid by means of a conductive rubber roller, a conductive brush, corona discharge, or the like, and electrodes disposed in the vicinity of the recording medium; (c) a combination of charging of the recording medium and the treatment liquid by means of electron beam irradiation or ion irradiation onto the recording medium or treatment liquid on the recording medium, and electrodes disposed in the vicinity of the recording medium; (d) a combination of charging of the droplets (the droplets of treatment liquid) themselves by passing the projected droplets through an electric field, and electrodes disposed in the vicinity of the recording medium; and the like. 
     Preferably, the image forming apparatus further comprises: a recording medium type identification device which identifies a type of the recording medium; and an electric field control device which controls the electric field created by the electric field application device in accordance with the type of the recording medium identified by the recording medium type identification device. 
     Since the permeability of the liquid or the droplets having been deposited on the recording medium varies depending on conditions such as the type and thickness of the recording medium, the dielectric constant thereof, and so on, it is then desirable that the type of recording medium is ascertained by means of a recording medium type identification device, and the electric field is controlled appropriately in accordance with the type of medium, by application (ON)/non-application (OFF) of the electric field, or the electric field intensity when an electric field is applied, or a combination of these. Accordingly, it is possible to print under optimal conditions in relation to the type of recording medium. 
     For example, if a highly permeable medium is used, then an electric field is applied, thereby suppressing the permeation of the treatment liquid, whereas if a medium of low permeability (non-permeable medium or low-permeability medium) is used, then an electric field is not applied. 
     A more desirable mode of a device which controls the electric field intensity when an electric field is applied is one having a composition where the electric field intensity is adjusted automatically on the basis of information obtained by the recording medium type identification device, but it is also possible to adopt a composition in which the electric field intensity is switched or changed by manual operation performed by an operator, or the like. 
     The recording medium type identification device may comprise, for example, a device which measures the reflectivity of the recording medium, or a device which reads in the type of the recording medium used from the ID, or the like, of the supply magazine. Furthermore, the recording medium type identification device is not limited to a device which obtains information automatically by means of sensors, an information reading device, or the like, and it may also be constituted in such a manner that information relating to the type of recording medium or the like is input by a user by means of a prescribed input apparatus (user interface), or the like. 
     Preferably, the treatment liquid further contains a coloring material dispersion inhibitor which prevents dispersion of the coloring material. 
     By mixing of the treatment liquid and the ink of this mode, the dispersion of the coloring material on the recording medium is suppressed and hence it is possible to prevent bleeding. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus, comprising: a first treatment liquid deposition device which deposits a first treatment liquid onto a recording medium, the first treatment liquid containing a polymerization initiator and particles introducing electrorheological properties; an electric field application device which applies an electric field to the first treatment liquid having been deposited on the recording medium; a second treatment liquid deposition device which deposits a second treatment liquid onto the recording medium, the second treatment liquid containing a polymerization initiator and having no electrorheological properties; a recording medium type identification device which identifies a type of the recording medium; a treatment liquid selection control device which controls operation of the first treatment liquid deposition device and the second treatment liquid deposition device, in such a manner that one of the first treatment liquid and the second treatment liquid is selectively deposited onto the recording medium, in accordance with the type of the recording medium identified by the recording medium type identification device; an ink ejection device which ejects ink toward the recording medium on which the one of the first treatment liquid and the second treatment liquid has been deposited, the ink containing a coloring material and a radiation-curable polymerizable compound; and a radiation irradiation device which irradiates radiation to cure the ink having been deposited on the recording medium. 
     According to the present invention, at least two types of treatment liquids (a first and a second treatment liquids) are provided, and the treatment liquids are switched in accordance with the type of recording medium used. The first and second treatment liquids are similar in that they both contain a polymerization initiator, but the first treatment liquid contains particles which introduce electrorheological properties and is therefore an electrorheological fluid, which has electrorheological properties, whereas the second treatment liquid is a non-electrorheological fluid which does have electrorheological properties. 
     According to this aspect of the present invention, it is also possible to deposit a suitable treatment liquid according to the type of recording medium. Furthermore, if the first treatment liquid is selected, then by applying an electric field, the permeation of the first treatment liquid into the recording medium is suppressed by an electrorheological effect. In this way, by ejecting ink in a state where there is sufficient treatment liquid remaining on the recording medium, it is possible to achieve reliable mixing of the two liquids, and it is also possible to avoid a phenomenon in which the ink droplets combine together (unite) (landing interference). Furthermore, since the curing reaction is promoted by irradiating radiation, it is possible to achieve curing and fixing of the ink in a short period of time, as well as forming images of high quality and achieving high-speed printing. 
     Preferably, the image forming apparatus further comprises an electric field control device which controls the electric field created by the electric field application device in accordance with the type of the recording medium identified by the recording medium type identification device. 
     Desirably, the electric field is controlled suitably in accordance with the selection of the treatment liquid in accordance with the type of recording medium, by means of the application (ON) or non-application (OFF) of the electric field, or the electric field intensity when an electric field is applied, or a combination of these, or the like. Since the permeability of the liquid with respect to the recording medium depends on the type of the recording medium, then a suitable treatment liquid is selected in accordance with the type of the recording medium, as well as being able to print under optimal conditions according to the recording medium, by controlling the electric field suitably. 
     For example, if a medium of high permeability is used, then the first treatment liquid is selected and an electric field is applied, thereby suppressing permeation of the first treatment liquid by means of an electrorheological effect. On the other hand, if a medium of low permeability (non-permeable medium or low-permeability medium) is used, then the second treatment liquid is selected and no electric field is applied. 
     Preferably, each of the first treatment liquid and the second treatment liquid further contains a coloring material dispersion inhibitor which prevents dispersion of the coloring material. 
     By mixing of the treatment liquid and the ink in this mode, the dispersion of the coloring material on the recording medium is suppressed and hence it is possible to prevent bleeding. 
     The treatment liquid deposition device of the image forming apparatus may be a device which ejects droplets of the treatment liquid, by using an inkjet-type ejection head, a device which applies the treatment liquid by means of a roller, a brush, a blade-shaped member, a porous member, or the like, a device which deposits a treatment liquid by spraying a mist, or a suitable combination of these. 
     For the ink ejection device, it is suitable to use an inkjet droplet ejection head which ejects ink liquid on the basis of image information for printing (print data). 
     The inkjet recording apparatus according to one mode of the image recording apparatus of the present invention comprises: a liquid ejection head (corresponding to a “recording head”) having a droplet ejection element row in which a plurality of droplet ejection elements are arranged in a row, each droplet ejection element comprising a nozzle for ejecting an ink droplet in order to form a dot and a pressure generating device (piezoelectric element, heating element, or the like) which generates an ejection pressure; and an ejection control device which controls the ejection of droplets from the recording head on the basis of droplet ejection arrangement data (dot data) generated from the image data. An image is formed on a recording medium by means of the ink droplets ejected from the nozzles. 
     One compositional example of a recording head (ink ejection device) is a full line type head in which a plurality of nozzles are arranged through a length corresponding to the full width of the recording medium. In this case, a mode may be adopted in which a plurality of relatively short recording head modules having nozzles rows which do not reach a length corresponding to the full width of the recording medium are combined and joined together, thereby forming nozzle rows of a length that correspond to the full width of the recording medium. 
     A full line type head is usually disposed in a direction that is perpendicular to the relative feed direction (relative conveyance direction) of the recording medium, but a mode may also be adopted in which the recording head is disposed following an oblique direction that forms a prescribed angle with respect to the direction perpendicular to the conveyance direction. 
     A “recording medium” is a medium onto which the liquid ejected from the liquid ejection head (recording head) is deposited, and it receives the recording of an image by the action of the recording head. More specifically, the “recording medium” indicates a print medium, image forming medium, image receiving medium, ejection receiving medium, or the like. This term includes various types of media, irrespective of material and size, such as continuous paper, cut paper, sealed paper, resin sheets, such as OHP sheets, film, cloth, a printed circuit board on which a wiring pattern, or the like, is formed, and an intermediate transfer medium, and the like. 
     The “conveyance device” may include a mode where the recording medium is conveyed with respect to a stationary (fixed) recording head, or a mode where a recording head is moved with respect to a stationary recording medium, or a mode where both the recording head and the recording medium are moved. 
     When forming color images by means of an inkjet head, it is possible to provide recording heads for a plurality of colored inks (recording liquids), or it is possible to eject inks of a plurality of colors, from one recording head. 
     Furthermore, the present invention is not limited to a full line head, and may also be applied to a shuttle scanning type recording head (a recording head which ejects droplets while moving reciprocally in a direction substantially perpendicular to the conveyance direction of the recording medium). 
     In order to attain the aforementioned object, the present invention is also directed to an image forming method, comprising: a treatment liquid deposition step of depositing treatment liquid onto a recording medium, the treatment liquid containing a polymerization initiator and particles introducing electrorheological properties; an electric field application step of applying an electric field to the treatment liquid having been deposited on the recording medium; an ink ejection step of ejecting ejects ink toward the recording medium on which the treatment liquid has been deposited, the ink containing a coloring material and a radiation-curable polymerizable compound; and a radiation irradiation step of irradiating radiation to cure the ink having been deposited on the recording medium. 
     Preferably, the image forming method further comprises: a recording medium type identification step of identifying a type of the recording medium, wherein the electric field created in the electric field application step is controlled in accordance with the type of the recording medium identified in the recording medium type identification step. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming method of forming an image on a recording medium, the method comprising: a treatment liquid preparation step of preparing a first treatment liquid and a second treatment liquid, the first treatment liquid containing a polymerization initiator and particles introducing electrorheological properties, the second treatment liquid containing a polymerization initiator and having no electrorheological properties; a recording medium type identification step of identifying a type of the recording medium; a treatment liquid selection step of selecting one of the first treatment liquid and the second treatment liquid in accordance with the type of recording medium identified in the recording medium type identification step; a treatment liquid deposition step of depositing the one of the first treatment liquid and the second treatment liquid selected in the treatment liquid selection step, onto the recording medium; an electric field application step of, if the first treatment liquid is selected in the treatment liquid selection step, applying an electric field to the first treatment liquid having been deposited on the recording medium; an ink ejection step of ejecting ink toward the recording medium on which the one of the first treatment liquid and the second treatment liquid has been deposited, the ink containing a coloring material and a radiation-curable polymerizable compound; and a radiation irradiation step of irradiating radiation to cure the ink having been deposited on the recording medium. 
     Preferably, the electric field created in the electric field application step is controlled in accordance with the type of the recording medium identified in the recording medium type identification step. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus, comprising: a treatment liquid deposition device which deposits treatment liquid onto a recording medium, the treatment liquid containing a polymerization initiator, electrorheological property introducing particles introducing electrorheological properties, and a solvent, the electrorheological property introducing particles and the solvent both being made of materials that are colorless and transparent and have mutually proximate refractive indices; an electric field application device which applies an electric field to the treatment liquid having been deposited on the recording medium; an ink ejection device which ejects ink toward the recording medium on which the treatment liquid has been deposited, the ink containing a coloring material and a radiation-curable polymerizable compound; and a radiation irradiation device which irradiates radiation to cure the ink having been deposited on the recording medium. 
     According to the present invention, before ink is deposited onto the recording medium, a treatment liquid (pre-treatment liquid) is deposited onto the recording medium, and an electric field is applied to the treatment liquid on the recording medium. The viscosity of the treatment liquid is increased by an electrorheological effect, thereby suppressing permeation of the liquid into the recording medium (lowering the speed of permeation into the recording medium). In this way, it is possible to achieve reliable mixing of the two liquids by ejecting ink in a state where a sufficient amount of treatment liquid is remaining on the recording medium, and furthermore, by curing and fixing by irradiating radiation onto the mixed liquid, it is possible to form images of high quality. More specifically, it is possible to suppress permeation of treatment liquid into the recording medium (and in particular, into a permeable medium of high permeability) by increasing the viscosity of the treatment liquid by means of an electrorheological effect. Therefore, attenuation of the functional effects of the treatment liquid due to permeation of the treatment liquid on the recording medium can be prevented. Furthermore, it is possible to suppress spreading of the coloring material in the treatment liquid, by increasing the viscosity of the treatment liquid by means of an electrorheological effect. 
     Moreover, in the present invention, both the solvent and the electrorheological property introducing particles of the treatment liquid are both colorless and transparent, and are made of materials having mutually proximate refractive indices. Therefore, the treatment liquid is prevented from becoming clouded, and a colorless and transparent treatment liquid can be achieved. Consequently, it is possible to reproduce the ink colors faithfully, and hence deterioration of image quality can be prevented. 
     Moreover, by causing the curing reaction to proceed by irradiation of radiation after deposition of the ink, it is possible to achieve curing and fixing of the ink in a short period of time, and hence high-speed printing can be achieved. 
     Since the permeability of the liquid and the behavior of the deposited droplets on the recording medium vary, depending on conditions such as the material and thickness of the recording medium and the dielectric constant, and the like, then it is desirable that the image forming apparatus further comprises a recording medium type identification device which identifies a type of the recording medium; and an electric field control device which controls the electric field created by the electric field application device in accordance with the type of the recording medium identified by the recording medium type identification device. 
     The type of recording medium is ascertained by using the recording medium type identification device, and by controlling the electric field suitably in accordance with the type of medium, namely, by controlling the application (ON) or non-application (OFF) of the electric field, or the intensity of the electric field in cases where an electric field is applied, or a combination of these, it is possible to print under optimal conditions in accordance with the recording medium used. 
     The treatment liquid deposition device of the image forming apparatus according to the present invention may be a device which ejects droplets of the treatment liquid by using an inkjet-type ejection head (a device using inkjet nozzles), a device which applies the treatment liquid by means of a roller, a brush, a blade-shaped member, a porous member, or the like, a device which deposits the treatment liquid by spraying a mist, or a suitable combination of these. 
     For the ink ejection device, it is suitable to use an inkjet droplet ejection head which ejects ink liquid on the basis of image information for printing (print data). 
     Preferably, the treatment liquid further contains a coloring material dispersion inhibitor which prevents dispersion of the coloring material. 
     By mixing of the treatment liquid and the ink in this mode, the dispersion of the coloring material on the recording medium is prevented reliably and hence it is possible to prevent bleeding. 
     Preferably, an average size of the electrorheological property introducing particles is 0.3 μm to 10 μm; and the treatment liquid deposition device includes an application device which applies the treatment liquid while making contact with the recording medium. 
     If the average particle size of the electrorheological property introducing particles dispersed in the treatment liquid is 0.3 μm to 10 μm, then from the viewpoint of the liquid characteristics of the treatment liquid, a mode where the treatment liquid is applied by using an application member, such as a roller, is easier to implement than a mode where droplets of the treatment liquid are ejected by means of an inkjet method. There is no restriction from the viewpoint of the suitability of inkjet ejection, and since particles having a relatively large size, which are suitable for introducing electrorheological properties, can be used, then it is possible to produce a sufficiently large electrorheological effect. 
     By using a radiation-curable polymerizable compound as the solvent of the treatment liquid, a merit is obtained in that the treatment liquid deposited onto the non-image sections can also be cured. 
     Alternatively, it is also preferable that an average size of the electrorheological property introducing particles is 100 nm to 1 μm; and the treatment liquid deposition device includes an ejection device which ejects droplets of the treatment liquid by an inkjet method. 
     If the average particle size of the electrorheological property introducing particles dispersed in the treatment liquid is 100 nm to 1 μm, then the characteristics of the treatment liquid are suitable for ejection in the form of droplets by an inkjet method, and therefore, in such cases, a mode where the treatment liquid is deposited by means of an inkjet-type ejection device can be selected. According to this mode, since the treatment liquid can be deposited selectively, only on those regions of the recording medium where it is required, then it is possible to reduce wasteful consumption of treatment liquid in comparison with an application device. 
     Preferably, the image forming apparatus further comprises: a treatment liquid tank which stores the treatment liquid to be supplied to the treatment liquid deposition device; and a stirring device which stirs the treatment liquid in the treatment liquid tank. 
     By providing a stirring device inside the treatment liquid tank forming the device which accumulates the treatment liquid, and by stirring the treatment liquid inside the treatment liquid tank by means of this stirring device, it is possible to suppress aggregation and settling of the dispersed particles in the treatment liquid. Therefore, the storage stability of the treatment liquid can be improved. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming method, comprising: a treatment liquid deposition step of depositing treatment liquid onto a recording medium, the treatment liquid containing a polymerization initiator, electrorheological property introducing particles introducing electrorheological properties, and a solvent, the electrorheological property introducing particles and the solvent both being made of materials that are colorless and transparent and have mutually proximate refractive indices; an electric field application step of applying an electric field to the treatment liquid having been deposited on the recording medium; an ink ejection step of ejecting ejects ink toward the recording medium on which the treatment liquid has been deposited, the ink containing a coloring material and a radiation-curable polymerizable compound; and a radiation irradiation step of irradiating radiation to cure the ink having been deposited on the recording medium. 
     In order to attain the aforementioned object, the present invention is also directed to an ink set, comprising: a treatment liquid which contains a polymerization initiator, electrorheological property introducing particles introducing electrorheological properties, and a solvent, the electrorheological property introducing particles and the solvent both being made of materials that are colorless and transparent and have mutually proximate refractive indices; and an ink which contains a coloring material and a radiation-curable polymerizable compound. 
     According to the present invention, it is possible to make a treatment liquid (first treatment liquid) remain on the medium of high permeability, by means of an electrorheological effect, and hence it is possible to cause the two liquids to mix together reliably, regardless of the type of recording medium. By irradiating radiation onto the mixed liquid, it is possible to achieve reliable curing and fixing, as well as high-quality image formation. 
     Furthermore, in the composition of the treatment liquid, the solvent and the dispersed particles (particles introducing electrorheological properties) are both made of materials which are colorless and transparent and have mutually proximate refractive indices. Therefore, it is possible to prevent the treatment liquid from becoming clouded, and hence the treatment liquid can be made transparent. Consequently, the colors of the ink can be reproduced faithfully. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a general compositional diagram of an inkjet recording apparatus according to a first embodiment of the present invention; 
         FIGS. 2A and 2B  are plan view perspective diagrams showing an embodiment of the composition of an ink head; 
         FIG. 3  is a plan view perspective diagram showing a further embodiment of the composition of a full line head; 
         FIG. 4  is a cross-sectional diagram showing the three-dimensional composition of a droplet ejection element of one channel (an ink chamber unit corresponding to one nozzle); 
         FIG. 5  is an enlarged view showing a nozzle arrangement in the ink head in  FIGS. 2A and 2B ; 
         FIG. 6  is a schematic drawing showing the composition of an ink supply system in the inkjet recording apparatus; 
         FIG. 7  is a plan view schematic drawing showing an embodiment of an electrode arrangement structure in an electrode unit; 
         FIG. 8  is a cross-sectional view along line  8 - 8  in  FIG. 7 ; 
         FIG. 9  is a principal block diagram showing the system configuration of the inkjet recording apparatus according to the first embodiment; 
         FIG. 10  is a flowchart showing a control procedure of the inkjet recording apparatus according to the first embodiment; 
         FIG. 11  is a principal compositional diagram of an inkjet recording apparatus according to a second embodiment of the present invention; 
         FIG. 12  is a principal block diagram showing the system configuration of the inkjet recording apparatus according to the second embodiment; 
         FIG. 13  is a flowchart showing a control procedure of the inkjet recording apparatus according to the second embodiment; 
         FIG. 14  is a general compositional diagram of an inkjet recording apparatus according to a third embodiment of the present invention; 
         FIG. 15  is a cross-sectional view along line  8 - 8  in  FIG. 7 , corresponding to the third embodiment; 
         FIG. 16  is a principal block diagram showing the system configuration of the inkjet recording apparatus according to the third embodiment; 
         FIG. 17  is a principal compositional diagram of the inkjet recording apparatus according to the third embodiment; and 
         FIG. 18  is a principal compositional diagram of an inkjet recording apparatus according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     General Composition of Inkjet Recording Apparatus 
       FIG. 1  is a general schematic drawing of an inkjet recording apparatus  10  which forms a first embodiment of an image forming apparatus according to the present invention. As shown in  FIG. 1 , the inkjet recording apparatus  10  comprises: a treatment liquid head  11  (corresponding to a “treatment liquid deposition device”), which ejects a first liquid serving as a treatment liquid (pre-treatment liquid); a plurality of ink ejection heads (corresponding to “ink ejection devices”; hereinafter referred to as “ink heads”)  12 C,  12 M,  12 Y and  12 K, provided respectively to correspond to the inks (second liquids) of colors of cyan (C), magenta (M), yellow (Y), black (K); a treatment liquid storing and loading unit  13 , which stores the treatment liquid to be supplied to the treatment liquid head  11 ; an ink storing and loading unit  14 , which stores the inks to be supplied to the ink heads  12 C,  12 M,  12 Y and  12 K; an ultraviolet light source (corresponding to a “radiation irradiation device”; hereinafter referred to as “ultraviolet light source”)  16  forming a fixing promotion device; a medium supply unit  22 , which supplies a recording medium  20 ; a decurling unit  24 , which removes curl from the recording medium  20 ; a medium type determination unit (corresponding to a “recording medium type identification device”)  25 , which determines the type of recording medium  20 ; a conveyance unit  26 , disposed facing the ejection surface (nozzle surface) of the heads  11 ,  12 C,  12 M,  12 Y and  12 K, and the light emission surface of the ultraviolet light source  16 , which conveys the recording medium while keeping the medium flat; and an electrode unit (corresponding to an “electric field application device”)  28 , attached to the conveyance unit  26 , which applies an electric field to the liquid on the recording medium  20 . 
     The treatment liquid storing and loading unit  13  has a treatment liquid tank for storing the treatment liquid, and the treatment liquid tank is connected to the treatment liquid head  11  through a tubing channel  30 P. The treatment liquid storing and loading unit  13  has a warning device (for example, a display device or an alarm sound generator) for warning when the remaining amount of the treatment liquid is low, and has a mechanism for preventing loading errors between types of liquid. 
     The ink storing and loading unit  14  has ink tanks  14 C,  14 M,  14 Y and  14 K for storing the inks of the colors corresponding to the respective ink heads  12 C,  12 M,  12 Y and  12 K, and the tanks are connected to the heads  12 C,  12 M,  12 Y and  12 K, through prescribed channels  30 C,  30 M,  30 Y and  30 K. The ink storing and loading unit  14  has a warning device (for example, a display device or an alarm sound generator) for warning when the remaining amount of any ink is low, and has a mechanism for preventing loading errors among the colors. 
     In the present embodiment, an electrorheological fluid having electrorheological properties is used for the first liquid serving as the treatment liquid. An electrorheological fluid is a fluid in which the apparent viscosity increases instantaneously when an electric field is applied. The change in viscosity is reversible by switching the electric field on and off. There are two types of electrorheological fluids: particle-dispersed fluids and homogeneous fluids. 
     A particle-dispersed fluid is one in which dielectric micro-particles are dispersed in an electrically insulating solvent. This fluid behaves in such a manner that when no electric field is applied, the micro-particles remain in a dispersed state and the viscosity of the fluid is low, but when an electric field is applied, the polarized particles form chain-like structures (“bridges”) linked in the direction of the electric field, and these bridges act so as to increase the viscosity of the fluid. Particle-dispersed electrorheological fluids include aqueous and non-aqueous fluids. 
     Homogeneous electrorheological fluids are fluids having anisotropic properties in which molecules or domains are oriented in the direction of the electric field, such as in liquid crystals, or the like. Since homogeneous electrorheological fluids currently display little change in viscosity, it is thought that particle-dispersed electrorheological fluids are more suitable for use in inkjet printers. 
     In the present embodiment, a treatment liquid is imparted with electrorheological properties, and a treatment liquid of this kind may be manufactured, for example, by dispersing solid micro-particles (silica gel, starch, dextrin, carbon, gypsum, gelatin, alumina, cellulose, mica, zeolite, kaolite, or the like) in a liquid containing at least a polymerization initiator, by using micro-particles (particles formed into micro-capsules, or the like), providing insulation on the surface thereof, as a dispersant for introducing electrorheological properties, or by combining a homogeneous electrorheological fluid, or the like. 
     Here, for the first liquid or the treatment liquid, a transparent treatment liquid (which contains no coloring material) containing “a polymerization initiator, a coloring material dispersion inhibitor, an oil acting as a high-boiling-point organic solvent, and particles that introduce electrorheological properties (hereinafter referred to as “electrorheological property introducing particles”)” is used. 
     Furthermore, for the second liquids or the inks, inks having a liquid composition containing “an ultraviolet-curable polymerizable compound (monomer, oligomer, or the like), and a pigment forming a coloring material” are used, in equal number to the number of colors used (in the present embodiment, four colors of C, M, Y and K). The details of the ink set used in the present embodiment are described below. 
     When the ink and the treatment liquid mix together, the dispersion of the coloring material in the ink is suppressed by the coloring material dispersion inhibitor in the treatment liquid, and the polymerization reaction of the liquids progresses due to the mixing of the two liquids and the irradiation of the radiation onto the mixed liquids, thereby curing and fixing the ink. 
     It is possible to adjust the curing speed and the physical properties of the liquids (surface tension, viscosity, and the like) by adjusting the respective compositions, component densities, and the like, of the ink and treatment liquid, and hence prescribed fixing properties of the ink (curing speed, fixing speed, and the like) can be achieved. 
     In  FIG. 1 , a magazine  32  for rolled paper (continuous paper) is shown as an example of the medium supply unit  22 ; however, a plurality of magazines  32  with papers of different paper width and quality may be jointly provided. Moreover, papers may be supplied in cassettes that contain cut papers loaded in layers and that are used jointly or in lieu of magazines for rolled papers. 
     The recording medium  20  delivered from the medium supply unit  22  retains curl due to having been loaded in the magazine  32 . In order to remove the curl, heat is applied to the recording medium  20  in the decurling unit  24  by a heating drum  34  in the direction opposite from the curl direction in the magazine  32 . The heating temperature at this time is preferably controlled so that the recording medium  20  has a curl in which the surface on which the print is to be made is slightly round outward. 
     In the case of the configuration in which roll paper is used, a cutter  38  is provided as shown in  FIG. 1 , and the continuous paper is cut into a desired size by the cutter  38 . The cutter  38  has a stationary blade  38 A, whose length is not less than the width of the conveyor pathway of the recording medium  20 , and a round blade  38 B, which moves along the stationary blade  38 A. The stationary blade  38 A is disposed on the reverse side of the printed surface of the recording medium  20 , and the round blade  38 B is disposed on the printed surface side across the conveyor pathway. When cut papers are used, the cutter  38  is not required. 
     After decurling in the decurling unit  24 , the cut recording medium  20  is delivered to the conveyance unit  26 . The medium type determination unit  25  is disposed at a suitable position in the conveyance path of the recording medium  20 , in a stage before the treatment liquid ejection head  11  (on the upstream side in the recording medium conveyance path). This medium type determination unit  25  is a device which acquires information relating to the medium type, and it is constituted by a device which determines the paper type, wettability, size, and the like, of the recording medium  20  (for example, it is a sensor which determines the optical reflectivity of the paper, a paper width determination sensor, a sensor which determines the thickness of the paper, or a suitable combination of these). The type of recording medium  20  is judged automatically by the medium type determination unit  25 , and control is implemented in such a manner that suitable treatment liquid deposition, control of the electric field and ink ejection are achieved, in accordance with the medium type. 
     The device which acquires information relating to the medium type is not limited to the composition described above. For example, it is also possible to adopt a composition in which an information recording body, such as a barcode or radio tag, which records medium type information, is attached to the magazine  32  of the medium supply unit  22 , and the type of medium used is identified automatically by reading in the information of this information recording body by means of a prescribed reading apparatus (information reading device). Furthermore, it is also possible to adopt a composition in which recording medium information relating to the paper type, wettability, size, or the like, is specified by means of an input made through a prescribed user interface, instead of or in conjunction with such automatic determination devices. 
     The conveyance unit  26  has a configuration in which a minimally conductive endless belt  43  is set around rollers  41  and  42  in such a manner that at least the portion of the endless belt  43  facing the nozzle faces of the heads  11 ,  12 C,  12 M,  12 Y and  12 K forms a horizontal plane (flat plane). 
     The minimally conductive belt  43  has a broader width than the recording medium  20 , and the electrode unit  28  is disposed on the rear side of the portion of the belt which supports the recording medium  20 . Although described in more detail below, by applying a DC high voltage to the electrode unit  28  by means of a DC high-voltage generator  78  (not shown in  FIG. 1 , but shown in  FIG. 7 ), the recording medium  20  is attracted to and held on the minimally conductive belt  43  due to an electrostatic force, and an electric field is applied to the treatment liquid and the ink deposited on the recording medium  20 . 
     The minimally conductive belt  43  is driven in the counter-clockwise direction in  FIG. 1  by means of the motive force of a motor  138  (not shown in  FIG. 1 , but shown in  FIG. 9 ) being transmitted to at least one of the rollers  41  and  42 , which the belt  43  is set around, and the recording medium  20  is thus conveyed from right to left in  FIG. 1 . 
     The treatment liquid head  11  and the ink heads  12 C,  12 M,  12 Y and  12 K are full line heads having a length corresponding to the maximum width of the recording medium  20  used with the inkjet recording apparatus  10 , and comprising a plurality of nozzles (ejection ports) arranged on a nozzle face through a length exceeding at least one edge of the maximum-size recording medium  20  (namely, the full width of the printable range). 
     The ink heads  12 C,  12 M,  12 Y and  12 K are arranged in color order (cyan (C), magenta (M), yellow (Y), black (K)) from the upstream side in the delivery direction of the recording medium  20 , and these ink heads  12 C,  12 M,  12 Y and  12 K are fixed extending in a direction substantially perpendicular to the conveyance direction of the recording medium  20 . 
     A color image can be formed on the recording medium  20  by ejecting inks of different colors from the ink heads  12 C,  12 M,  12 Y and  12 K, respectively, onto the recording medium  20  while the recording medium  20  is conveyed at a uniform speed by the conveyance unit  26 . 
     By adopting a configuration in which full line heads  12 C,  12 M,  12 Y and  12 K having nozzle rows covering the full paper width are provided for the separate colors in this way, it is possible to record an image on the full surface of the recording medium  20  by performing just one operation of moving the recording medium  20  relatively with respect to the heads  12 C,  12 M,  12 Y and  12 K in the paper conveyance direction (the sub-scanning direction), (in other words, by means of one sub-scanning action). The single-pass inkjet recording apparatus  10  of this kind is able to print at high speed in comparison with a shuttle scanning system in which an image is printed by moving a recording head back and fourth reciprocally in the main scanning direction, and hence print productivity can be improved. 
     Although a configuration with four standard colors, C M Y and K, is described in the present embodiment, the combinations of the ink colors and the number of colors are not limited to these, and light and/or dark inks, or special color inks can be added as required. For example, a configuration is possible in which heads for ejecting light-colored inks such as light cyan and light magenta are added. Furthermore, there are no particular restrictions of the sequence in which the heads of respective colors are arranged. 
     The ultraviolet light source  16  disposed at a downstream stage of the ink head  12 K of the last color has a length corresponding to the maximum width of the recording medium  20 , similarly to the heads  11 ,  12 C,  12 M,  12 Y and  12 K, and is fixed extending in a direction substantially perpendicular to the conveyance direction of the recording medium  20 . An ultraviolet lamp, for example, is used as the ultraviolet light source  16 , and it irradiates ultraviolet light for promoting the curing of the ink having been deposited on the recording medium  20 . Instead of the ultraviolet lamp, it is also possible to use a composition in which ultraviolet light-emitting diode (LED) elements or ultraviolet laser diode (LD) elements are arranged in a line shape. According to this composition, since light emission can be controlled selectively in each individual light-emitting element, it is possible readily to adjust the number of light emitting elements that light up, and the amount of light generated, and hence a prescribed irradiation range and light volume (intensity) can be achieved in the ultraviolet irradiation area. 
     A mode may also be adopted in which the ink droplets having been deposited on the recording medium  20  are not necessarily cured and fixed completely (to a state where the curing reaction has completed fully), but rather the ultraviolet light source  16  cures and fixes the ink droplets to a level whereby no image degradation occurs in subsequent handling, and a step of performing full curing is performed separately, as a later step. Here, this “handling” means, for example, (1) rubbing of the image surface against the rollers, conveyance guides, and the like, in the conveyance steps downstream of the ultraviolet light source  16 , (2) rubbing between prints in the print stacking section, and (3) rubbing of a finished print against various objects when it is actually handled for use. 
     In this way, the recording medium  20  which has passed under the ultraviolet light source  16  (the generated printed object) is outputted from the paper output unit, by means of a toothed idle roller (not illustrated) and a nip roller, or the like. Although not shown in  FIG. 1 , the paper output unit is provided with a sorter for collecting images according to print orders. 
     The electrode unit  28  attached to the conveyance unit  26  is disposed at least in a region which extends from the treatment liquid deposition start position of the treatment liquid head  11  to the ultraviolet light irradiation position of the ultraviolet light source  16 , and it is able to generate an electric field in this region. 
     Structure of Heads 
     Next, the structure of the ink heads  12 C,  12 M,  12 Y and  12 K is described. The heads provided for the respective ink colors each have a common structure, and a reference numeral  50  is hereinafter designated to any of the ink heads  12 C,  12 M,  12 Y and  12 K. 
       FIG. 2A  is a plan view perspective diagram showing an example of the structure of an ink head  50 , and  FIG. 2B  is an enlarged diagram of a portion of same. In order to minimize the pitch of the dots printed onto the surface of the recording medium  20 , it is necessary to minimize the nozzle pitch in the ink head  50 . As shown in  FIGS. 2A and 2B , the ink head  50  according to the present embodiment has a structure in which a plurality of ink chamber units (droplet ejection elements)  53 , each having a nozzle  51  forming an ink droplet ejection port, a pressure chamber  52  corresponding to the nozzle  51 , and the like, are disposed two-dimensionally in the form of a staggered matrix, and hence the effective nozzle interval (the projected nozzle pitch) as projected in the lengthwise direction of the head (the direction perpendicular to the paper conveyance direction) is reduced (high nozzle density is achieved). 
     The invention is not limited to the present embodiment of a mode for constituting nozzle rows equal to or exceeding a length corresponding to the full width Wm of the recording medium  20  in a direction (indicated by arrow M; main scanning direction) which is substantially perpendicular to the feed direction of the recording medium  20  (indicated by arrow S; sub-scanning direction). For example, instead of the composition in  FIG. 2A , as shown in  FIG. 3 , a line head having nozzle rows of a length corresponding to the entire width of the recording medium  20  can be formed by arranging and combining, in a staggered matrix, short head units  50 ′ each having a plurality of nozzles  51  arrayed in a two-dimensional fashion. 
     As shown in  FIGS. 2A and 2B , the planar shape of the pressure chamber  52  provided for each nozzle  51  is substantially a square, and the nozzle  51  and an inlet for supplied ink (supply port)  54  are disposed in both corners on a diagonal line of the square. The shape of the pressure chamber  52  is not limited to that of the present embodiment and various modes are possible in which the planar shape is a quadrilateral shape (rhombic shape, rectangular shape, or the like), a pentagonal shape, a hexagonal shape, or other polygonal shape, or a circular shape, elliptical shape, or the like. 
       FIG. 4  is a cross-sectional diagram (along line  4 - 4  in  FIG. 2A ) showing the three-dimensional composition of the droplet ejection element of one channel (an ink chamber unit corresponding to one nozzle  51 ). As shown in  FIG. 4 , each pressure chamber  52  is connected to a common flow channel  55  through the supply port  54 . The common flow channel  55  is connected to an ink tank  60  (not shown in  FIG. 4 , but shown in  FIG. 6 ), which is a base tank that supplies ink, and the ink supplied from the ink tank  60  is delivered through the common flow channel  55  shown in  FIG. 4  to the pressure chambers  52 . 
     An actuator  58  provided with an individual electrode  57  is bonded to a pressure plate (a diaphragm that also serves as a common electrode)  56  which forms the surface of one portion (in  FIG. 4 , the ceiling) of the pressure chambers  52 . When a drive voltage is applied to the individual electrode  57  and the common electrode, the actuator  58  deforms, thereby changing the volume of the pressure chamber  52 . This causes a pressure change which results in ink being ejected from the nozzle  51 . For the actuator  58 , it is possible to adopt a piezoelectric element using a piezoelectric body, such as lead zirconate titanate, barium titanate, or the like. When the displacement of the actuator  58  returns to its original position after ejecting ink, the pressure chamber  52  is replenished with new ink from the common flow channel  55  through the supply port  54 . 
     As shown in  FIG. 5 , the high-density nozzle head according to the present embodiment is achieved by arranging a plurality of ink chamber units  53  having the above-described structure in a lattice fashion based on a fixed arrangement pattern, in a row direction which coincides with the main scanning direction, and a column direction which is inclined at a fixed angle of θ with respect to the main scanning direction, rather than being perpendicular to the main scanning direction. 
     More specifically, by adopting a structure in which a plurality of ink chamber units  53  are arranged at a uniform pitch d in line with a direction forming an angle of θ with respect to the main scanning direction, the pitch P of the nozzles projected so as to align in the main scanning direction is d×cos θ, and hence the nozzles  51  can be regarded to be equivalent to those arranged linearly at the fixed pitch P along the main scanning direction. Such configuration results in a nozzle row having a high nozzle density. 
     In a full-line head comprising rows of nozzles that have a length corresponding to the entire width of the image recordable width, the “main scanning” is defined as printing one line (a line formed of a row of dots, or a line formed of a plurality of rows of dots) in the width direction of the recording paper (the direction perpendicular to the conveyance direction of the recording paper) by driving the nozzles in one of the following ways: (1) simultaneously driving all the nozzles; (2) sequentially driving the nozzles from one side toward the other; and (3) dividing the nozzles into blocks and sequentially driving the nozzles from one side toward the other in each of the blocks. 
     In particular, when the nozzles  51  arranged in a matrix such as that shown in  FIG. 5  are driven, the main scanning according to the above-described (3) is preferred. More specifically, the nozzles  51 - 11 ,  51 - 12 ,  51 - 13 ,  51 - 14 ,  51 - 15  and  51 - 16  are treated as a block (additionally; the nozzles  51 - 21 ,  51 - 22 , . . . ,  51 - 26  are treated as another block; the nozzles  51 - 31 ,  51 - 32 , . . . ,  51 - 36  are treated as another block; . . . ); and one line is printed in the width direction of the recording medium  20  by sequentially driving the nozzles  51 - 11 ,  51 - 12 , . . . ,  51 - 16  in accordance with the conveyance velocity of the recording medium  20 . 
     On the other hand, “sub-scanning” is defined as to repeatedly perform printing of one line (a line formed of a row of dots, or a line formed of a plurality of rows of dots) formed by the main scanning, while moving the full-line head and the recording paper relatively to each other. 
     The direction indicated by one line (or the lengthwise direction of a band-shaped region) recorded by main scanning as described above is called the “main scanning direction”, and the direction in which sub-scanning is performed, is called the “sub-scanning direction”. In other words, in the present embodiment, the conveyance direction of the recording medium  20  is called the sub-scanning direction and the direction perpendicular to same is called the main scanning direction. 
     In implementing the present invention, the arrangement of the nozzles is not limited to that of the embodiment illustrated. Moreover, a method is employed in the present embodiment where an ink droplet is ejected by means of the deformation of the actuator  58 , which is typically a piezoelectric element; however, in implementing the present invention, the method used for discharging ink is not limited in particular, and instead of the piezo jet method, it is also possible to apply various types of methods, such as a thermal jet method where the ink is heated and bubbles are caused to form therein by means of a heat generating body such as a heater, ink droplets being ejected by means of the pressure applied by these bubbles. 
     Although not illustrated here, the structure of the treatment liquid head  11  is generally the same as that of the ink head  50  described above. Since it is sufficient that the treatment liquid is deposited on the recording medium  20  in a substantially uniform (even) fashion in the region where ink droplets are to be deposited, then it is not necessary to form treatment liquid droplets to a high density, in comparison with the ink. Consequently, the treatment liquid head  11  may also be composed with a reduced number of nozzles (a reduced nozzle density) in comparison with the ink head  50  for ejecting ink. Furthermore, a composition may also be adopted in which the nozzle diameter of the treatment liquid head  11  is greater than the nozzle diameter of the ink head  50  for ejecting ink. 
     Configuration of Ink Supply System 
       FIG. 6  is a schematic drawing showing the configuration of the ink supply system in the inkjet recording apparatus  10 . The ink tank  60  is a base tank that supplies ink to the ink head  50  and is set in the ink storing and loading unit  14  described with reference to  FIG. 1 . In other words, the ink supply tank  60  in  FIG. 6  is equivalent to the ink storing and loading unit  14  in  FIG. 1 . The aspects of the ink tank  60  include a refillable type and a cartridge type: when the remaining amount of ink is low, the ink tank  60  of the refillable type is filled with ink through a filling port (not shown) and the ink tank  60  of the cartridge type is replaced with a new one. In order to change the ink type in accordance with the intended application, the cartridge type is suitable, and it is preferable to represent the ink type information with a bar code or the like on the cartridge, and to perform ejection control in accordance with the ink type. 
     A filter  62  for removing foreign matters and bubbles is disposed between the ink tank  60  and the ink head  50  as shown in  FIG. 6 . The filter mesh size in the filter  62  is preferably equivalent to or less than the diameter of the nozzle. Although not shown in  FIG. 6 , it is preferable to provide a sub-tank integrally to the ink head  50  or nearby the ink head  50 . The sub-tank has a damper function for preventing variation in the internal pressure of the head and a function for improving refilling of the head. 
     The inkjet recording apparatus  10  is also provided with a cap  64  as a device to prevent the nozzles  51  from drying out or to prevent an increase in the ink viscosity in the vicinity of the nozzles  51 , and a cleaning blade  66  as a device to clean the nozzle face  50 A. A maintenance unit (restoring device) including the cap  64  and the cleaning blade  66  can be relatively moved with respect to the ink head  50  by a movement mechanism (not shown), and is moved from a predetermined holding position to a maintenance position below the ink head  50  as required. 
     The cap  64  is displaced up and down relatively with respect to the ink head  50  by an elevator mechanism (not shown). When the power of the inkjet recording apparatus  10  is turned OFF or when in a print standby state, the cap  64  is raised to a predetermined elevated position so as to come into close contact with the ink head  50 , and the nozzle face  50 A is thereby covered with the cap  64 . 
     The cleaning blade  66  is composed of rubber or another elastic member, and can slide on the nozzle surface  50 A (nozzle plate surface) of the ink head  50  by means of a blade movement mechanism (not shown). If there are ink droplets or foreign matter adhering to the nozzle plate surface, then the nozzle plate surface is wiped clean by causing the cleaning blade  66  to slide over the nozzle plate. 
     During printing or during standby, if the use frequency of a particular nozzle has declined and the ink viscosity in the vicinity of the nozzle has increased, or if the ink has degenerated, then a preliminary ejection is performed onto the cap  64  (which also serves as an ink receptacle), in order to remove the degraded ink. 
     If the ink head  50  continues in a state of not ejecting ink for a prescribed time or longer, then the viscosity of the ink in the vicinity of the nozzles increases, and it becomes impossible to eject ink from the nozzles  51 , even if the actuators  58  for driving ejection are actuated. Therefore, before reaching such a state, the actuators  58  are operated toward an ink receptacle (while the ink viscosity is within a range that allows ejection by the operation of the actuators  58 ), and a “preliminary ejection” is performed which causes the ink in the vicinity of the nozzles whose viscosity has increased to be ejected. Furthermore, after cleaning away soiling on the surface of the nozzle surface  50 A by means of a wiper, such as a cleaning blade  66 , provided as a cleaning device on the surface of the nozzle plate, a preliminary ejection is also carried out in order to prevent infiltration of foreign matter into the nozzles  51  due to the rubbing action of the wiper. The preliminary ejection is also referred to as “dummy ejection”, “purge”, “liquid ejection”, and so on. 
     On the other hand, if air bubbles become intermixed into the nozzle  51  or pressure chamber  52 , or if the increase in the viscosity of the ink inside the nozzle  51  exceeds a certain level, then it may not be possible to eject ink in the preliminary ejection operation described above. In cases of this kind, a cap  64  forming a suction device is pressed against the nozzle surface  50 A of the ink head  50 , and the ink inside the pressure chambers  52  (namely, the ink containing air bubbles of the ink of increased viscosity) is suctioned by a suction pump  67 . The ink suctioned and removed by means of this suction operation is sent to a recovery tank  68 . The ink collected in the recovery tank  68  may be used, or if reuse is not possible, it may be discarded. 
     Since the suctioning operation is performed with respect to all of the ink in the pressure chambers  52 , it consumes a large amount of ink, and therefore, desirably, preliminary ejection is carried out while the increase in the viscosity of the ink is still minor. The suction operation is also carried out when ink is loaded into the ink head  50  for the first time, and when the head starts to be used after being idle for a long period of time. 
     The supply system for the treatment liquid is substantially the same as the composition of the ink supply system shown in  FIG. 6 , and is not illustrated. 
     Structure of Electrode Unit 
       FIG. 7  is a plan diagram showing an embodiment of the structure of an electrode arrangement in the electrode unit  28  described in  FIG. 1 . As shown in  FIG. 7 , the electrode unit  28  has a structure in which bar-shaped positive electrodes  72  and negative electrodes  74  extending substantially in parallel with a direction perpendicular to the conveyance direction of the recording medium  20  (direction S) are arranged alternately at a prescribed electrode pitch Wp in the medium conveyance direction. In  FIG. 7 , the number of electrodes is reduced and a schematic illustration is provided in order to simplify the drawing; however, a large number of electrodes are arranged in a dense configuration in practice. 
     The bar-shaped positive electrodes  72  and negative electrodes  74  are each formed to a longer dimension WL than the width Wm of the recording medium  20 , in such a manner that they apply a uniform electric field to the treatment liquid deposited on the recording medium  20 . 
     The electrode unit  28  has a pair of positive and negative electrode patterns  72 - 1  and  74 - 1 , which are connected to a DC high-voltage generator  78  through the switches SW 11  and SW 12 . The positive electrode pattern  72 - 1  has a comb shape in which one end (the upper end in  FIG. 7 ) of each of a plurality of bar-shaped positive electrodes  72 - 1   a  is connected to a common base end electrode unit  72 - 1   b . Similarly, the negative electrode pattern  74 - 1  has a comb shape in which one end (the lower end in  FIG. 7 ) of each of a plurality of bar-shaped negative electrodes  74 - 1   a  is connected to a common base end electrode unit  74 - 1   b . The positive electrode pattern  72 - 1  and the negative electrode pattern  74 - 1  are disposed in such a manner that the sides of the bar-shaped electrodes formed in comb shapes are positioned respectively alongside each other. The positive side base electrode section  72 - 1   b  is connected to the positive electrode of the DC high-voltage generator  78  through the switch SW 11 . The negative side base electrode section  74 - 1   b  is connected to the negative electrode of the DC high-voltage generator  78  through the switch SW 12 . A composition is adopted wherein, by controlling the switches SW 11  and SW 12 , the application (ON) or non-application (OFF) of voltage is controlled. 
       FIG. 8  is a cross-sectional view along line  8 - 8  in  FIG. 7 . As shown in  FIG. 8 , the electrode unit  28  is positioned below the minimally conductive belt  43 , which supports the recording medium  20 . The electrode unit  28  forms a layered structure in which an electrode layer  82  is provided on top of an insulating supporting layer  80 . The positive and negative electrodes  72  and  74  described in  FIG. 7  are formed within the same plane in the electrode layer  82 . Furthermore, the spaces between the electrodes  72  and  74  in the electrode layer  82  are filled with an insulating material  84 , thereby providing an electrical insulation between the electrodes. 
     The minimally conductive belt  43  covers the upper surface of the electrode unit  28  and makes contact with the rear surface of the recording medium  20 . Desirably, the electrical resistivity of the minimally conductive belt  43  is approximately 10 8  Ohm/cm to 10 12  Ohm/cm. Furthermore, desirably, the thickness of the minimally conductive belt  43  is approximately 0.01 mm to 10 mm. 
     The minimally conductive belt  43  covers the surface of the electrode layer  82  adjacent to the recording medium  20 , and serves to prevent human injury resulting from electrical shock, or the like, as well as protecting the positive and negative electrodes  72  and  74 . Furthermore, the minimally conductive belt  43  is prevented from remaining in a charged state when no printing operation is being performed, in other words, when the power supply is switched off. 
     When a prescribed voltage from the DC high-voltage generator  78  shown in  FIG. 7  is applied between the electrodes  72  and  74 , an electric field is generated between the adjacent electrodes  72  and  74 , as shown in  FIG. 8 . In  FIG. 8 , the lines of electric force  86  of the electric field generated in this case are shown by double-dotted broken lines. As shown in  FIG. 8 , the lines of electric force  86  of the electric field created between mutually adjacent electrodes  72  and  74  form approximately arc-shaped lines, and an electric field is also created above the print surface of the recording medium  20 . Consequently, an electric field is applied to the treatment liquid  88  having been deposited on the recording medium  20 . In this case, a minimal current flows through the treatment liquid  88  deposited on the recording medium  20 , through the minimally conductive belt  43  and the recording medium  20 . An electrorheological effect is thus produced in the treatment liquid  88  deposited on the recording medium  20 , thereby increasing the viscosity of the deposited treatment liquid  88 . This state of increased viscosity due to the aforementioned electrorheological effect is sustained while the electric field continues to be applied. Accordingly, the deposited treatment liquid droplet is maintained in a liquid state in a substantially hemispherical shape on the recording medium  20 , and its permeation into the recording medium  20 , landing interference, bleeding, or the like, are suppressed. 
     In the present embodiment, the intensity of the electric field applied to the recording medium  20  is dependent on the electrode pitch Wp between the adjacently disposed positive electrodes  72  and negative electrodes  74 , and the voltage applied between the electrodes. At a constant applied voltage, the smaller the electrode pitch Wp, the greater the intensity of the electric field at the recording medium  20 . Consequently, from the viewpoint of reducing the applied voltage, it is desirable that the electrode pitch Wp should be small, and more desirable that it should be approximately 0.1 mm to 20 mm. 
     Furthermore, the smaller the thickness of the electrodes  72  and  74  (namely, electrode width) Ws, the evener (substantially uniform) the intensity distribution of the electric field created on the recording medium  20 . Therefore, desirably, the electrode width Ws is small, and more desirably, it is approximately 0.01 mm to 10 mm. 
     Experimentation reveals that when the intensity of the electric field applied to the recording medium  20  lies within the range of 0.1 kV/mm to 10 kV/mm, a large electrorheological effect is obtained with respect to the treatment liquid droplets deposited on the recording medium  20 . Therefore, desirably, the electrode pitch Wp, electrode width Ws and applied voltage are set in such a manner that the intensity of the electric field applied to the recording medium  20  lies in the range of 0.1 kV/mm to 10 kV/mm. 
     Description of Ink Set 
     Next, an ink set (of treatment liquid and inks) used in the inkjet recording apparatus  10  according to the present embodiment is described. 
     The inkjet recording apparatus  10  in the present embodiment uses an ink set comprising: a treatment liquid containing a polymerization initiator, a coloring material dispersion inhibitor, a high-boiling-point organic solvent and electrorheological property introducing particles; and inks of respective colors including a polymerizable compound and a coloring material. 
     The polymerizable compound includes a compound having a function of generating a polymerization reaction and curing, by means of initial seeds, such as radicals generated by the polymerization initiator, which is described below. 
     Desirably, the polymerizable compound is an addition-polymerizable compound having at least one ethylenically unsaturated double bond, and preferably, it is selected from a multi-functional compound having at least one, or two or more, terminal ethylenically unsaturated bonds. This group of compounds is widely known in the related industrial field, and such compounds can be used without any particular restrictions. This group includes compounds having various chemical forms, such as monomers, pre-polymers, in other words, dimers, trimers and oligomers, or mixtures of these, and copolymers of these. 
     Desirably, the polymerizable compound has a polymerizable group, such as an acryloyl group, a methacryloyl group, an allyl group, a vinyl group, an inner double bonding group (maleic acid), or the like, and of these, compounds containing an acryloyl group or a methacryloyl group are more desirable, since they can generate a curing reaction at low energy. 
     It is possible to use either one type of polymerizable compound only, or two or more types of polymerizable compounds. 
     The content ratio of the polymerizable compound in the second liquid containing a coloring material (here, the second liquid serving as the ink of each color) is desirably in the range of 50 wt % to 99 wt % in the second liquid, more desirably, in the range of 70 wt % to 99 wt %, and even more desirably, in the range of 80 wt % to 99 wt %. 
     Here, the “polymerization initiator” indicates a compound which generates initial seeds, such as radicals, due to light energy, heat energy, or both light and heat energy, thereby starting and promoting the polymerization of the polymerizable compound, and it is possible to use selectively a commonly known thermal polymerization initiator, a compound or polymerization initiator having bonds of low bond disassociation energy, or light polymerization initiator or the like. 
     As a radical generating agent of this kind, it is possible to use, for example, an organic halide compound, a carbonyl compound, an organic peroxide compound, an azo type polymerization initiator, an azide compound, a metallocene compound, a hexaallyl biimidazole compound, an organic borate compound, a disulfonate compound, an onium compound, or the like. 
     In the ink set according to the present embodiment, of the plurality of types of liquids used, a polymerization initiator which causes curing of the polymerizable compound is contained in the first liquid serving as the treatment liquid (the pre-treatment liquid). 
     From the viewpoints of stability over time, curing characteristics, and curing speed, it is desirable that the content ratio of the polymerization initiator is 0.5 wt % to 20 wt % with respect to the total amount of polymerizable compound used in the ink set, and more desirably, 1 wt % to 15 wt %, and even more desirably, 3 wt % to 10 wt %. 
     For the polymerization initiator, it is possible to use one type of initiator, or a combination of two or more types of initiator. Furthermore, provided that the required beneficial effects are not impaired, it is also possible to use a commonly known sensitizing agent, conjointly, with the object of improving sensitivity. 
     There are no particular restrictions on the coloring material used in the present embodiment, and provided that it achieves a color hue and color density that matches the object of use of the ink, it is possible to select a coloring material appropriately from commonly known oil-based dyes and pigments. 
     There are no particular restrictions on the oil-based dyes which are usable in the present invention, and any desired oil-based dye may be used. Desirably, the content (converted to solid) ratio of the dye in a case where an oil-based dye is used as the coloring material is in the range of 0.05 wt % to 20 wt %, more desirably, 0.1 wt % to 15 wt %, and especially desirably, 0.2 wt % to 6 wt %. 
     A mode which uses a pigment as the coloring material is desirable from the viewpoint of enabling easy aggregation when mixing the plurality of types of liquids. 
     For the pigment used in the present embodiment, it is possible to use either an organic pigment or an inorganic pigment, and as regards a black pigment, a carbon black pigment, or the like, is desirable. Furthermore, in general, pigments of black, and three primary colors of cyan, magenta and yellow, are used; however, depending on the required objective, it is also possible to use pigments having color hues, such as red, green, blue, brown, white, or the like, or a metallic lustrous pigment, such as gold or silver, or a colorless or weakly colored body pigment, or the like. 
     Moreover, for a pigment, it is also possible to use particles having a core material including a particle of silica, alumina, or resin, with dye or pigment affixed to the surface thereof, or an insoluble lake compound of a dye, a colored emulsion, a colored latex, or the like. 
     Furthermore, it is also possible to use a pigment coated with a resin. This is called a micro-capsule pigment, and can be acquired as commercial products, from Dainippon Ink and Chemicals Incorporated, Toyo Ink Manufacturing Co., Ltd., and the like. 
     From the viewpoint of achieving a balance between optical density and stability during storage, it is desirable that the volume-average particle size of the pigment particles contained in the liquid of the present embodiment is in the range of 30 nm to 250 nm, and more desirably, 50 nm to 200 nm. Here, the volume-average particle size of the pigment particles can be measured by a measurement apparatus, such as LB-500 of Horiba, Ltd. 
     When a pigment is used as a coloring material, it is desirable from the viewpoint of optical density and ejection stability that the content (converted to solid) ratio is in the range of 0.1 wt % to 20 wt % in the second liquid (here, the second liquid serving as the ink of each color), and more desirably, in the range of 1 wt % to 10 wt %. 
     It is possible to use only one type of coloring material and it is also possible to combine two or more types of coloring material. Furthermore, it is possible to use different coloring materials or the same coloring material, for the respective liquids. 
     In the present embodiment, the coloring material dispersion inhibitor indicates a material contained in the first liquid, with the object of preventing the dispersion or bleeding of the second liquid having a coloring material (in other words, the second liquid serving as the ink) ejected in the form of droplets onto the first liquid that has been deposited on the recording medium (in other words, the first liquid serving as the treatment liquid). 
     For the coloring material dispersion inhibiting agent, at least one agent selected from a group including a polymer having an amino group, a polymer having an onium group, a polymer having a nitrogen-containing hetero ring, and a metal compound, is used. 
     It is possible to use only one type of polymer, and the like, or it is possible to combine a plurality of types of polymers. Here, “a plurality of types” includes, for example, a case of polymers which belong to the category of polymers having the same amino group, but which have different structures, or a case of polymers belonging to different types, such as a polymer having an amino group and a polymer having an onium group. Furthermore, it is also possible to make two or more of an amino group, an onium group, a nitrogen-containing hetero ring, and a metal compound coexist within the same molecule. 
     In the present embodiment, the high-boiling-point organic solvent (oil) means an organic solvent having a viscosity at 25° C. of 100 mPa·s or below or a viscosity at 60° C. of 30 mPa·s or below, and a boiling point above 100° C. Here, the viscosity is measured with a viscometer RE80 of Toki Sangyo Co., Ltd. The RE80 viscometer is based on a conical rotor/flat plate measurement system equivalent to an E type, and measurement is carried out using a Code No. 1 rotor, at a rotational speed of 10 rpm. In case of material having a viscosity greater than 60 mPa·s, according to requirements, measurement is carried out by changing the rotational speed to 5 rpm, 2.5 rpm, 1 rpm, 0.5 rpm, and the like. 
     Desirably, the amount of the high-boiling-point organic solvent used (converted to a coating amount) is 5 wt % to 2000 wt % and more desirably, 10 wt % to 1000 wt %. 
     In the ink set according to the present embodiment, it is possible to add a storage stabilizing agent, with the aim of suppressing unwanted polymerization during storage of the plurality of types of liquids. Desirably, a storage stabilizing agent is used by being contained in the same liquid as the polymerizable compound, and furthermore, desirably, a storage stabilizing agent that is soluble in the liquid or other components in which it is contained is used. 
     For the storage stabilizing agent, it is possible to use, a quaternary ammonium salt, a hydroxylamine, a cyclic amide, a nitrile, a substituted urea derivative, a complex ring compound, an organic acid, hydroquinone, hydroquinone monoether, an organic phosphine, a copper compound, or the like. 
     Desirably, the added amount of the storage stabilizing agent is adjusted suitably on the basis of the activity of the polymerization initiator used, the polymerization characteristics of the polymerizable compound, and the type of storage stabilizing agent, and it is desirable that the amount (in solid conversion) in the liquid is 0.005 wt % to 1 wt %, and more desirably, 0.01 wt % to 0.5 wt %, and even more desirably, 0.01 wt % to 0.2 wt %, from the viewpoint of achieving a balance between storage stability and curability of the ink when the liquids are mixed. 
     Description of Control System 
       FIG. 9  is a principal block diagram showing the system composition of the inkjet recording apparatus  10 . The inkjet recording apparatus  10  comprises a communication interface  100 , a system controller  102 , an image memory  104 , a ROM  106 , the medium type determination unit  25 , a motor driver  116 , a heater driver  18 , an electric field controller  120 , a light source controller  122 , a print controller  130 , an image buffer memory  132 , a treatment liquid controller  133 , a head driver  134 , and the like. 
     The communication interface  100  is an interface unit (image input unit) which functions as an image input device for receiving image data transmitted by a host computer  136 . For the communication interface  100 , a serial interface, such as USB (Universal Serial Bus), IEEE 1394, an Ethernet (registered tradename), or a wireless network, or the like, or a parallel interface, such as a Centronics interface, or the like, can be used. It is also possible to install a buffer memory (not illustrated) for achieving high-speed communications. 
     The image data sent from the host computer  136  is received by the inkjet recording apparatus  10  through the communication interface  100 , and is temporarily stored in the image memory  104 . The image memory  104  is a storage device for temporarily storing images input through the communication interface  100 , and data is written and read to and from the image memory  104  through the system controller  102 . The image memory  104  is not limited to a memory composed of semiconductor elements, and a hard disk drive or another magnetic medium may be used. 
     The system controller  102  is constituted by a central processing device (CPU) and peripheral circuits thereof, and the like, and it functions as a control device for controlling the whole of the inkjet recording apparatus  10  in accordance with a prescribed program, as well as a calculation device for performing various calculations. 
     More specifically, the system controller  102  is a control unit which controls the various sections, such as the communication interface  100 , image memory  104 , motor driver  116 , heater driver  118 , electric field controller  120 , light source controller  122 , print controller  130 , and the like, and as well as controlling communications with the host computer  136  and writing and reading to and from the image memory  104 , it also generates control signals for controlling the motor  138  and heater  139  of the conveyance system. 
     The program executed by the CPU of the system controller  102  and the various types of data which are required for control procedures are stored in the ROM  106 . The ROM  106  may be a non-rewriteable storage device, or it may be a rewriteable storage device, such as an EEPROM. The image memory  104  is used as a temporary storage region for the image data, and it is also used as a program development region and a calculation work region for the CPU. 
     As shown in  FIG. 1 , the medium type determination unit  25  acquires information relating to the medium type, and includes a device which determines the type, wettability, size, and the like, of the recording medium  20 . Furthermore, alternatively, it is also possible to adopt a composition in which information relating to the paper type, size, or the like, is specified by means of an input through a prescribed user interface, instead of or in conjunction with a sensor, or the like. 
     The information obtained by the medium type determination unit  25  is sent to the system controller  102  in  FIG. 9 . The system controller  102  calculates the on and off switching of the electric field, control target values for electric field application, and control target values for ultraviolet light irradiation, on the basis of the information obtained from the medium type determination unit  25 , and the image data for printing, and it controls the electric field controller  120  and the light source controller  122  in accordance with the calculation results. 
     The motor driver  116  is a driver (drive circuit) which drives the motor  138  in accordance with instructions from the system controller  102 . The heater driver  118  is a driver for driving the heater  139  of the heating drum  34  (see  FIG. 1 ), and other sections, in accordance with instructions from the system controller  102 . 
     The electric field controller  120  in  FIG. 9  controls the voltage generated by the DC high-voltage generator  78  in accordance with instructions from the system controller  102 , as well as outputting control signals for switching the switches SW 11  and SW 12  shown in  FIG. 7  on and off, and controlling the application or non-application of the electric field by the electrode unit  28  (see  FIGS. 1 ,  7  and  8 ), and the electric field intensity when the electric field is applied. More specifically, in the present embodiment, the combination of the electric field controller  120  and the system controller  102  in  FIG. 9  corresponds to the “electric field control device”. 
     The light source controller  122  in  FIG. 9  comprises a light source control circuit for controlling the on/off operation, the lighting position, and the amount of light generated in the ultraviolet light source  16 . The light source controller  122  controls light emission by the ultraviolet light source  16  in accordance with instructions from the system controller  102 . 
     The print controller  130  functions as a signal processing device which performs corrections and other types of processing in order to generate a signal for controlling ink ejection and a signal for controlling treatment liquid ejection, from the image data in the image memory  104  (multiple-value input image data), in accordance with the control implemented by the system controller  102 . Furthermore, the print controller  130  functions as an ink ejection control device which controls the ejection driving of the ink heads  50  by supplying the generated ink ejection data to the ink head driver  134 , as well as functioning as a treatment liquid deposition control device which controls the ejection operation of the treatment liquid head  11  by generating data for treatment liquid ejection in conjunction with the treatment liquid controller  133 . 
     The head driver  134  drives the actuators  58  which drive ejection in the respective heads  50 , on the basis of the ink ejection data supplied from the print controller  130 . A feedback control system for maintaining constant drive conditions for the print heads may be included in the head driver  134 . 
     Prescribed signal processing is applied to the input image data in the print controller  130 , and the treatment liquid ejection volume and ejection timing of the treatment liquid head  11  are controlled by means of the treatment liquid controller  133 , and the ink ejection volume and ejection timing of the ink heads  50  of the respective colors are controlled by means of the head driver  134 , on the basis of the image data. By this means, prescribed dot size and dot positions can be achieved. 
     The print controller  130  is provided with the image buffer memory  132 ; and image data, parameters, and other data are temporarily stored in the image buffer memory  132  when image data is processed in the print controller  130 . The aspect shown in  FIG. 9  is one in which the image buffer memory  132  accompanies the print controller  130 ; however, the image memory  104  may also serve as the image buffer memory  132 . Also possible is an aspect in which the print controller  130  and the system controller  102  are integrated to form a single processor. 
     To give a general description of the sequence of processing from image input to print output, image data to be printed (original image data) is input from an external source through a communication interface  100 , and is accumulated in the image memory  104 . At this stage, multiple-value RGB input image data is stored in the image memory  104 , for example. 
     In the inkjet recording apparatus  10 , an image which appears to have a continuous tonal gradation to the human eye is formed by changing the droplet ejection density and the dot size of fine dots created by ink (coloring material), and therefore, it is necessary to convert the input digital image into a dot pattern which reproduces the tonal gradations of the image (namely, the light and shade toning of the image) as faithfully as possible. Therefore, original image data (RGB data) stored in the image memory  104  is sent to the print controller  130  through the system controller  102 , and is converted to the dot data (droplet ejection arrangement data) for each ink color by a halftoning technique, using dithering, error diffusion, or the like. 
     In other words, the print controller  130  performs processing for converting the input RGB image data into dot data for the four colors of C, M, Y and K. The dot data generated by the print controller  130  in this way is stored in the image buffer memory  132 . This dot data of the respective colors is converted into C, M, Y, K droplet ejection data for ejecting inks from the nozzles of the ink heads  50 , thereby establishing the ink ejection data to be printed. 
     The ink head driver  134  outputs drive signals for driving actuators  58  corresponding to the respective nozzles  51  of the ink heads  50  on the basis of the ink ejection data supplied by the print controller  130 . 
     Similarly, the treatment liquid controller  133  outputs drive signals for driving the actuators corresponding to the respective nozzles of the treatment liquid head  11 , on the basis of the treatment liquid ejection data generated from the image data (treatment liquid dot data generated in correlation with the ink ejection volume). More specifically, the treatment liquid controller  133  also serves as the treatment liquid head driver. 
     By supplying the drive signals output by the treatment liquid controller  133  to the treatment liquid head  11 , treatment liquid is ejected from the corresponding nozzles. By supplying the drive signals output by the ink head driver  134  to the ink heads  50 , ink is ejected from the corresponding nozzles  51 . By controlling the ejection of treatment liquid from the treatment liquid head  11  and the ejection of ink from the ink heads  50  in synchronism with the conveyance speed of the recording medium  20 , an image is formed on the recording medium  20 . 
     As described above, the ejection volume and the ejection timing of the droplets from the treatment liquid head  11  and the ink heads  50  are controlled, on the basis of the treatment liquid ejection data and ink ejection data generated by implementing prescribed signal processing in the print controller  130 . By this means, prescribed dot size and dot positions can be achieved. 
     An electrorheological fluid (for example, particle-dispersed liquid) applied with an electric field by an external source, such as the electrode unit  28 , has a property whereby it does not flow unless the externally applied stress τ exceeds a certain uniform value τy (the yield stress). Furthermore, the value of this yield stress τy depends on the properties of the electrorheological fluid and the intensity of the electric field applied to the electrorheological fluid. By setting the yield stress τy to an appropriate value, it is possible to suppress the flow of the treatment liquid  88  and the permeation of the treatment liquid  88  into the recording medium  20  after its deposition on the recording medium  20 , and hence beneficial effects can be obtained in terms of improving printing quality. 
     For example, in respect of treatment liquid bleeding and spreading, the yield stress τy is set so as to satisfy the relationship:
 
“Capillary force between treatment liquid and medium”&lt;“Yield stress τy of treatment liquid”.  (Condition 1)
 
     Furthermore, in respect of interference between the droplets on the medium, and movement of the droplets, the yield stress τy is set so as to satisfy the relationship:
 
“Aggregation force between treatment liquid droplets”&lt;“Yield stress τy of treatment liquid”.  (Condition 2)
 
     Moreover, by setting the yield stress τy in such a manner that it satisfies both the conditions (1) and (2), and then applying an electric field of intensity corresponding to this yield stress value, it is possible to prevent bleeding and spreading of the treatment liquid  88  at the same time as avoiding interference between the droplets on the recording medium  20 , or movement of the droplets, on the surface of the recording medium  20 . 
     According to the inkjet recording apparatus  10  having the above-described composition, when an electric field is applied to the treatment liquid  88  having been deposited on the recording medium  20  (electric field ON), an electrorheological effect is generated, and the viscosity of the treatment liquid  88  increases, thereby suppressing the permeation of the treatment liquid  88  into the recording medium  20  and thus maintaining the shape of the droplets. In this state, bleeding and spreading of the treatment liquid  88  is suppressed, and interference between mutually adjacent treatment liquid droplets on the recording medium  20 , and movement of the treatment liquid and the like, is also restricted. 
     By ejecting inks containing coloring materials from the ink heads  50 , in this state, then it is possible to deposit the ink liquids onto an area where sufficient treatment liquid is present. Accordingly, it is possible reliably to cause mixing of the liquids of different types. In a state where the treatment liquid and the ink are reliably mixed together, the ink can be cured and fixed by irradiating light from the ultraviolet light source  16  (see  FIG. 1 ). 
     Since the permeability of the treatment liquid (the maintainability of the treatment liquid droplets on the recording medium) varies depending on the type of recording medium used, then, in the inkjet recording apparatus  10  according to the present embodiment, the on and off switching of the electric field, and the intensity of the application voltage (electric field intensity) when the electric field is applied, are controlled in accordance with the type of the recording medium  20 . 
       FIG. 10  is a flowchart indicating the control sequence of the inkjet recording apparatus  10  according to the present embodiment. Firstly, a medium type judgment process is implemented (step S 10 ). This judgment may be based, for example, on automatic determination by measuring the optical reflectivity of the recording medium  20 , or on determination of the paper magazine, or specification of a paper type through a user interface menu, or the like. 
     On the basis of the medium type judgment result in step S 10 , the judgment value corresponding to the type of recording medium  20  used is established to be A (step S 12 ). The inkjet recording apparatus  10  is provided with an information storage device (internal memory or external memory) which stores data for a medium type table that associates media types with judgment values. The judgment value is determined by referring to the medium type table. 
     Thereupon, the on/off switching of the electric field is selected in accordance with the judgment value=A determined at step S 12  (step S 14 ). In this, if a medium of greater permeability than a prescribed reference value (a permeable medium) is used, then switching on of the electric field is selected. On the other hand, if a medium having permeability not greater than the prescribed reference value (a medium of low permeability or a non-permeable medium) is used, then switching off of the electric field is selected. The permeation characteristics (permeation time) of the treatment liquid are evaluated previously for each type of medium, and a judgment value corresponding to the permeability is set, by taking account of the electric field on/off selection made in step S 12 . Furthermore, table data which associates the value of the judgment value=A with the on or off switching of the electric field is stored in the inkjet recording apparatus  10 , and the on/off switching of the electric field is selected on the basis of this table. 
     Next, the procedure advances to step S 16 , and an electric field application intensity corresponding to the judgment value=A is established. Table data which associates the value of the judgment value=A with the electric field intensity is stored in the inkjet recording apparatus  10 , and a suitable intensity is determined on the basis of this table. 
     According to the processing results of steps S 14  and S 16 , the electric field produced by the electrode unit  28  (see  FIGS. 1 ,  7  and  8 ) is controlled, and an image is formed on the recording medium  20  by controlling the deposition of treatment liquid and the ejection of ink on the basis of the image data (S 18  in  FIG. 10 ). 
     According to the above-described embodiment of the present invention, due to the electrorheological effect in the treatment liquid, the permeation of the treatment liquid into the recording medium  20  is suppressed, and hence ink can be deposited in a state where there is a sufficient amount of treatment liquid remaining on the surface of the recording medium. Therefore, the two liquids can be made to mix together reliably. Accordingly, high-quality image formation can be achieved. 
     Furthermore, the present embodiment also has the following advantages. Supposing that a composition is adopted in which particles that introduce electrorheological properties (electrorheological property introducing particles) are contained in an ultraviolet-curable ink, then if the coloring material is a pigment, it is necessary to disperse the pigment within the main ink component, and there is the possibility that the inclusion of a large amount of electrorheological property introducing particles (dispersed micro-particles), with the aim of producing a strong electrorheological effect, may impede the dispersion of the pigment. Furthermore, if there is a large content of electrorheological property introducing particles, then a problem arises in that the viscosity of the ink increases and it becomes impossible to eject the ink. On the other hand, according to this embodiment of the present invention, by combining dispersed electrorheological effect introducing micro-particles in the first liquid (treatment liquid) which does not contain coloring material, then even if the coloring material of the ultraviolet-curable ink is a pigment, it is possible to achieve a system which does not impede the dispersion of the pigment. Furthermore, even if a highly viscous treatment liquid becomes necessary in order to achieve a high electrorheological effect, it is still possible to deposit the treatment liquid by means of an application device or the like (a non-ejection device), rather than ejecting the treatment liquid by means of an inkjet method. 
     In other words, when implementing the present invention, it is also possible to use an application device, or other type of device, rather than a device which ejects liquid from inkjet nozzles, as a device which deposits the first liquid serving as the treatment liquid (pre-treatment liquid) onto the recording medium. 
     There are no particular restrictions on the device used for this application step, and it is possible to select a commonly known application device, according to the required objective. Possible examples of the application devices include: an air doctor coater, a blade coater, a rod coater, a knife coater, a squeeze coater, an immersion coater, a reverse roll coater, a transfer roll coater, a gravure coater, a kiss roll coater, a cast coater, a spray coater, a curtain coater, an extrusion coater, or the like. 
     Furthermore, in implementing the present invention, it is possible to use ultraviolet light, visible light, or the like, as an exposure light source which applies energy for promoting the polymerization of the polymerizable compound. Moreover, it is also possible to apply energy by means of radiation other than light, such as α rays, γ rays, X rays, an electron beam, or the like, but of the various options, the use of ultraviolet light or visible light is most desirable from the viewpoints of cost and safety, and use of ultraviolet light is especially desirable. The amount of energy required for the curing reaction varies depending on the type and amount of the polymerization initiator, and it is about 1 mJ/cm 2  to 500 mJ/cm 2  in general. 
     Second Embodiment 
     Next, a second embodiment of the present invention is described.  FIG. 11  is a principal compositional diagram of an inkjet recording apparatus  10 ′ according to a second embodiment of the present invention. In  FIG. 11 , the elements which are the same as or similar to the composition shown in  FIG. 1  are denoted with the same reference numerals and description thereof is omitted here. 
     The inkjet recording apparatus  10 ′ shown in  FIG. 11  uses two types of treatment liquid, as the first liquids serving as pre-treatment liquids. More specifically, the first treatment liquid (P 1 ) is an electrorheological fluid having a composition including a polymerization initiator, a coloring material dispersion inhibitor, an oil forming a high-boiling-point organic solvent, and particles that introduce electrorheological properties. The second treatment liquid (P 2 ) is a liquid which has no electrorheological properties (a non-electrorheological fluid) and contains a polymerization initiator, a coloring material dispersion inhibitor, and an oil forming a high-boiling-point organic solvent. 
     The first treatment liquid P 1  is used with respect to a recording medium having high permeability (permeable medium treatment liquid). On the other hand, the second treatment liquid P 2  is used with respect to a recording medium which is non-permeable or has low permeability (non-permeable medium treatment liquid). 
     The inkjet recording apparatus  10 ′ comprises an application roller  11 - 1  (corresponding to a “first treatment liquid deposition device”), as a first device for depositing the first treatment liquid P 1  on the recording medium (not shown in  FIG. 11 ), and an ejection head  11 - 2  (corresponding to a “second treatment liquid deposition device”), as a second device for deposing the second treatment liquid P 2  on the recording medium. 
     The inkjet recording apparatus  10 ′ further comprises a first treatment liquid storing and loading unit  13 - 1 , which stores the first treatment liquid P 1 , and a second treatment liquid storing and loading unit  13 - 2 , which stores the second treatment liquid P 2 . The application roller  11 - 1  receives a supply of the first treatment liquid P 1  from the first treatment liquid storing and loading unit  13 - 1 , through a tubing channel  30 P 1 , and the treatment liquid head  11 - 2  receives a supply of the second treatment liquid P 2  from the second treatment liquid storing and loading unit  13 - 2 , through a tubing channel  30 P 2 . 
     The application roller  11 - 1  is, for example, made of a porous member, and is composed in such a manner that the first treatment liquid P 1  is applied to a prescribed region of the recording medium (all or a portion of the recording medium), by moving the recording medium in the paper feed direction while causing the application roller  11 - 1  soaked with the treatment liquid P 1  to make contact with the recording medium. 
     The application roller  11 - 1  may have a length corresponding to the full width of the recording medium by means of one (a single) long roller member, and may also achieve the required length by aligning a plurality of roller modules divided in a direction (main scanning direction) substantially perpendicular to the conveyance direction of the recording medium. Furthermore, it is possible to adopt a composition in which a plurality of rows of application rollers are disposed in line with the conveyance direction of the recording medium. 
     Although not shown in  FIG. 11 , an elevator mechanism for raising and lowering the application roller  11 - 1  with respect to the recording medium is provided. By controlling the elevator mechanism in accordance with instructions from the system control system, thereby adjusting the height position of the application roller  11 - 1  (the relative position thereof in the direction perpendicular to the recording surface of the recording medium), it is possible to alter the contact pressure with respect to the recording medium, and the clearance with respect to the recording medium. In the case of a composition having a plurality of roller modules, a desirable mode is one in which a mechanism for controlling the vertical position is provided for each roller module. 
     The operation of depositing the treatment liquid P 2  by the treatment liquid head  11 - 2  shown in  FIG. 11  is the same as that of the treatment liquid head  11  described in  FIG. 1 , and description thereof is omitted here. 
     In the present embodiment, the first treatment liquid P 1  or the second treatment liquid P 2  is selected in accordance with the type of recording medium used, and the upward and downward movement of the application roller  11 - 1  is controlled, as well as the electric field applied by the electrode unit  28 , in conjunction with this selection. 
     In other words, the first treatment liquid P 1  is selected with respect to a recording medium of high permeability, and the application roller  11 - 1  is lowered to a position where it makes contact with the recording medium, thereby depositing the first treatment liquid P 1  onto the recording medium, in addition to which, a prescribed voltage is applied to the electrode unit  28 , thereby increasing the viscosity of the first treatment liquid P 1  on the recording medium, due to an electrorheological effect. On the other hand, the second treatment liquid P 2  is selected with respect to non-permeable medium or recording medium of low-permeability. In this case (when the first treatment liquid P 1  is not deposited), the application roller  11 - 1  is raised to a prescribed withdrawn position which is distanced from the recording medium, and furthermore, the electric field of the electrode unit  28  is switched to off, the treatment liquid head  11 - 2  is driven and the second treatment liquid P 2  is thereby deposited onto the recording medium. 
     From the viewpoint of arranging the electrode unit  28  satisfactorily in the required range, taking account of the region (range) in which an electric field is to be generated by the electrode unit  28 , a desirable mode is one in which the relative positional relationship of the application roller  11 - 1  and the treatment liquid head  11 - 2  is such that, as shown in  FIG. 11 , the treatment liquid head  11 - 2  is disposed on the upstream side (right-hand side in  FIG. 11 ) in terms of the conveyance direction of the recording medium, and the application roller  11 - 1  is disposed to the downstream side (at a later stage) from the treatment liquid head  11 - 2 . 
     Instead of the application roller structure which comprises the porous member described above, it is also possible to adopt a treatment liquid application mechanism (device) using an application roller made of a rubber member, or the like, and having a structure in which the treatment liquid is caused to flow onto the recording medium along the circumferential surface of the application roller, while rotating the application roller in a prescribed direction. 
     The above-described device that applies the treatment liquid by using a member such as the application roller  11 - 1 , or the like, has a merit in that it enables handling of a liquid of high viscosity of a level which is difficult to eject by means of an inkjet ejection head, as well as also enabling a large amount of liquid to be deposited in a short period of time. 
     On the other hand, if the composition that deposits the treatment liquid by means of the ejection head (the treatment liquid head  11  in  FIG. 1  or the treatment liquid head  11 - 2  in  FIG. 11 ) is adopted, then it is possible to apply the treatment liquid selectively to the required region of the recording medium (for example, only to the regions to be printed with ink), on the basis of the image data, and therefore a beneficial effect is obtained in that the amount of treatment liquid consumed can be reduced in comparison with the application device using a roller, or the like. 
       FIG. 12  is a principal block diagram showing the system configuration of the inkjet recording apparatus  10 ′ according to the second embodiment shown in  FIG. 11 . In  FIG. 12 , the elements which are the same as or similar to the composition described in  FIG. 9  are denoted with the same reference numerals and description thereof is omitted here. 
     The print controller  130  shown in  FIG. 12 , in conjunction with the treatment liquid controller  133 , selects the treatment liquid in accordance with the type of the recording medium, as well as controlling the application roller  11 - 1  and the treatment liquid head  11 - 2  in accordance with this selection. More specifically, in the present embodiment, the combination of the print controller  130  and the treatment liquid controller  133  corresponds to the “treatment liquid selection control device”. 
       FIG. 13  is a flowchart indicating the control sequence of the inkjet recording apparatus  10 ′ according to the second embodiment. In  FIG. 13 , the steps which are the same as or similar to those of the flowchart described in  FIG. 10  are denoted with the same step numbers and description thereof is omitted here. 
     In the flowchart in  FIG. 13 , step S 13  is introduced between steps S 12  and S 14  in  FIG. 10 . As shown in  FIG. 13 , after step S 12  the procedure advances to step S 13 . At step S 13 , the type of pre-treatment liquid corresponding to the judgment value=A determined at step S 12  is selected. In this, if a medium of greater permeability than a prescribed reference value (a permeable medium) is used, then the first treatment liquid is selected. On the other hand, if a medium having permeability not greater than the prescribed reference value (a medium of low permeability or a non-permeable medium) is used, then the second treatment liquid is selected. The permeation characteristics (permeation time) of the treatment liquid are evaluated previously for each type of medium, and a judgment value corresponding to the permeability is set, by taking account of the judgment on the selection of liquid type made in step S 13 . Furthermore, data for a table which specifies associations between the judgment value=A and the type of treatment liquid used is stored in the inkjet recording apparatus  10 , and the treatment liquid is determined on the basis of this table. 
     After step S 13 , the procedure advances to step S 14 , and switching on or off of the electric field is selected. The following steps are the same as those in  FIG. 10 , and description thereof is omitted here. 
     According to the second embodiment described above, it is possible to deposit the suitable treatment liquid in accordance with the type of recording medium. 
     Third Embodiment 
       FIG. 14  is a general schematic drawing of an inkjet recording apparatus  210  which forms a third embodiment of an image forming apparatus according to the present invention. In  FIG. 14 , the elements which are the same as or similar to the composition shown in  FIG. 1  are denoted with the same reference numerals and description thereof is omitted here. 
     As shown in  FIG. 14 , the inkjet recording apparatus  210  comprises: a treatment liquid application mechanism  211  (corresponding to the “treatment liquid deposition device”) for applying the first liquid serving as the treatment liquid (pre-treatment liquid); the plurality of ink ejection heads (corresponding to the “ink ejection devices”; hereinafter referred to as “ink heads”)  12 M,  12 C,  12 Y and  12 K, provided respectively to correspond to the inks (second liquids) of colors of magenta (M), cyan (C), yellow (Y), black (K); a treatment liquid tank  213  which stores the treatment liquid to be supplied to the treatment liquid application mechanism  211 ; the ink storing and loading unit  14  which stores the inks to be supplied to the ink heads  12 M,  12 C,  12 Y and  12 K; the ultraviolet light source (corresponding to the “radiation irradiation device”; hereinafter referred to as “ultraviolet light source”)  16  forming the fixing promotion device; the medium supply unit  22 ; the decurling unit  24 ; the medium type determination unit  25  (corresponding to the “recording medium type identification device”) which determines the type of recording medium  20 ; the conveyance unit  26  for conveying the recording medium  20 ; and the electrode unit  28  (corresponding to the “electric field deposition device”). 
     The treatment liquid application mechanism  211  comprises: an application roller  231 , which makes contact with the recording medium  20 ; a treatment liquid supply roller  232 , which makes external contact with the application roller  231  and supplies the treatment liquid to the application roller  231 ; and a container (immersion container)  233 , which holds the treatment liquid into which the treatment liquid supply roller  232  is immersed. 
     The treatment liquid tank  213  supplies the treatment liquid to the immersion container  233 , through a prescribed channel  35 T. The treatment liquid unit  213  has a warning device (for example, a display device or an alarm sound generator) for warning when the remaining amount of the treatment liquid is low, and it has a mechanism for preventing loading errors between types of liquid. Furthermore, in order to prevent aggregation and settling of the dispersed micro-particles contained in the treatment liquid, a stirring blade  236  is provided as a stirring device inside the treatment liquid tank  213 . The details of composition of the treatment liquid are described hereinafter. 
     The ink storing and loading unit  14  has the ink tanks  14 M,  14 C,  14 Y and  14 K for storing the inks of the colors corresponding to the respective ink heads  12 M,  12 C,  12 Y and  12 K, and the tanks are connected to the heads  12 M,  12 C,  12 Y and  12 K, through prescribed channels  35 M,  35 C,  35 Y and  35 K. The ink storing and loading unit  14  has a warning device (for example, a display device or an alarm sound generator) for warning when the remaining amount of any ink is low, and has a mechanism for preventing loading errors among the colors. 
     Here, for the first liquid or the treatment liquid, a transparent treatment liquid (which contains no coloring material) containing “a polymerization initiator, a coloring material dispersion inhibitor, an oil acting as a high-boiling-point organic solvent, and particles that introduce electrorheological properties (hereinafter referred to as “electrorheological property introducing particles”)” is used. 
     Here, the high-boiling-point organic solvent (oil) is used as the solvent; however, instead of this, it is also possible to use a radiation-curable monomer or oligomer as the solvent. 
     Moreover, the treatment liquid used in the third embodiment (and a fourth embodiment described hereinafter) has characteristics in which the materials selected for the solvent and the electrorheological property introducing particles are both colorless and transparent, and have mutually proximate refractive indices. In order to guarantee the transparency of the treatment liquid, it is preferable through experimental observation that the difference between the refractive indices of the solvent and the electrorheological property introducing particles is within 0.1, and more desirably within 0.05. 
     An example of such a combination is one where diethyl phthalate (refractive index=1.505) forming a treatment liquid oil, or hexanediol diacrylate (HDDA) (refractive index=1.456) forming a monomer liquid, is used as the solvent, and silica (refractive index=1.46) or mica (refractive index=1.56) is used for the electrorheological property introducing particles. 
     Furthermore, for the second liquids or the inks, inks having a liquid composition containing “an ultraviolet-curable polymerizable compound (monomer, oligomer, or the like), and a pigment forming a coloring material” are used, in equal number to the number of colors used (in the present embodiment, four colors of M, C, Y and K). The details of the ink set used in the present embodiment are described below. 
     When the ink and the treatment liquid mix together, the dispersion of the coloring material between deposited droplets is suppressed by the coloring material dispersion inhibitor in the treatment liquid, and the polymerization reaction of the liquids progresses due to the mixing of the two liquids and the irradiation of the radiation onto the mixed liquids, thereby curing and fixing the ink. 
     The application roller  231  shown in  FIG. 14  is constituted by a member made of rubber, or the like, and has a structure in which the treatment liquid is caused to flow onto the recording medium  20  along the circumferential surface of the application roller  231 , while rotating the application roller  231  in a prescribed direction. 
     This application roller  231  may have a length corresponding to the full width of the recording medium  20  by means of one (a single) long roller member, and may also achieve the required length by aligning a plurality of roller modules divided in a direction (main scanning direction) substantially perpendicular to the conveyance direction of the recording medium  20 . Furthermore, it is possible to adopt a composition in which a plurality of rows of application rollers are disposed in line with the conveyance direction of the recording medium  20 . 
     Although not shown in  FIG. 14 , an elevator mechanism for raising and lowering the application roller  231  with respect to the recording medium  20  is provided. By controlling the elevator mechanism in accordance with instructions from the system control system, thereby adjusting the height position of the application roller  231  (the relative position thereof in the direction perpendicular to the recording surface of the recording medium  20 ), it is possible to alter the content pressure with respect to the recording medium  20 , and the clearance with respect to the recording medium  20 . In the case of a composition having a plurality of roller modules, a desirable mode is one in which a mechanism for controlling the vertical position is provided for each roller module. 
     The electrode unit  28  attached to the conveyance unit  26  is disposed at least in a region which extends from the treatment liquid deposition start position of the application roller  231  to the ultraviolet light irradiation position of the ultraviolet light source  16 , and it is able to generate an electric field in this region. 
     As described with reference to  FIG. 7 , when a prescribed voltage from the DC high-voltage generator  78  is applied between the electrodes  72  and  74 , an electric field is generated between the adjacent electrodes  72  and  74 , as shown in  FIG. 15 . In  FIG. 15 , the lines of electric force  86  of the electric field generated in this case are shown by double-dotted broken lines. As shown in  FIG. 15 , the lines of electric force  86  of the electric field created between mutually adjacent electrodes  72  and  74  form approximately arc-shaped lines, and an electric field is also created above the print surface of the recording medium  20 . Consequently, an electric field is applied to the treatment liquid  88  having been deposited on the recording medium  20 . In this case, a minimal current flows through the treatment liquid  88  on the recording medium  20 , through the minimally conductive belt  43  and the recording medium  20 . An electrorheological effect is thus produced in the treatment liquid  88  deposited on the recording medium  20 , thereby increasing the viscosity of the deposited treatment liquid  88 . This state of increased viscosity due to the aforementioned electrorheological effect is sustained while the electric field continues to be applied. Accordingly, the deposited treatment liquid  88  is maintained in a liquid state, and its permeation into the recording medium  20 , bleeding, or the like, are suppressed. 
       FIG. 16  is a principal block diagram showing the system configuration of the inkjet recording apparatus  210  according to the third embodiment. In  FIG. 16 , the elements which are the same as or similar to the compositions described in  FIGS. 9 and 12  are denoted with the same reference numerals and description thereof is omitted here. 
     As shown in  FIG. 16 , the inkjet recording apparatus  210  comprises the treatment liquid controller  133  for controlling the application roller  231 . 
     The print controller  130  functions as the treatment liquid application control device which generates treatment liquid application data in conjunction with the treatment liquid controller  133 , and controls the application operation performed by the application roller  231 . 
     Prescribed signal processing is applied to the input image data in the print controller  130 , and application of the treatment liquid is controlled by means of the treatment liquid controller  133 , and the ink ejection volume and ejection timing of the ink heads  50  of the respective colors are controlled by means of the head driver  134 , on the basis of the image data. By this means, prescribed dot size and dot positions can be achieved. 
     Specifically, the treatment liquid controller  133  outputs drive signals for driving the application roller  231 , on the basis of the treatment liquid application data generated from the image data (treatment liquid volume data generated in correlation with the ink ejection volume). 
     The treatment liquid is deposited onto the recording medium  20  by means of the application roller  231  rotating while making contact with the recording medium  20 . Furthermore, by supplying the drive signals output by the ink head driver  134  to the ink heads  50 , ink is ejected from the corresponding nozzles  51 . By controlling the deposition of treatment liquid from the application roller  231  and the ejection of ink from the ink heads  50  in synchronism with the conveyance speed of the recording medium  20 , an image is formed on the recording medium  20 . 
     As described above, the treatment liquid application volume and application timing from the application roller  231 , and the ejection volume and the ejection timing of the droplets from the ink heads  50  are controlled, on the basis of the treatment liquid application data and ink ejection data generated by implementing prescribed signal processing in the print controller  130 . By this means, prescribed dot size and dot positions can be achieved. 
     According to the inkjet recording apparatus  10  having the above-described composition, as shown in  FIG. 17 , the treatment liquid  88  is applied to the recording medium  20  by the application roller  231 , and an electric field is applied through the conductive belt  43  to the treatment liquid  88  having been applied on the recording medium  20 . Due to the action of this electric field, an electrorheological effect is produced in the treatment liquid  88 , thereby increasing the viscosity of the treatment liquid  88 , and hence the permeation of the treatment liquid  88  into the recording medium  20  is suppressed and the treatment liquid  88  is maintained in a liquid state on the recording medium  20 . In this state, bleeding and spreading of the treatment liquid  88  is suppressed, and movement of the treatment liquid  88  on the recording medium  20  is also prevented. 
     By ejecting inks containing coloring materials from the ink heads  50  onto the treatment liquid  88  having been deposited on the recording medium  20 , it is possible to deposit the ink liquids onto an area where sufficient treatment liquid is present. Accordingly, it is possible reliably to cause mixing of the liquids of different types. Furthermore, due to the presence of the treatment liquid  88 , it is possible to avoid the phenomenon of “landing interference” which causes image degradation due to the ink droplets moving and becoming fixed in positions displaced from the original landing positions, and/or deformation and disruption of the shape of the ink droplets, as a result of the ink droplets combining together immediately after landing on the recording medium and before fixing on the recording medium, and it is also possible to inhibit the dispersion of the coloring material in the treatment liquid  88  by increasing the viscosity of the treatment liquid  88 . 
     In a state where the treatment liquid and the ink are reliably mixed together in this way, the ink can be cured and fixed by irradiating light from the ultraviolet light source  16  (see  FIG. 14 ). Since the permeability of the treatment liquid (the maintainability of the treatment liquid droplets on the recording medium) varies depending on the type of recording medium used, then, in the inkjet recording apparatus  10  according to the present embodiment, the on and off switching of the electric field, and the intensity of the application voltage (electric field intensity) when the electric field is applied, are controlled in accordance with the type of the recording medium  20 . 
     According to the above-described embodiment of the present invention, due to the electrorheological effect in the treatment liquid, the permeation of the treatment liquid into the recording medium  20  is suppressed, and hence ink can be deposited in a state where there is a sufficient amount of treatment liquid remaining on the surface of the recording medium. Therefore, the two liquids can be made to mix together reliably. Accordingly, high-quality image formation can be achieved. 
     Moreover, according to the present embodiment, materials are selected for both the solvent and the electrorheological property introducing particles in the treatment liquid that are colorless and transparent and have mutually proximate refractive indices. Therefore, it is possible to prevent the treatment liquid from becoming clouded, and hence the colorless and transparent treatment liquid can be achieved. Consequently, the colors of the ink can be reproduced faithfully. 
     Furthermore, the present embodiment also has the following advantages. Supposing that a composition is adopted in which particles that introduce electrorheological properties (electrorheological property introducing particles) are contained in an ultraviolet-curable ink, then if the coloring material is a pigment, it is necessary to disperse the pigment within the main ink component, and there is the possibility that the inclusion of a large amount of electrorheological property introducing particles (dispersed micro-particles), with the aim of producing a strong electrorheological effect, may impede the dispersion of the pigment. Furthermore, if there is a large content of electrorheological property introducing particles, then a problem arises in that the viscosity of the ink increases and it becomes impossible to eject the ink. On the other hand, according to this embodiment of the present invention, by combining dispersed electrorheological effect introducing micro-particles in the first liquid (treatment liquid) which does not contain coloring material, then even if the coloring material of the ultraviolet-curable ink is a pigment, it is possible to achieve a system which does not impede the dispersion of the pigment. Furthermore, even if a highly viscous treatment liquid becomes necessary in order to achieve a high electrorheological effect, it is still possible to deposit the treatment liquid by means of an application device or the like (a device other than one based on ejection by an inkjet method). 
     If the average particle size of the electrorheological property introducing particles, which are dispersed in the treatment liquid, is relatively large, for example, 0.3 μm to 10 μm, then an application device is suitable for the treatment liquid deposition device, and if the average particle size of the electrorheological property introducing particles dispersed in the treatment liquid is relatively small, for example, 100 nm to 1 μm, then a liquid ejection device based on an inkjet method is suitable. 
     If the average particle size of the electrorheological property introducing particles dispersed in the treatment liquid lies in the overlap between these ranges (the average particle size of 0.3 μm to 1 μm), then it is possible to use either an application device or a liquid ejection device. 
     Instead of the structure shown in  FIG. 14  using the application roller  231  made of the rubber member, or the like, it is also possible to adopt a treatment liquid application mechanism (device) which uses an application roller made of a porous member, whereby the treatment liquid is applied to a prescribed region of the recording medium (either the whole surface or a portion thereof), by moving the recording medium in the paper conveyance direction, while causing the application roller soaked with the treatment liquid to make contact with the recording medium  20 . 
     There are no particular restrictions on the device used for this application step, and it is possible to select a commonly known application device, according to the required objective. Possible examples of the application devices include: an air doctor coater, a blade coater, a rod coater, a knife coater, a squeeze coater, an immersion coater, a reverse roll coater, a transfer roll coater, a gravure coater, a kiss roll coater, a cast coater, a spray coater, a curtain coater, an extrusion coater, or the like. 
     Furthermore, in implementing the present invention, it is possible to use ultraviolet light, visible light, or the like, as an exposure light source which applies energy for promoting the polymerization of the polymerizable compound. Moreover, it is also possible to apply energy by means of radiation other than light, such as α rays, γ rays, X rays, an electron beam, or the like, but of the various options, the use of ultraviolet light or visible light is most desirable from the viewpoints of cost and safety, and use of ultraviolet light is especially desirable. The amount of energy required for the curing reaction varies depending on the type and amount of the polymerization initiator, and it is about 1 mJ/cm 2  to 500 mJ/cm 2  in general. 
     Fourth Embodiment 
     Next, a fourth embodiment of the present invention is described.  FIG. 18  is a principal compositional diagram of an inkjet recording apparatus  210 ′ according to a fourth embodiment of the present invention. In  FIG. 18 , the elements which are the same as or similar to the composition shown in  FIGS. 1 ,  14  and  17  are denoted with the same reference numerals and description thereof is omitted here. 
     The inkjet recording apparatus  210 ′ shown in  FIG. 18  comprises a treatment liquid ejection head (hereinafter referred to as “treatment liquid head”)  11 T of inkjet-type as a device for depositing the first liquid serving as the treatment liquid, rather than the treatment liquid application mechanism  211  shown in  FIGS. 14 and 17 . 
     When the treatment liquid is ejected toward the recording medium  20  by the treatment liquid head  11 T, an electrorheological effect is generated in the treatment liquid having been deposited on the recording medium  20  by applying an electric field to the deposited treatment liquid, thereby increasing the viscosity of the deposited treatment liquid. This state of increased viscosity due to the aforementioned electrorheological effect is sustained while the electric field continues to be applied. Accordingly, the deposited treatment liquid droplet is maintained in a liquid state in a substantially hemispherical shape on the recording medium  20 , and its permeation into the recording medium  20 , landing interference, bleeding, movement or the like, are suppressed. 
     In this high-viscosity state of the treatment liquid, ink droplets are deposited from the ink heads  12 M,  12 C,  12 Y and  12 K, onto the treatment liquid. 
     Similarly to the ink heads, the treatment liquid head  11 T is a full line head having a length corresponding to the maximum width of the recording medium  20  used with the inkjet recording apparatus  210 ′, and has a plurality of nozzles (ejection ports) arranged on a nozzle face through a length exceeding at least one edge of the maximum-size recording medium  20  (namely, the full width of the printable range). 
     Although not shown in the drawings, the structure of the treatment liquid head  11 T is generally the same as that of the ink head  50  shown in  FIGS. 2A to 5 . Since it is sufficient that the treatment liquid is deposited on the recording medium  20  in a substantially uniform (even) fashion in the region where ink droplets are to be deposited, then it is not necessary to form treatment liquid droplets to a high density, in comparison with the ink. Consequently, the treatment liquid head  11 T may also be composed with a reduced number of nozzles (a reduced nozzle density) in comparison with the ink head  50  for ejecting ink. Furthermore, a composition may also be adopted in which the nozzle diameter of the treatment liquid head  11 T is greater than the nozzle diameter of the ink head  50  for ejecting ink. 
     The treatment liquid tank  213  is connected to the treatment liquid head  11 T through a tubing channel  35 T, and the treatment liquid is supplied to the treatment liquid head  11 T through the tubing channel  35 T. The supply system for the treatment liquid and the cleaning device (restoration device) for the treatment liquid head  11 T have substantially the same composition as the ink supply system and cleaning device shown in  FIG. 6 , and they are not illustrated. 
     The system configuration of the inkjet recording apparatus  210 ′ of the fourth embodiment shown in  FIG. 18  is the same as the configuration described with reference to  FIG. 9 , and the description thereof is omitted here. 
     If the composition that deposits the treatment liquid by means of the inkjet-type liquid ejection head (treatment liquid head  11 T) as shown in  FIG. 9  or  18  is adopted, then it is possible to apply the treatment liquid selectively to the required region of the recording medium (for example, only to the regions to be printed with ink), on the basis of the image data, and therefore a beneficial effect is obtained in that wasteful consumption of treatment liquid can be reduced in comparison with the application device using a roller, or the like. 
     Modification 1 
     The above-described embodiments are related to the mixing of two liquids, namely, the pre-treatment liquid and the ink, but the present invention may also be applied to a case where a plurality of types of liquids, such as three or more types of liquids, are mixed together. 
     Furthermore, a mode is also possible in which a plurality of different types of pre-treatment liquids, such as two or more types, are prepared in advance, and one type of the treatment liquids or a suitable combination of two or more types of the treatment liquids are selected to be used, according to the type of recording medium used. 
     Modification 2 
     It is also possible to adopt a composition using an endless belt embedded with electrode pairs for generating electric field, instead of the conveyance unit  26  illustrated in  FIGS. 1 ,  7  and  8 . In this case, for example, the cross-sectional structure of the belt can be made similar to that shown in  FIG. 8 . Moreover, for the conveyance unit  26 , it is also possible to use a structure that conveys a table that supports the medium (a table conveyance mechanism), instead of the belt conveyance mechanism. 
     Modification 3 
     The inkjet recording apparatus  10  described with reference to  FIG. 1  has a composition in which the treatment liquid head  11  is arranged only on the upstream side of the ink head  12 C in terms of the conveyance direction of the recording medium (the right-hand side in  FIG. 1 ), and the inkjet recording apparatus  210 ′ described with reference to  FIG. 18  has a composition in which the treatment liquid head  11 T is arranged only on the upstream side of the ink head  12 M in terms of the conveyance direction of the recording medium (the right-hand side in  FIG. 18 ). However, in implementing the present invention, it is also possible to adopt a composition in which a plurality of treatment liquid heads are respectively disposed on the upstream sides of the ink heads  12 M,  12 C,  12 Y and  12 K. According to this composition, it is possible to deposit a suitable amount of treatment liquid for each color of ink. 
     Modification 4 
     The ultraviolet-curable ink is used in the above-described embodiments, but in implementing the present invention, the ink is not limited to a light-curable ink, of which ultraviolet-curable ink is a typical example, and other radiation-curable inks which are cured by electron beams, X rays, or the like, may also be used. In this case, a fixing promotion processing unit (radiation irradiation device) using a radiation source suitable for activating the hardening agent (namely, activating polymerization) is provided, according to the type of ink used. 
     Furthermore, the inkjet recording apparatus using the page-wide full line type head having a nozzle row of a length corresponding to the entire width of the recording medium is described in the embodiments, but the scope of application of the present invention is not limited to this, and the present invention may also be applied to an inkjet recording apparatus using a shuttle head which performs image recording while moving a short recording head reciprocally. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.