Abstract:
A drop-on-demand ink jet printing system includes an ink channel having a nozzle orifice through which ink droplets are ejected when ink in the ink channel is subjected to a momentary positive pressure wave. An ink feed passage opens into the ink channel to transport ink into the channel from an ink reservoir. A selectively-actuated valve, associated with the ink feed passage, restricts the flow of ink through the ink feed passage when actuated. The valve is actuated in timed association with the momentary pressure wave, whereby flow of ink past the valve from the ink channel towards the reservoir is inhibited. The ink feed passage may be a microfluidic channel, and the selectively-actuated valve a heater in thermal contact with at least a portion of the associated microfluidic channel, whereby thermally-responsive ink in the ink feed passage can selectively be heated by the heater such that the thermally-responsive ink will be caused to increase in viscosity to thereby restrict backward ink flow through the ink feed passage. The ink may be comprised of a carrier having a tri-block copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned co-pending U.S. patent application Ser. No. 09/735,322 filed in the names of Yang et al. on Dec. 12, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to a drop-on-demand ink jet printer in which the flow of ink toward the ink reservoir during droplet ejection is controlled. 
     BACKGROUND OF THE INVENTION 
     Drop-on-demand ink jet printers selectively eject droplets of ink toward a receiver to create an image. Such printers typically include a print head having an array of nozzles, each of which is supplied with ink from a reservoir. Each of the nozzles communicates with a chamber that can be pressurized in response to an electrical impulse to induce the generation of an ink droplet from the outlet of the nozzle. Some such printers, commercial and theoretically-known, use piezoelectric transducers to create the momentary forces necessary to generate an ink droplet. A squeezing action by the piezoelectric transducers causes ink to flow out of the nozzles, but also causes some ink to flow backward toward the ink reservoir. Considerable energy is wasted, as not all of the pressure generated by the piezoelectric transducers results in droplet formation. Thus, a higher voltage must be applied to compensate for the loss. 
     The amount of backward flow of ink may be reducible by providing a narrow entry channel into the ink chamber from the reservoir. However, this would result in an undesirable increase in chamber refill time. 
     SUMMARY OF THE INVENTION 
     According to the present invention, the amount of backward flow of ink is reduced, while allowing free forward flow into the ink chamber by providing a valve in the entry channel to the ink chamber. During droplet ejection, the valve chokes back flow to improve efficiency. During chamber refill, the valve is opened, reducing refill time. 
     While any valve would be useful, response time of the valve should be better than the refill time for the chamber. According to a preferred embodiment of the present invention, a thermally activated valve, in which heat causes a thermal-reversible gel to form in the fluid channel, is provided to impede ink flow. When the heat is reduced, the gel returns to a freely-flowing fluid. By timing the heat pulse and the piezo device, drop ejection efficiency and refill time can be optimized. 
     According to one feature of the present invention, a drop-on-demand ink jet printing system includes a channel having a nozzle orifice through which ink droplets are ejected when ink in the channel is subjected to a momentary positive pressure wave. An ink feed passage opens into the ink channel to transport ink into the channel from an ink reservoir. A selectively-actuated valve, associated with the ink feed passage, restricts the flow of ink through the ink feed passage when actuated. The valve is actuated in timed association with the momentary pressure wave, whereby flow of ink past the valve from the ink channel towards the reservoir is inhibited. 
     According to another feature of the present invention, the ink feed passage is a microfluidic channel, and the selectively-actuated valve comprises a heater in thermal contact with at least a portion of the associated microfluidic channel. Thermally-responsive ink in the ink feed passage can selectively be heated by the heater such that the thermally-responsive ink will be caused to increase in viscosity to thereby restrict ink flow through the ink feed passage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with the claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following detailed description when taken in conjunction with the following drawings wherein: 
     FIG. 1 is a simplified schematic view of an ink jet print head, showing ejection of a liquid droplet onto a receiver; 
     FIG. 2 is a graph of voltage versus time, illustrating the shape of an electrical drive waveform applied to an ink jet print head such as illustrated in FIG. 1; 
     FIG. 3 a  is a photomicrograph of liquid structure being ejected, at a time just before the liquid structure detaching from the nozzle plate, as a result of applying the electrical drive waveform in FIG. 2; 
     FIG. 3 b  is a photomicrograph of the liquid structures that are ejected, at a time 30 microseconds after the time shown in FIG. 3 a;    
     FIG. 4 is a cross-sectional side view of an inkjet print head of FIG. 1 showing in greater detail a single channel of the ink jet print head; and 
     FIG. 5 is a partial perspective view of the ink jet print head structure of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, an apparatus and method and in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     Referring to FIG. 1, an inkjet print head  200  is shown, ejecting a liquid droplet  20  through a nozzle plate  233 , onto a surface  14  of a moving receiver  16 . Print head  200  is supplied with ink to be ejected, and is activated by an electrical drive signal  30  produced by a signal generator. The ink jet print head may contain a piezoelectric actuator, whose electrodes are connected to receive drive signal  30 . The electrode polarities are chosen such that the downward-going voltage edge  301 , see FIG. 2, causes an outward mechanical expansion of an actuator, drawing ink  22  into print head  200 . The upward-going voltage edges  302  and  303  cause inward compression of the actuator, expelling liquid from the nozzles. Finally, the downward-going voltage edge  304  returns the actuator to its original state, in readiness for the next actuation. 
     FIG. 3 a  is a photomicrograph of a liquid structure ejected from nozzle plate  233  upon application electrical drive signal  30  to print head  200 . The liquid structure takes the form of a ligament  26 . The photomicrograph is taken at a time close to, but just before, detachment of the liquid structure from nozzle plate  233 . FIG. 3 b  is a photomicrograph, taken thirty microseconds after the time of FIG. 3 a , of the liquid structure. The ligament  26  has broken off one small drop, which then quickly combines with the main droplet  20 , as the shown in FIG. 3 b.    
     FIG. 4 is a cross-sectional side view of a single channel of ink jet print head  200 . Print head structure  200  comprises a transducer  202 , formed of piezoelectric material, into which is cut an ink channel  229  bordered along one end by nozzle plate  233  having a nozzle orifice  238  there through. A rear cover plate  248  is suitably secured to the other end of ink channel  229 . A cover  231  and a base portion  236  complete the enclosure of the ink channel, which is supplied with ink from an ink reservoir  210  through ink feed passage  247  in rear cover plate  248 . Actuation of transducer  202  results in the expulsion of ink droplets from ink channel  229  through nozzle orifice  238 . 
     FIG. 5 shows the print head transducer of FIG. 4 in greater detail. The print head transducer comprises a first wall portion  232 , a second wall portion  234 , and a base portion  236 . The upper surfaces of first and second wall portions  232  and  234  define a first face  207  of transducer  202 , and the lower surface of base portion  236  defines a second opposite face  209  of transducer  202 . Ink channel  229  is defined on three sides by the inner surface of base portion  236  and the inner wall surfaces of wall portions  232  and  234 , and is an elongated channel cut into the piezoelectric material of transducer  202 . This leaves a lengthwise opening along the upper first face of transducer  202 . One end of ink channel  229  is closed by nozzle plate  233 , while the other end is closed by rear cover plate  248 . A metallization layer  224  coats the inner surfaces of ink channel  229  and is also deposited along the upper surfaces of first wall portion  232  and second wall portion  234 . Cover  231  is bonded over the first face of transducer  202  to close the lengthwise lateral opening in ink channel  229 . A second metallization layer  222  coats the outer surfaces of base portion  236 , and also extends approximately half way up each of the outer surfaces of first and second wall portions  232  and  234 . 
     Metallization layer  222  defines an addressable electrode  260 , which is connected to signal generator  30   a  (FIG. 1) to provide electrical drive signals to actuate the piezoelectric material of transducer  202 . Metallization layer  224  defines a common electrode  262  that is maintained at ground potential. 
     The print head of FIGS. 4 and 5 works upon the principle of the piezoelectric effect, where the application of an electrical signal across certain faces of piezoelectric material produces a corresponding mechanical distortion or strain in that material. In general, an applied voltage of one polarity will cause material to bend in the first direction, and an applied voltage of the opposite polarity will cause material to bend in the second direction opposite that of the first. Application of a positive voltage to electrodes  260  results in movement of the base portion  236  and wall portions  232  and  234  inward, toward channel  229 , resulting in a diminishment of the interior volume of ink channel  229 . Upon application of negative voltage to addressable electrode  260  there is a resulting net volume increase in the interior volume of ink channel  229 . This change in volume within channel  229  generates an acoustic pressure wave within ink channel  229 , and this pressure wave within channel  229  provides energy to expel ink from orifice  238  of print head structure  220  onto receiver  16 . Typically, signals from an external encoder  35  are provided to a microprocessor  36  which outputs control signals to the signal generator linked to the motion of the print head so that the expelled ink droplets are ejected with optimal timing to impact the receiver at the correct position. 
     One or more heaters  300  are positioned on the inner surfaces of ink feed passage  247  in rear cover plate  248  such that a microfluidic valve is formed in the ink feed passage  247 . A single heater could extend substantially around the ink feed passage. The terms “microfluidic”, “microscale” and “microfabricated” generally refer to structural elements or features of a device, such as ink feed passage  247 , having at least one fabricated dimension in the range from about 0.1 μm to about 500 μm. In devices according to the present invention, microscale ink feed passage  247  preferably has at least one internal cross-section dimension, e.g., depth, width, length, diameter, etc., between about 0.1 μm to about 500 μm, preferably between about 1 μm to about 200 μm. 
     Heaters  300 , preferably made from appropriately doped polysilicon, are fabricated on the inner surfaces of ink feed passage  247 . A conducting material, not shown, such as aluminum or copper, is also integrated to serve as wires to connect the heaters to an external power supply. In a preferred embodiment of the invention, the microfluidic devices are fabricated using CMOS compatible fabrication techniques, and the heaters are integrated with a CMOS circuit controller  302  on the chip. The controller is adapted to actuate the valve by signals or voltages applied to the heaters. 
     Various techniques using chip technology for the fabrication of microfluidic devices, and particularly micro-capillary devices, with silicon and glass substrates have been discussed by Manz, et al. ( Trends in Anal. Chem.  1990, 10, 144, and  Adv. In Chromatog.  1993, 33, 1). Other techniques such as laser ablation, air abrasion, injection molding, embossing, etc., are also known to be used to fabricate microfluidic devices, assuming compatibility with the selected substrate materials. 
     The function of a microfluidic valve is to control the flow rate or volume flux of a liquid through a micro-capillary channel. In general, for a fluid with a viscosity of μ that is driven through a micro-capillary channel with a length of L by a pressure of P, the volume flux, Q, of the liquid pass through the channel is:          Q   =       P     μ                 L       ·   f       ,                          
     where ƒ is the dimension factor of the cross-section for the microfluidic channel. 
     For a circular cross-section capillary channel with a radius r:            f   c     =       π                   r   4       8       ,                          
     while for a rectangular cross-section channel with a width α, height b and aspect ratio η=b/α (η≧1)            f   R     =         a   4          [       η   12     -       16     π   5            tanh        (       π   2        η     )           ]       .                                         
     It is generally true that the flow rate or the volume flux is inversely proportional to the internal viscosity of fluid in the channel. Therefore, if one can control the viscosity of the fluid in the channel, one can indeed control the flow rate of the fluid passing though the channel. 
     The microfluidic ink feed system of the present invention has a microfluidic valve that utilizes the property of a specially formulated thermally-responsive fluid serving as the carrier fluid for transport of subject materials through a microfluidic channel such as ink feed passage  247 . The viscosity of the formulated thermally-responsive fluid is sensitive to the temperature, and preferably increases with applied heat. 
     The “subject materials” simply refers to the materials, such as chemical or biological compounds, of interest, which may also include a variety of different compounds, including chemical compounds, mixtures of chemical compounds, e.g., a dye, a pigment, a protein, DNA, a peptide, an antibody, an antigen, a cell, an organic compound, a surfactant, an emulsion, a dispersion, a polysaccharide, colloidal particles, organic or inorganic compounds, nucleic acids, or extracts made from biological materials, such as bacteria, plains, fungi, or animal cells or tissues, naturally occurring or synthetic compositions. In the preferred embodiment of the present invention, the subject material is a printing dye or pigment 
     The thermally-responsive material may comprise at least one kind of block copolymer with at least one block comprising poly(ethylene oxide), commonly referred to as PEO. In another form, the thermally-responsive material comprises a tri-block copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), commonly referred to as PEO-PPO-PEO dissolved in an aqueous solution. The preferred concentrations of the solutions are from about 5% to about 80%, preferably from 10% to 40% in weight. 
     The solutions at room temperature, e.g., 22° C., are fluidic with a typical viscosity less than 10 centipoise. The viscosity of the formulated solutions increases dramatically when raising the temperature from about 30° C. to about 80° C., as the solutions rapidly form non-fluidic gels at the elevated temperature. The viscosity change of the formulated solutions in response of temperature change is entirely reversible as the solutions turn to fluidic having the original viscosity when cooled down to its initial temperature. 
     In yet another form, a methyl cellulose polymer may be used as a thermally-responsive material in the carrier fluid. For example, 2.75 wt. % solution of METHOCEL® K100LV (Dow Chemical Co.) having a viscosity of about 1 poise at 50° C. and a viscosity of more than 10 poise at 75° C. can be used. 
     The ink used in the invention usually contains a colorant such as a pigment or dye. Suitable dyes include acid dyes, direct dyes, solvent dyes or reactive dyes listed in the COLOR INDEX but is not limited thereto. Metallized and non-metallized azo dyes may also be used as disclosed in U.S. Pat. No. 5,482,545, the disclosure of which is incorporated herein by reference. Other dyes which may be used are found in EP 802246-A1 and JP 09/202043, the disclosures of which are incorporated herein by reference. 
     Any of the known organic pigments can be used to prepare inkjet inks used in the invention. Pigments can be selected from those disclosed, for example, in U.S. Pat. Nos. 5,026,427; 5,085,698; 5,141,556; 5,160,370 and 5,169,436. The exact choice of pigment will depend upon the specific color reproduction and image stability requirements of the printer and application. For four-color printers, combinations of cyan, magenta, yellow and black (CMYK) pigments are used. An exemplary four color set is a cyan pigment, bis(phthalocyanyl-alumino)tetraphenyldisiloxane, quinacridone magenta (pigment red 122), pigment yellow 74 and carbon black (pigment black 7). 
     In addition to the thermally responsive material, a humectant may be employed in the inkjet compositions used in the invention to help prevent the ink from drying out or crusting in the orifices of the printhead. Examples of humectants which can be used include polyhydric alcohols, such as ethylene glycol, diethylene glycol(DEG), triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol, 2-ethyl-2-hydroxymethyl-1,3-propanediol(EHMP), 1,5 pentanediol, 1,2-hexanediol, 1,2,6-hexanetriol and thioglycol; lower alkyl mono- or di-ethers derived from alkylene glycols, such as ethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol mono-methyl or mono-ethyl ether, propylene glycol mono-methyl or mono-ethyl ether, triethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol di-methyl or di-ethyl ether, poly(ethylene glycol) monobutyl ether (PEGMBE), and diethylene glycol monobutylether(DEGMBE); nitrogen-containing compounds, such as urea, 2-pyrrolidinone, N-methyl-2-pyrrolidinone, and 1,3-dimethyl-2-imidazolidinone; and sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone. 
     Penetrants may also be added to the inks employed in the invention to help the ink penetrate the receiving substrate, especially when the substrate is a highly sized paper. Examples of such penetrants include alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; ketones or ketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol; ethers, such as tetrahydrofuran and dioxane; and esters, such as, ethyl lactate, ethylene carbonate and propylene carbonate. 
     Polymeric binders can also be added to the ink employed in the invention to improve the adhesion of the colorant to the support by forming a film that encapsulates the colorant upon drying. Examples of polymers that can be used include polyesters, polystyrene/acrylates, sulfonated polyesters, polyurethanes, polyimides and the like. The polymers may be present in amounts of from about 0.01 to about 15 percent by weight and more preferably from about 0.01 to about 5 percent by weight based on the total amount of components in the ink. 
     Surfactants may be added to the ink to adjust the surface tension to an appropriate level. The surfactants may be anionic, cationic, amphoteric or nonionic and used at levels of 0.01 to 1% of the ink composition. Preferred surfactants include Surfynol 465® (available from Air Products Corp.) and Tergitol 15-S-5® (available from Union Carbide). 
     A biocide may be added to the ink composition employed in the invention to suppress the growth of micro-organisms such as molds, fungi, etc. in aqueous inks. A preferred biocide for the ink composition employed in the present invention is Proxel® GXL (Zeneca Specialties Co.) at a final concentration of 0.0001-0.5 wt. %. 
     The pH of the aqueous ink compositions employed in the invention may be adjusted by the addition of organic or inorganic acids or bases. Useful inks may have a preferred pH of from about 2 to 10, depending upon the type of dye being used. Typical inorganic acids include hydrochloric, phosphoric and sulfuric acids. Typical organic acids include methanesulfonic, acetic and lactic acids. Typical inorganic bases include alkali metal hydroxides and carbonates. Typical organic bases include ammonia, triethanolamine and tetramethylethlenediamine. 
     A typical ink composition employed in the invention may comprise, for example, the following components by weight: colorant (0.05-20%), water (0-90%), a humectant (0-70%), the thermally responsive material (0.1-40%), penetrants (0-20%), surfactant (0-10%), biocide (0.05-5%) and pH control agents (0.1-10%). 
     Additional additives which may optionally be present in the inkjet ink compositions employed in the invention include thickeners, conductivity enhancing agents, anti-kogation agents, drying agents, waterfast agents, dye solubilizers, chelating agents, binders, light stabilizers, viscosifiers, buffering agents, anti-mold agents, anti-rusting agents, anti-curl agents, dispersants and defoamers. Examples of buffering agents include, but are not limited to sodium borate, sodium hydrogen phosphate, sodium dihydrogen phosphate, mixtures thereof and the like. 
     EXAMPLE 1 
     Viscosity vs. Temperature of Thermally-responsive Solutions 
     Thermally-responsive solutions were formulated by dissolving a tri-block copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), or PEO-PPO-PEO in an aqueous solution. A series of the PEO-PPO-PEO tri-block copolymers were obtained from BASF under the product trade name of Pluronic®. 
     A Rheometrics ARES Fluids Spectrometer, from Rheometric Scientific, Inc., equipped with a corvette geometry, was used to measure the oscillatory shear properties of the Pluronic® solutions. Dynamic viscosity was measured continuously as the temperature was ramped from 20° C. to 80° C. The typical ramp rate was 1° C. per minute. The fluids were initially characterized at 20° C. in a continuous shear experiment covering a typical range of shear rates from 1 to 100 per second. All were found to have low viscosity and Newtonian response. For the temperature scan experiments, a monitoring frequency of 10 radians per second was used. 
     The results are shown in the following tables: 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Viscosity (Poise) of Pluronic ® P85 Solutions 
               
             
          
           
               
                 Temperature (° C.) 
                 20% 
                 15% 
                 10% 
               
               
                   
               
             
          
           
               
                 25 
                 0.09 
                 0.037 
                 0.022 
               
               
                 30 
                 0.112 
                 0.033 
                 0.017 
               
               
                 35 
                 0.113 
                 0.031 
                 0.014 
               
               
                 40 
                 0.096 
                 0.026 
                 0.012 
               
               
                 45 
                 0.079 
                 0.022 
                 0.01 
               
               
                 50 
                 0.066 
                 0.019 
                 0.008 
               
               
                 55 
                 0.054 
                 0.016 
                 0.007 
               
               
                 60 
                 0.05 
                 0.014 
                 0.006 
               
               
                 62 
                 0.069 
                 0.016 
                 0.007 
               
               
                 64 
                 0.143 
                 0.029 
                 0.011 
               
               
                 66 
                 0.382 
                 0.065 
                 0.022 
               
               
                 68 
                 1.283 
                 0.185 
                 0.059 
               
               
                 70 
                 5.176 
                 0.792 
                 0.194 
               
               
                 72 
                 15.018 
                 3.684 
                 0.821 
               
               
                 74 
                 31.802 
                 11.303 
                 3.534 
               
               
                 76 
                 46.005 
                 21.505 
                 9.134 
               
               
                 78 
                 52.008 
                 28.574 
                 13.39 
               
               
                 80 
                 51.921 
                 30.369 
                 17.917 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Viscosity of 25% Pluronic ® L62 Solution 
               
             
          
           
               
                   
                 Temperature (° C.) 
                 Viscosity (Poise) 
               
               
                   
                   
               
             
          
           
               
                   
                 22 
                 0.072 
               
               
                   
                 25 
                 0.068 
               
               
                   
                 28 
                 0.069 
               
               
                   
                 30 
                 0.073 
               
               
                   
                 32 
                 0.081 
               
               
                   
                 34 
                 0.1 
               
               
                   
                 36 
                 0.136 
               
               
                   
                 38 
                 0.237 
               
               
                   
                 40 
                 0.44 
               
               
                   
                 42 
                 0.834 
               
               
                   
                 44 
                 0.976 
               
               
                   
                 46 
                 1.777 
               
               
                   
                 48 
                 5.864 
               
               
                   
                 49 
                 26.704 
               
               
                   
                 50 
                 37.107 
               
               
                   
                 52 
                 40.677 
               
               
                   
                 54 
                 35.045 
               
               
                   
                 56 
                 31.245 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Viscosity of 22% Pluronic ® F87 Solution 
               
             
          
           
               
                   
                 Temperature (° C.) 
                 Viscosity (Poise) 
               
               
                   
                   
               
             
          
           
               
                   
                 22 
                 0.201 
               
               
                   
                 25 
                 0.242 
               
               
                   
                 30 
                 0.525 
               
               
                   
                 32 
                 0.696 
               
               
                   
                 34 
                 0.968 
               
               
                   
                 36 
                 1.225 
               
               
                   
                 37 
                 1.505 
               
               
                   
                 38 
                  385 
               
               
                   
                 39 
                 13873 
               
               
                   
                 40 
                 17046 
               
               
                   
                 41 
                 15056 
               
               
                   
                 42 
                 14963 
               
               
                   
                 45 
                 14512 
               
               
                   
                 50 
                 15008 
               
               
                   
                 55 
                 15509 
               
               
                   
                   
               
             
          
         
       
     
     The above results show that the Pluronic® P85 solutions with the concentrations from 10% to 20% have viscosity increases of more than 3 orders of magnitude when the temperature increases from 60° C. to 80° C., the 25% Pluronic® L62 solution has a 3 orders of magnitude viscosity increase with temperature from 30° C. to 50° C., and the 22% Pluronic® F87 solution has a more than 5 orders of magnitude viscosity increase with temperature from 30° C. to 40° C. The results demonstrated that these fluids are thermally-responsive and can be used in the device and method of the invention. 
     EXAMPLE 2 
     A Set of Thermally Responsive Inks with CMYK Colors 
     The thermally responsive inks were formulated by dissolving 15% wt of Pluronic® P85 in an aqueous solution. For black ink, a 5% wt dye of Food Black2 was added, for cyan ink a 6% wt dye of Avecia ProJet® Cyan Fast2 was added, for magenta ink a 5% wt dye of Tricon acid Red52 was added, and for yellow ink a 5% wt dye of acid Yellow was added. The viscosity vs. temperature measurements of thermally responsive inks were carried as described above in Example 1 and the results are shown in Table 4. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Viscosity vs. temperature of the thermally responsive inks 
               
             
          
           
               
                   
                 Viscosity (CentiPoise) of Thermally 
               
               
                   
                 Responsive Inks 
               
             
          
           
               
                 Temperature (° C.) 
                 Black 
                 Cyan 
                 Magenta 
                 Yellow 
               
               
                   
               
             
          
           
               
                 25 
                 6.9 
                 5.1 
                 5.1 
                 6.1 
               
               
                 60 
                 3.2 
                 2.0 
                 2.1 
                 2.8 
               
               
                 85 
                 3200 
                 3100 
                 41 
                 30 
               
               
                   
               
             
          
         
       
     
     The above results show that all the formulated thermally responsive inks have viscosities less than 7 centipoise from room temperature to about 60° C. and have viscosities more than 30 centipoise at 85° C. The black and cyan inks even have viscosities more than 3000 centipoise at 85° C. The results demonstrated that these inks are thermally responsive and can be used in the method of the invention. 
     In operation, dye or pigment in a specially formulated thermally-responsive carrier fluid is transported through ink feed passage  247  past microfluidic valve heaters  300  during the downward-going voltage edge  301 , in FIG. 2, which causes an outward mechanical expansion of the actuator. Thus, ink is drawn into the interior volume of ink channel  229 . Coordinated with the upward-going voltage edges  302  and  303 , which cause an inward mechanical compression of the actuator to expel ink from the nozzle, heaters  300  receive electrical pulses to cause heat to be transmitted to the solution in ink feed passage  247 . The viscosity of the formulated solution increases dramatically when raising the temperature from about 30° C. to about 80° C., as the solutions rapidly form non-fluidic gels at the elevated temperature. The increased viscosity quickly forms a gel, blocking ink feed passage  247 . The viscosity change of the formulated solutions in response of temperature change is entirely reversible as the solutions turn to fluidic having the original viscosity when cooled down to its initial temperature. Flow resumes through passage  247  and the pressure returns to a level incapable of droplet formation. 
     By blocking the ink feed passage during compression of the actuator, backward flow of ink is inhibited, while allowing free forward flow into the ink chamber. During droplet ejection, the valve chokes back flow to improve efficiency. During chamber refill, the valve is opened, reducing refill time. By timing the heat pulse and the piezo device, dorp ejection efficiency and refill time can be optimized. 
     While different embodiments, applications and advantages of the invention have been shown and described with sufficient clarity to enable one skilled in the art to make and use the invention, it would be equally apparent to those skilled in the art that many more embodiments, applications and advantages are possible without deviating from the inventive concepts disclosed, described, and claimed herein. The invention, therefore, should only be restricted in accordance with the spirit of the claims appended hereto or their equivalents, and is not to be restricted by the specification, drawings or the description of the preferred embodiments. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.