Patent Publication Number: US-6340216-B1

Title: Ballistic aerosol marking apparatus for treating a substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to U.S. patent application Ser. Nos. 09/163,893, 09/164,124, 09/163,808, 09/163,765, 09/163,839 now U.S. Pat. No. 6,290,342, Ser. Nos. 09/163,954, 09/163,924, 09/163,904 now U.S. Pat. No. 6,116,718, Ser. Nos. 09/163,799, 09/163,664 now U.S. Pat. No. 6,265,050, Ser. Nos. 09/163,518, 09/164,104, 09/163,825, issued U.S. Pat. No. 5,717,986, and U.S. Pat. Nos. 5,422,698, 5,893,015, 5,968,674, and 5,853,906, each of the above being incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates generally to the field of making devices, and more particularly to a device capable of applying a treatment material to a substrate by introducing the treatment material into a high-velocity propellant stream. 
     Ink jet is currently a common printing technology. There are a variety of types of ink jet printing, including thermal ink jet (TIJ), piezo-electric ink jet, etc. In general, liquid ink droplets are ejected from an orifice located at a one terminus of a channel. In a TIJ printer, for example, a droplet is ejected by the explosive formation of a vapor bubble within an ink-bearing channel. The vapor bubble is formed by means of a heater, in the form of a resistor, located on one surface of the channel. 
     We have identified several disadvantages with TIJ (and other ink jet) systems known in the art. For a 300 spot-per-inch (spi) TIJ system, the exit orifice from which an ink droplet is ejected is typically on the order of about 64 μm in width, with a channel-to-channel spacing (pitch) of about 84 μm, and for a 600 dpi system width is about 35 μm and pitch of about 42 μm. A limit on the size of the exit orifice is imposed by the viscosity of the fluid ink used by these systems. It is possible to lower the viscosity of the ink by diluting it in increasing amounts of liquid (e.g., water) with an aim to reducing the exit orifice width. However, the increased liquid content of the ink results in increased wicking, paper wrinkle, and slower drying time of the ejected ink droplet, which negatively affects resolution, image quality (e.g., minimum spot size, inter-color mixing, spot shape), etc. The effect of this orifice width limitation is to limit resolution of TIJ printing, for example to well below 900 spi, because spot size is a function of the width of the exit orifice, and resolution is a function of spot size. 
     Another disadvantage of known ink jet technologies is the difficulty of producing greyscale printing. That is, it is very difficult for an ink jet system to produce varying size spots on a printed substrate. If one lowers the propulsive force (heat in a TIJ system) so as to eject less ink in an attempt to produce a smaller dot, or likewise increases the propulsive force to eject more ink and thereby to produce a larger dot, the trajectory of the ejected droplet is affected. This in turn renders precise dot placement difficult or impossible, and not only makes monochrome greyscale printing problematic, it makes multiple color greyscale ink jet printing impracticable. In addition, preferred greyscale printing is obtained not by varying the dot size, as is the case for TIJ, but by varying the dot density while keeping a constant dot size. 
     Still another disadvantage of common ink jet systems is rate of marking obtained. Approximately 80% of the time required to print a spot is taken by waiting for the ink jet channel to refill with ink by capillary action. To a certain degree, a more dilute ink flows faster, but raises the problem of wicking, substrate wrinkle, drying time, etc. discussed above. 
     One problem common to ejection printing systems is that the channels may become clogged. Systems such as TIJ which employ aqueous ink colorants are often sensitive to this problem, and routinely employ non-printing cycles for channel cleaning during operation. This is required since ink typically sits in an ejector waiting to be ejected during operation, and while sitting may begin to dry and lead to clogging. 
     Importantly, such prior art marking systems are not employed to apply a treatment material, such as a finish material, surface texture material, etc., to a substrate. Rather, such treatment materials are applied typically in the fabrication of the substrate or by dedicated process and equipment following marking. 
     Other technologies which may be relevant as background to the present invention include electrostatic grids, electrostatic ejection (so-called tone jet), acoustic ink printing, and certain aerosol and atomizing systems such as dye sublimation. 
     SUMMARY 
     The present invention is a novel system for applying a treatment material to a substrate, directly or indirectly, which overcomes the disadvantages referred to above, as well as others discussed further herein. In particular, the present invention is a system of the type including a propellant which travels through a channel, and a treatment material which is controllably (i.e., modifiable in use) introduced, or metered, into the channel such that energy from the propellant propels the treatment material to the substrate. The propellant is usually a dry gas which may continuously flow through the channel while the apparatus is in an operative configuration (i.e., in a power-on or similar state ready to mark). The system is referred to as “ballistic aerosol marking” in the sense that application of a material to a substrate (defined herein as “marking”) is achieved by in essence launching a non-colloidal, solid or semi-solid particulate, or alternatively a liquid, marking material at a substrate. The shape of the channel may result in a collimated (or focused) flight of the propellant and treatment material (or equivalently marking material) onto the substrate. 
     The following summary and detailed description describe many of the general features of a ballistic aerosol marking apparatus, and method of employing same. The present invention is, however, a subset of the complete description contained herein as will be apparent from the claims hereof. 
     In our system, the propellant may be introduced at a propellant port into the channel to form a propellant stream. A marking material may then be introduced into the propellant stream from one or more marking material inlet ports. The propellant may enter the channel at a high velocity. Alternatively, the propellant may be introduced into the channel at a high pressure, and the channel may include a constriction (e.g., de Laval or similar converging/diverging type nozzle) for converting the high pressure of the propellant to high velocity. In such a case, the propellant is introduced at a port located at a proximal end of the channel (defined as the converging region), and the marking material ports are provided near the distal end of the channel (at or further down-stream of a region defined as the diverging region), allowing for introduction of marking material into the propellant stream. 
     The port may provide for pre-marking treatment material (such as a marking material adherent), post-marking treatment material (such as a substrate surface finish material, e.g., matte or gloss coating, etc.), marking material not otherwise visible to the unaided eye (e.g., magnetic particle-bearing material, ultra violet-fluorescent material, etc.) or other marking material to be applied to the substrate. The marking material is imparted with kinetic energy from the propellant stream, and ejected from the channel at an exit orifice located at the distal end of the channel in a direction toward a substrate. 
     One or more such channels may be provided in a structure which, in one embodiment, is referred to herein as a print head. The width of the exit (or ejection) orifice of a channel is generally on the order of 250 μm or smaller, preferably in the range of 100 μm or smaller. Where more than one channel is provided, the pitch, or spacing from edge to edge (or center to center) between adjacent channels may also be on the order of 250 μm or smaller, preferably in the range of 100 μm or smaller. Alternatively, the channels may be staggered, allowing reduced edge-to-edge spacing. The exit orifice and/or some or all of each channel may have a circular, semicircular, oval, square, rectangular, triangular or other cross sectional shape when viewed along the direction of flow of the propellant stream (the channel&#39;s longitudinal axis). 
     The material to be applied to the substrate may be transported to a port by one or more of a wide variety of ways, including simple gravity feed, hydrodynamic, electrostatic, or ultrasonic transport, etc. The material may be metered out of the port into the propellant stream also by one of a wide variety of ways, including control of the transport mechanism, or a separate system such as pressure balancing, electrostatics, acoustic energy, ink jet, etc. 
     The material to be applied to the substrate may be a solid or semi-solid particulate material, a suspension of such a material in a carrier, a suspension of such a material in a carrier with a charge director, a phase change material, etc. One preferred embodiment employs a marking material which is particulate, solid or semi-solid, and dry or suspended in a liquid carrier. Such a marking material is referred to herein as a particulate marking material. This is to be distinguished from a liquid marking material, dissolved marking material, atomized marking material, or similar non-particulate material, which is generally referred to herein as a liquid marking material. However, the present invention is able to utilize such a liquid marking material in certain applications, as otherwise described herein. 
     In addition, the ability to use a wide variety of marking materials (e.g., not limited to aqueous marking material) allows the present invention to mark on a wide variety of substrates. For example, the present invention allows direct marking on non-porous substrates such as polymers, plastics, metals, glass, treated and finished surfaces, etc. The reduction in wicking and elimination of drying time also provides improved marking on porous substrates such as paper, textiles, ceramics, etc. In addition, the present invention may be configured for indirect marking, for example marking to an intermediate transfer roller or belt, marking to a viscous binder film and nip transfer system, etc. 
     The material to be deposited on a substrate may be subjected to post ejection modification, for example fusing or drying, curing, etc. In the case of fusing, the kinetic energy of the material to be deposited may itself be sufficient to effectively either soften or melt (generically referred to herein as “melt”) the marking material upon impact with the substrate and fuse it to the substrate. The substrate may be heated to enhance this process. Pressure rollers may be used to cold-fuse the marking material to the substrate. In-flight phase change (solid-liquid-solid) may alternatively be employed. A heated wire in the particle path is one way to accomplish the initial phase change. Alternatively, propellant temperature may accomplish this result. In one embodiment, a laser may be employed to heat and melt the particulate material in-flight to accomplish the initial phase change. The melting and fusing may also be electrostatically assisted (i.e., retaining the particulate material in a desired position to allow ample time for melting and fusing into a final desired position). The type of particulate may also dictate the post ejection modification. For example, UV curable materials may be cured by application of UV radiation, either in flight or when located on the material-bearing substrate. 
     Since propellant may continuously flow through a channel, channel clogging from the build-up of material is reduced or eliminated (the propellant effectively continuously cleans the channel). In addition, a closure may be provided which isolates the channels from the environment when the system is not in use. Alternatively, the print head and substrate support (e.g., platen) may be brought into physical contact to effect a closure of the channel. Initial and terminal cleaning cycles may be designed into operation of the printing system to optimize the cleaning of the channel(s). Waste material cleaned from the system may be deposited in a cleaning station. However, it is also possible to engage the closure against an orifice to redirect the propellant stream through the port and into the reservoir to thereby flush out the port. 
     Thus, the present invention and its various embodiments provide numerous advantages discussed above, as well as additional advantages which will be described in further detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings. The drawings, briefly described below, are not to scale. 
     FIG. 1 is a schematic illustration of a system for marking a substrate according to the present invention. 
     FIG. 2 is a cross sectional illustration of a marking apparatus according to one embodiment of the present invention. 
     FIG. 3 is another cross sectional illustration of a marking apparatus according to one embodiment of the present invention. 
     FIG. 4 is a plan view of one channel, with nozzle, of the marking apparatus shown in FIG.  3 . 
     FIGS. 5A through 5F are cross sectional views, in the longitudinal direction, of several examples of channels according to the present invention. 
     FIG. 6 is another plan view of one channel of a marking apparatus, without a nozzle, according to the present invention. 
     FIGS. 7A through 7D are cross sectional views, along the longitudinal axis, of several additional examples of channels according to the present invention. 
     FIGS. 8A and 8B are end views of non-staggered and two-dimensionally staggered arrays of channels according to the present invention. 
     FIG. 9 is a plan view of an array of channels of an apparatus according to one embodiment of the present invention. 
     FIGS. 10A and 10B are plan views of a portion of the array of channels shown in FIG. 9, illustrating two embodiments of ports according to the present invention. 
     FIGS. 11A and 11B are cross sectional illustrations of a marking apparatus with a removable body according to two different embodiments of the present invention. 
     FIG. 12 is a process flow diagram for the marking of a substrate according to the present invention. 
     FIG. 13A is a cross-sectional side view, and FIG. 13B is a top view, of a marking material metering device according to one embodiment of the present invention, employing an annular electrode. 
     FIG. 14 is cross-sectional side view of a marking material metering device according to another embodiment of the present invention, employing two electrodes. 
     FIG. 15 is a cross-sectional side view of a marking material metering device according to yet another embodiment of the present invention, employing an acoustic ink ejector. 
     FIG. 16 is a cross-sectional side view of a marking material metering device according to still another embodiment of the present invention, employing a TIJ ejector. 
     FIG. 17 is a cross-sectional side view of a marking material metering device according to a further embodiment of the present invention, employing a piezo-electric transducer/diaphragm. 
     FIG. 18 is a schematic illustration of an array of marking material metering devices connected for matrix addressing. 
     FIG. 19 is another schematic illustration of an array of marking material metering devices connected for matrix addressing. 
     FIG. 20 is a cross-sectional view of an embodiment for generating a fluidized bed of marking material in a cavity. 
     FIG. 21 is a plot of pressure versus time for a pressure balanced cavity embodiment. 
     FIG. 22 illustrates an embodiment of the present invention employing an alternative marking material delivery system. 
     FIG. 23 is a cross-sectional side view of a marking material transport device according to one embodiment of the present invention, employing an electrode grid and electrostatic traveling wave. 
     FIG. 24 is a cross sectional illustration of a combined marking material transport and metering assembly according to a further embodiment of the present invention. 
     FIGS. 25A and 25B illustrate one embodiment for replenishing a fluidized bed of marking material according to the present invention. 
     FIG. 26 is a plan view of an array of channels and addressing circuitry according to one embodiment of the present invention. 
     FIG. 27 is an illustration of the distribution of colors per spot size or (spot density) obtained by one embodiment of a ballistic aerosol marking apparatus of the present invention. 
     FIG. 28 is an illustration of one example of the propellant flow patterns upon their interfacing with a substrate, viewed perpendicular to the substrate. 
     FIG. 29 is a side view of one of the propellant flow patterns of FIG. 28, and also an illustration of the marking material particle distribution as a function of position within the propellant stream. 
     FIG. 30 is a model used for the derivation of a worst case scenario for marking material lateral offset from a spot centroid. 
     FIG. 31 is a model used for the derivation of an example of laser power required for laser-assisted post-ejection marking material modification, such as assisted fusing. 
     FIG. 32 is an illustration of a ballistic aerosol marking apparatus having electrostatically assisted marking material extraction and/or pre-fusing retention. 
     FIG. 33 is cross sectional illustration of one embodiment of the present invention employing solid marking material particles suspended in a liquid carrier medium. 
     FIG. 34 is a plot of the number of particles versus kinetic energy, illustrating the kinetic fusion threshold for one embodiment of the present invention. 
     FIG. 35 is a plot of propellant velocity at an exit orifice versus propellant pressure for channels with and without converging/diversing regions according to the present invention. 
     FIG. 36 is a cut-away plan view of a channel and beam of light, arranged to provide light-assisted post-ejection marking material modification. 
     FIG. 37 is a plot of light source power versus marking material particle size, demonstrating the feasibility of employing light-assisted post-ejection marking material modification. 
     FIG. 38 is an illustration of a ballistic aerosol marking apparatus employing a closure structure for reducing or preventing clogging, humidity effects, etc. according to one embodiment of the present invention. 
     FIG. 39 is an illustration of a channel closure obtained by moving a platen into contact with an exit orifice according to one embodiment of the present invention. 
     FIGS. 40A-F are illustrations of one process for producing a print head according to the present invention. 
     FIG. 41 is an illustration of selected portions of another embodiment of a ballistic aerosol marking apparatus according to the present invention 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numeric ranges are provided for various aspects of the embodiments described, such as pressures, velocities, widths, lengths, etc. These recited ranges are to be treated as examples only, and are not intended to limit the scope of the claims hereof. In addition, a number of materials are identified as suitable for various facets of the embodiments, such as for marking materials, propellants, body structures, etc. These recited materials are also to be treated as exemplary, and are not intended to limit the scope of the claims hereof. 
     With reference now to FIG. 1, shown therein is a schematic illustration of a ballistic aerosol marking device  10  according to one embodiment of the present invention. As shown therein, device  10  consists of one or more ejectors  12  to which a propellant  14  is fed. A marking material  16 , which may be transported by a transport  18  under the control of control  20  is introduced into ejector  12 . (Optional elements are indicated by dashed lines.) The marking material is metered (that is controllably introduced) into the ejector by metering means  21 , under control of control  22 . The marking material ejected by ejector  12  may be subject to post ejection modification  23 , optionally also part of device  10 . Each of these elements will be described in further detail below. It will be appreciated that device  10  may form a part of a printer, for example of the type commonly attached to a computer network, personal computer or the like, part of a facsimile machine, part of a document duplicator, part of a labeling apparatus, or part of any other of a wide variety of marking devices. 
     The embodiment illustrated in FIG. 1 may be realized by a ballistic aerosol marking device  24  of the type shown in the cut-away side view of FIG.  2 . According to this embodiment, the materials to be deposited will be one or more pre- or post-marking material, of a type described further herein, which may be deposited individually, concomitantly, either mixed or unmixed, successively, or otherwise. While the illustration of FIG.  2  and the associated description contemplates a device for marking with four materials (either one at a time or in mixtures thereof), a device for marking with a fewer or a greater number of materials, or other or additional materials such as printing materials not visible to the unaided eye (such as magnetic particles, ultra violet-fluorescent particles, etc.) or other material associated with a marked substrate, is clearly contemplated herein. 
     Device  24  consists of a body  26  within which is formed one or more cavities  28 ′,  28 ″,  28 ′″, and  28 ″″, etc. (generically referred to as cavities  28 ) for receiving materials to be deposited. Also formed in body  26  may be a propellant cavity  30 . A fitting  32  may be provided for connecting propellant cavity  30  to a propellant source  33  such as a compressor, a propellant reservoir, or the like. Body  26  may be connected to a print head  34 , comprised of among other layers, substrate  36  and channel layer  37  that will be discussed later. 
     With reference now to FIG. 3, shown therein is a cut-away cross section of a portion of device  24 . Each of cavities  28  include a port  42 ′,  42 ″,  42 ′″, and  42 ″″, etc. (generically referred to as ports  42 ) respectively, of circular, oval, rectangular or other cross-section, providing communication between said cavities and a channel  46  which adjoins body  26 . Ports  42  are shown having a longitudinal axis roughly perpendicular to the longitudinal axis of channel  46 . However, the angle between the longitudinal axes of ports  42  and channel  46  may be other than 90 degrees, as appropriate for the particular application of the present invention. 
     Likewise, propellant cavity  30  includes a port  44 , of circular, oval, rectangular or other cross-section, between said cavity and channel  46  through which propellant may travel. Alternatively, print head  34  may be provided with a port  44 ′ in substrate  36  or port  44 ″ in channel layer  37 , or combinations thereof, for the introduction of propellant into channel  46 . As will be described further below, marking material is caused to flow out from cavities  28  through ports  42  and into a stream of propellant flowing through channel  46 . The marking material and propellant are directed in the direction of arrow A toward a substrate  38 , for example paper, supported by a platen  40 , as shown in FIG.  2 . We have experimentally demonstrated a propellant marking material flow pattern from a print head employing a number of the features described herein which remains relatively collimated for a distance of up to 10 millimeters, with an optimal printing spacing on the order of between one and several millimeters. For example, the print head produces a marking material stream which does not deviate by more than between 20 percent, and preferably by not more than 10 percent, from the width of the exit orifice for a distance of at least 4 times the exit orifice width. However, the appropriate spacing between the print head and the substrate is a function of many parameters, and does not itself form a part of the present invention. 
     Referring again to FIG. 3, according to one embodiment of the present invention, print head  34  consists of a substrate  36  and channel layer  37  in which is formed channel  46 . Additional layers, such as an insulating layer, capping layer, etc. (not shown) may also form a part of print head  34 . Substrate  36  is formed of a suitable material such as glass, ceramic, etc., on which (directly or indirectly) is formed a relatively thick material, such as a thick permanent photoresist (e.g., a liquid photosensitive epoxy such as SU-8, from Microlithography Chemicals, Inc; see also U.S. Pat. No. 4,882,245) and/or a dry film-based photoresist such as the Riston photopolymer resist series, available from DuPont Printed Circuit Materials, Research Triangle Park, N.C. (see, www.dupont.com/pcm/) which may be etched, machined, or otherwise in which may be formed a channel with features described below. 
     Referring now to FIG. 4, which is a cut-away plan view of print head  34 , in one embodiment channel  46  is formed to have at a first, proximal end a propellant receiving region  47 , an adjacent converging region  48 , a diverging region  50 , and a marking material injection region  52 . The point of transition between the converging region  48  and diverging region  50  is referred to as throat  53 , and the converging region  48 , diverging region  50 , and throat  53  are collectively referred to as a nozzle. The general shape of such a channel is sometimes referred to as a de Laval expansion pipe. An exit orifice  56  is located at the distal end of channel  46 . 
     In the embodiment of the present invention shown in FIGS. 3 and 4, region  48  converges in the plane of FIG. 4, but not in the plane of FIG. 3, and likewise region  50  diverges in the plane of FIG. 4, but not in the plane of FIG.  3 . Typically, this determines the cross-sectional shape of the exit orifice  56 . For example, the shape of orifice  56  illustrated in FIG. 5A corresponds to the device shown in FIGS. 3 and 4. However, the channel may be fabricated such that these regions converge/diverge in the plane of FIG. 3, but not in the plane of FIG. 4 (illustrated in FIG.  5 B), or in both the planes of FIGS. 3 and 4 (illustrated in FIG.  5 C), or in some other plane or set of planes, or in all planes (examples illustrated in FIGS. 5D-5F) as may be determined by the manufacture and application of the present invention. 
     In another embodiment, shown in FIG. 6, channel  46  is not provided with a converging and diverging region, but rather has a uniform cross section along its axis. This cross section may be rectangular or square (illustrated in FIG.  7 A), oval or circular (illustrated in FIG.  7 B), or other cross section (examples are illustrated in FIGS.  7 C- 7 D), as may be determined by the manufacture and application of the present invention. 
     Referring again to FIG. 3, propellant enters channel  46  through port  44 , from propellant cavity  30 , roughly perpendicular to the long axis of channel  46 . According to another embodiment, the propellant enters the channel parallel (or at some other angle) to the long axis of channel  46  by, for example, ports  44 ′ or  44 ″ or other manner not shown. The propellant may continuously flow through the channel while the marking apparatus is in an operative configuration (e.g., a “power on” or similar state ready to mark), or may be modulated such that propellant passes through the channel only when marking material is to be ejected, as dictated by the particular application of the present invention. Such propellant modulation may be accomplished by a valve  31  interposed between the propellant source  33  and the channel  46 , by modulating the generation of the propellant for example by turning on and off a compressor or selectively initiating a chemical reaction designed to generate propellant, or by other means not shown. 
     Marking material may controllably enter the channel through one or more ports  42  located in the marking material injection region  52 . That is, during use, the amount of marking material introduced into the propellant stream may be controlled from zero to a maximum per spot. The propellant and marking material travel from the proximal end to a distal end of channel  46  at which is located exit orifice  56 . 
     Print head  34  may be formed by one of a wide variety of methods. As an example, and with reference to FIGS. 40A-F, print head  34  may be manufactured as follows. Initially, a substrate  38 , for example an insulating substrate such as glass or a semi-insulating substrate such as silicon, or alternatively an arbitrary substrate coated with an insulating layer, is cleaned and otherwise prepared for lithography. One or more metal electrodes  54  may be formed on (e.g., photolithographically) or applied to a first surface of substrate  38 , which shall form the bottom of a channel  46 . This is illustrated in FIG.  40 A. 
     Next, a thick photoresist such as the aforementioned SU-8 is coated over substantially the entire substrate, typically by a spin-on process, although layer  310  may be laminated as an alternative. Layer  310  will be relatively quite thick, for example on the order of 100 μm or thicker. This is illustrated in FIG.  40 B. Well known processes such as lithography, ion milling, etc., are next employed to form a channel  46  in layer  310 , preferably with a converging region  48 , diverging region  50 , and throat  53 . The structure at this point is shown in a plan view in FIG.  40 C. 
     At this point, one alternative is to machine an inlet  44 ′ (shown in FIG. 3) for propellant through the substrate in propellant receiving region  47 . This may be accomplished by diamond drilling, ultrasonic drilling, or other technique well known in the art as a function of the selected substrate material. Alternatively, a propellant inlet  44 ″ (shown in FIG. 3) may be formed in layer  310 . However, a propellant inlet  44  may be formed in a subsequently applied layer, as described following. 
     Applied directly on top of layer  310  is another relatively thick layer of photoresist  312 , this preferably the aforementioned Riston or similar material. Layer  312  is preferably on the order of 100 μm thick or thicker, and is preferably applied by lamination, although it may alternatively be spun on or otherwise deposited. Layer  312  may alternatively be glass (such as Corning 7740) or other appropriate material bonded to layer  310 . The structure at this point is illustrated in FIG.  40 D. 
     Layer  312  is then patterned, for example by photolithography, ion milling, etc. to form ports  42  and  44 . Layer  312  may also be machined, or otherwise patterned by methods known in the art. The structure at this point is shown in FIG.  40 E. 
     One alternative to the above is to form channel  46  directly in the substrate, for example by photolithography, ion milling, etc. Layer  312  may still be applied as described above. Still another alternative is to form the print head from acrylic, or similar moldable and/or machinable material with channel  46  molded or machined therein. In addition to the above, layer  312  may also be a similar material in this embodiment, bonded by appropriate means to the remainder of the structure. 
     A supplement to the above is to preform electrodes  314  and  315 , which may be rectangular, annular (shown), or other shape in plan form, on layer  312  prior to applying layer  312  over layer  310 . In this embodiment, port  42 , and possibly port  44 , will also be preformed prior to application of layer  312 . Electrodes  314  may be formed by sputtering, lift-off, or other techniques, and may be any appropriate metal such as aluminum or the like. A dielectric layer  316  may be applied to protect the electrodes  314  and provide a planarized upper surface  318 . A second dielectric layer (not shown) may similarly be applied to a lower surface  319  of layer  312  to similarly protect electrode  315  and provide a planarized lower surface. The structure of this embodiment is shown in FIG.  40 F. 
     While FIGS. 4-7 illustrate a print head  34  having one channel therein, it will be appreciated that a print head according to the present invention may have an arbitrary number of channels, and range from several hundred micrometers across with one or several channels, to a page-width (e.g., 8.5 or more inches across) with thousands of channels. The width W of each exit orifice  56  may be on the order of 250 μm or smaller, preferably in the range of 100 μm or smaller. The pitch P, or spacing from edge to edge (or center to center) between adjacent exit orifices  56  may also be on the order of 250 μm or smaller, preferably in the range of 100 μm or smaller in non-staggered array, illustrated in end view in FIG.  8 A. In a two-dimensionally staggered array, of the type shown in FIG. 8B, the pitch may be further reduced. For example, Table 1 illustrates typical pitch and width dimensions for different resolutions of a non-staggered array. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Resolution 
                 Pitch 
                 Width 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 300 
                 84 
                 60 
               
               
                 600 
                 42 
                 30 
               
               
                 900 
                 32 
                 22 
               
               
                 1200 
                 21 
                 15 
               
               
                   
               
            
           
         
       
     
     As illustrated in FIG. 9, a wide array of channels in a print head may be provided with marking material by continuous cavities  28 , with ports  42  associated with each channel  46 . Likewise, a continuous propellant cavity  30  may service each channel  46  through an associated port  44 . Ports  42  may be discrete openings in the cavities, as illustrated in FIG. 10A, or may be formed by a continuous opening  43  (illustrated by one such opening  43 C) extending across the entire array, as illustrated in FIG.  10 B. 
     In an array of channels  46 , each channel may have similar dimensions and cross-sectional profiles so as to obtain identical or nearly identical propellant velocities therethrough. Alternatively, other arrays of channels  46  arranged in tandem in the process direction may be made to have different dimensions and/or cross sectional profiles to (or by other means such as selectively applied coatings or the like) provide channels having different propellant velocities. This may prove advantageous when seeking to employ different marking materials having significantly different masses, when seeking to have different marking effects, in the co-application of marking materials and other substrate treatment, or as might otherwise prove appropriate in a particular application of the present invention. 
     According to embodiments shown in FIGS. 11A and 11B, device  24  includes a replacably removable body  60 , retained to device  24  by operable means such as clips, clasps, catches, or other retaining means well known in the art (not shown). In the embodiment shown in FIG. 11A, body  60  is removable from print head  34  and the other components of device  24 . In the embodiment shown in FIG. 11B, body  60  and print head  34  form a unit replaceable removable from a mounting region  64  of device  24 . In either embodiment of FIGS. 11A or  11 B, electrical contacts may be provided between body  60  and device  24  for control of electrodes and other apparatus carried by or associated with body  60 . 
     In either case, body  60  may be a disposable cartridge carrying marking material and propellant, as described in the aforementioned application Ser. No. 09/163,765. Alternatively, the marking material and/or propellant cavities  28 ,  30  may be refillable. For example, openings  29 ′,  29 ″,  29 ′″, and  29 ″″ (generically referred to as openings  29 ) may be provided for the introduction of marking material into respective cavities. Also, cavity  30  may carry a propellant source  62 , such as solid carbon dioxide (CO 2 ), compressed gas cartridge (again such as CO 2 ), chemical reactants, etc. permanently, replacably removably, or refillably in body  60 . Alternatively, cavity  30  may carry a compact compressor or similar means (not shown) for generating a pressurized propellant. As a still further alternative, the propellant source may be removable and replaceable separately and independently from body  60 . Furthermore, device  24  may be provided with a means for generating propellant, such as a compressor, chemical reaction chamber, etc., in which case body  60  bears only cavities  28  and related components. 
     Device Operation 
     The process  70  involved in the marking of a substrate with marking material according to the present invention is illustrated by the steps shown in FIG.  12 . According to step  72 , a propellant is provided to a channel. A marking material is next metered into the channel at step  74 . In the event that the channel is to provide multiple marking materials to the substrate, the marking materials may be mixed in the channel at step  76  so as to provide a marking material mixture to the substrate. By this process, marking without the need for re-registration may be obtained. An alternative for marking is the sequential introduction of multiple marking materials while maintaining a constant registration between print head  34  and substrate  38 . Since, not every marking will be composed of multiple marking materials, this step is optional as represented by the dashed arrow  78 . At step  80 , the marking material is ejected from an exit orifice at a distal end of the channel, in a direction toward, and with sufficient energy to reach a substrate. The process may be repeated with reregistering the print head, as indicated by arrow  83 . Appropriate post ejection treatment, such as fusing, drying, etc. of the marking material is performed at step  82 , again optional as indicated by the dashed arrow  84 . Each of these steps will be discussed in further detail. 
     Providing Propellant 
     As previously mentioned, the role of the propellant is to impart the marking material with sufficient kinetic energy that the marking material at least impinges upon the substrate. The propellant may be provided by a compressor, refillable or non-refillable reservoir, material phase-change (e.g., solid to gaseous CO 2 ), chemical reaction, etc. associated with or separate from the print head, cartridge, or other elements of marking device  24 . In any event, the propellant must be dry and free of contaminants, principally so as not to interfere with the marking of the substrate by the marking material and so as not to cause or induce clogging of the channel. Thus, an appropriate dryer and/or filter (not shown) may be provided between the propellant source and the channel. 
     In one embodiment, the propellant is provided by a compressor of a type well known. This compressor ideally rapidly turns on to provide a steady state pressure or propellant. It may, however, be advantageous to employ a valve between the compressor and the channel so as to permit only propellant at operating pressure and velocity to enter into channel  46 . 
     While such an embodiment contemplates connecting the channel to an external compressor or similar external propellant source, there may be a need for the propellant to be generated by device  24  itself. Indeed, for a compact, desk-top type device, a compact propellant source must be employed. One approach would be to employ commonly available replaceable CO 2  cartridges in the device. However, such cartridges provide a comparatively small volume of propellant, and would need frequent replacing. And while it may also be possible to provide larger pressurized propellant containers, the size of the device (e.g., a compact, desk-top printer) may limit the propellant container size. Thus, a self-contained, physically small propellant generation unit would be employed. According to this embodiment, it would also then be possible to provide a replaceable combined propellant and marking material cartridge. 
     In another embodiment, the propellant is provided by means of a reaction. One goal of this embodiment is to provide a compact propellant source, of the type, for example, which may be included within a propellant cavity  30 . There are a great variety of spontaneous and non-spontaneous reactions of liquid or solid chemicals or compounds, thus being relatively compact, which produce gases. In the most simple, a reactant is heated to above its boiling point, producing a gas phase material. When the reaction or change occurs in a confined volume, a pressure change results within the volume. So, for a closed volume, one species of reaction is:            (   R   )     P1          →     Δ                   T   +                (   R   )     P2                     
     where R is a reactant, P 1  and P 2  are pressure, and P 2  is much greater than P 1 . To accomplish this, a heating element (such as filament  87  shown in FIG. 3) may be provided with propellant cavity  30  (or other reactant containing volume). 
     A variant of this non-spontaneous multiple reactant systems which may be heat activated, such as:            (       R   1     +     R   2     +   …                )     P1          →     Δ                   T   +                (       R   3     +     R   4     +   …                )     P2                     
     where R 1 —R . . . are reactants, and again P 2  is much greater than P 1 . 
     However, to avoid the effects which provide a heated propellant may have on the marking material (e.g., melting within the channel, which could lead to clogging of the channels) it may be more desirable to employ a reaction less dependent on added heat (and not overly exothermic), such as: 
     
       
         (R) P1 →(R) P2    
       
     
     as might occur in a phase change at room temperature (e.g., solid to gaseous CO 2 ), or 
     
       
         (R 1 +P 2 + . . . ) P1 →(R 3 +R 4 + . . . ) P2    
       
     
     There are many such reactions known in the art which may be employed to produce a gaseous propellant. 
     In general, the reaction may be moderatable, in that it may be possible to initiate and terminate the reaction at arbitrary times as a means for permitting the device to the turned on and off. Alternatively, the reaction may take place in a propellant cavity in communication with the channel  46  via a valve for modulating the flow of propellant. In general, in this embodiment it may also be necessary to provide a valve for regulating the propellant to a selected operating pressure. 
     The velocity and pressure at which the propellant must be provided depends on the embodiment of the marking device as explained below. In general, examples of appropriate propellants include CO 2 , clean and dry air, N 2 , gaseous reaction products, etc. Preferably, the propellant should be non-toxic (although in certain embodiments, such as devices enclosed in special chamber or the like, a broader range of propellants may be tolerated). Preferably, the propellant should be gaseous at room temperature, but gases at elevated temperatures may be used in appropriate embodiments. 
     However generated or provided, the propellant enters channel  46  and travels longitudinally through the channel so as to exit at exit orifice  56 . Channel  46  is oriented such that the propellant stream exiting exit orifice  56  is directed toward the substrate. 
     Marking Material 
     According to one embodiment of the present invention a solid, particulate marking material is employed for marking a substrate. The marking material particles may be on the order of 0.5 to 10.0 μm, preferably in the range of 1 to 5 μm, although sizes outside of these ranges may function in specific applications (e.g., larger or smaller ports and channels through which the particles must travel). 
     There are several advantages provided by the use of solid, particulate marking material. First, clogging of the channel is minimized as compared, for example, to liquid inks. Second, wicking and running of the marking material (or its carrier) upon the substrate, as well as marking material/substrate interaction may be reduced or eliminated. Third spot position problems encountered with liquid marking material caused by surface tension effects at the exit orifice are eliminated. Fourth, channels blocked by gas bubbles retained by surface tension are eliminated. Fifth, multiple marking materials (e.g., multiple colored toners) can be mixed upon introduction into a channel for single pass multiple material (e.g., multiple color) marking, without the risk of contaminating the channel for subsequent markings (e.g., pixels). Registration overhead (equipment, time, related print artifacts, etc.) is thereby eliminated. Sixth, the channel refill portion of the duty cycle (up to 80% of a TIJ duty cycle) is eliminated. Seventh, there is no need to limit the substrate throughput rate based on the need to allow a liquid marking material to dry. 
     However, despite any advantage of a dry, particulate marking material, there may be some applications where the use of a liquid marking material, or a combination of liquid and dry marking materials, may be beneficial. In such instances, the present invention may be employed, with simply a substitution of the liquid marking material for the solid marking material and appropriate process and device changes apparent to one skilled in the art or described herein, for example substitution of metering devices, etc. 
     In certain applications of the present invention, it may be desirable to apply a substrate surface pre-marking treatment. For example, in order to assist with the fusing of particulate marking material in the desired spot locations, it may be beneficial to first coat the substrate surface with an adherent layer tailored to retain the particulate marking material. Examples of such material include clear and/or colorless polymeric materials such as homopolymers, random copolymers or block copolymers that are applied to the substrate as a polymeric solution where the polymer is dissolved in a low boiling point solvent. The adherent layer is applied to the substrate ranging from 1 to 10 microns in thickness or preferably from about 5 to 10 microns thick. Examples of such materials are polyester resins either linear or branched, poly(styrenic) homopolymers, poly(acrylate) and poly(methacrylate) homopolymers and mixtures thereof, or random copolymers of styrenic monomers with acrylate, methacrylate or butadiene monomers and mixtures thereof, polyvinyl acetals, poly(vinyl alcohol), vinyl alcohol-vinyl acetal copolymers, polycarbonates and mixtures thereof and the like. This surface pre-treatment may be applied from channels of the type described herein located at the leading edge of a print head, and may thereby apply both the pre-treatment and the marking material in a single pass. Alternatively, the entire substrate may be coated with the pre-treatment material, then marked as otherwise described herein. See U.S. patent application Ser. No. 08/041,353, now U.S. Pat. No. 5,362,800 incorporated herein by reference. Furthermore, in certain applications it may be desirable to apply marking material and pre-treatment material simultaneously, such as by mixing the materials in flight, as described further herein. 
     Likewise, in certain applications of the present invention, it may be desirable to apply a substrate surface post-marking treatment. For example, it may be desirable to provide some or all of the marked substrate with a gloss finish. In one example, a substrate is provided with marking comprising both text and illustration, as otherwise described herein, and it is desired to selectively apply a gloss finish to the illustration region of the marked substrate, but not the text region. This may be accomplished by applying the post-marking treatment from channels at the trailing edge of the print head, to thereby allow for one-pass marking and post-marking treatment. Alternatively, the entire substrate may be marked as appropriate, then passed through a marking device according to the present invention for applying the post-marking treatment. Furthermore, in certain applications if may be desirable to apply marking material and post-treatment material simultaneously, such as by mixing the materials in flight, as described further herein. Examples of materials for obtaining a desired surface finish include polyester resins either linear or branched, poly(styrenic)homopolymers, poly(acrylate) and poly(methacrylate) homopolymers and mixtures thereof, or random copolymers of styrenic monomers with acrylate, methacrylate or butadiene monomers and mixtures thereof, polyvinyl acetals, poly(vinyl alcohol), vinyl alcohol-vinyl acetal copolymers, polycarbonates, and mixtures thereof and the like. 
     Other pre- and post-marking treatments include the underwriting/overwriting of markings with marked material not visible to the unaided eye, document tamper protection coatings, security encoding, for example with wavelength specific dyes or pigments that can only be detected at a specific wavelength (e.g., in the infrared or ultraviolet range) by a special decoder, and the like. See U.S. Pat. No. 5,208,630, U.S. Pat. No. 5,385,803, and U.S. Pat. No. 5,554,480, each incorporated herein by reference. Still other pre- and post-marking treatments include substrate or surface texture coatings (e.g. to create embossing effects, to simulate an arbitrarily rough or smooth substrate) materials designed to have a physical or chemical reaction at the substrate (e.g., two materials which, when combined at the substrate, cute or otherwise cause a reaction to affix the marking material to the substrate), etc. It should be noted, however, that references herein to apparatus and methods for transporting, metering, containing, etc. marking material are intended to be applicable to pre- and post-marking treatment material (and in general, to other non-marking material) unless otherwise noted or as may be apparent to one skilled in the art. 
     As has been alluded to, marking material may be either solid particulate material or liquid. However, within this set there are several alternatives. For example, apart from a mere collection of solid particles, a solid marking material may be suspended in a gaseous (i.e., aerosol) or liquid carrier. Other examples include multi-phase materials. With reference to FIG. 33, in one such material, solid marking material particles  286  are suspended in discrete agglomerations of a liquid carrier medium  288 . The combined particles and enveloping carrier may be located in a pool  290  of the carrier medium. The carrier medium may be a colorless dielectric which lends liquid flow properties to the marking material. The solid marking material particles  286  may be on the order of 1-2 μm, and provided with a net charge. By way of a process discussed further below, the charged marking material particles  286  may be attracted by the field generated by appropriate electrodes  292  located proximate the port  294 , and directed into channel  296 . A supplement electrode  298  may assist with the extraction of the marking material particles  286 . A meniscus  300  forms at the channel side of port  294 . When the particle  286 /carrier  288  combination is pulled through the meniscus  300 , surface tension causes particle  286  to pull out of the carrier medium  288  leaving only a thin film of carrier medium on the surface of the particle. This thin film may be beneficially employed, in that it may cause adhesion of the particle  286  to most substrate types, especially at low velocity, allowing for particle position retention prior to post-ejection modification (e.g., fusing). 
     Metering Marking Material 
     The next step in the marking process typically is metering the marking material into the propellant stream. While the following specifically discusses the metering of marking material, it will be appreciated that the metering of other material such as the aforementioned pre- and post-marking treatment materials is also contemplated by this discussion, and references following which exclusively discuss marking material do so for simplicity of discussion only. Metering, then, may be accomplished by one of a variety of embodiments of the present invention. 
     According to a first embodiment for metering the marking material, the marking material includes material which may be imparted with an electrostatic charge. For example, the marking material may be comprised of a pigment suspended in a binder together with charge capture or control additives. The charge capture additives may be charged, for example by way of a corona  66 ′,  66 ″,  66 ′″, and  66 ″″ (generically referred to as coronas  66 ), located in cavities  28 , shown in FIG.  3 . Another alternative is to initially charge the propellant gas, e.g., by way of a corona  45  in cavity  30  (or some other appropriate location such as port  44 , etc.) The charged propellant may be made to enter into cavities  28  through ports  42 , for the dual purposes of creating a fluidized bed  86 ′,  86 ″,  86 ′″, and  86 ″″ (generically referred to as fluidized bed  86 , and discussed further below), and imparting a charge to the marking material. Other alternatives include tribocharging, by other means external to cavities  28 , or other mechanism. 
     Referring again to FIG. 3, formed at one surface of channel  46 , opposite each of the ports  42  are electrodes  54 ′,  54 ″,  54 ′″, and  54 ″″ (generically referred to as electrodes  54 ). Formed within cavities  28  (or some other location such as at or within ports  44 ) are corresponding counter-electrodes  55 ′,  55 ″,  55 ′″, and  55 ″″ (generically referred to as counter-electrodes  55 ). When an electric field is generated by electrodes  54  and counter-electrodes  55 , the charged marking material may be attracted to the field, and exits cavities  28  through ports  42  in a direction roughly perpendicular to the propellant stream in channel  46 . The shape and location of the electrodes and the charge applied thereto, determine the strength of the electric field, and hence the force of the injection of the marking material into the propellant stream. In general, the force injecting the marking material into the propellant stream is chosen such that the momentum provided by the force of the propellant stream on the marking material overcomes the injecting force, and once into the propellant stream in channel  46 , the marking material travels with the propellant stream out of exit orifice  56  in a direct towards the substrate. 
     As an alternative or supplement to electrodes  54  and counter-electrodes  55 , each port  42  may be provided with an electrostatic gate. With reference to FIGS. 13A and 13B, this gate may take the form of a two-part ring or band electrode  90   a,    90   b  at the inside diameter of the ports  42 , connected via contact layers  91   a  and  91   b  to a controllably switchable power supply. The field generated by the ring electrode may attract or repel the charged marking material. Layers  91   a  and  91   b  may be photolithographically, mechanically or otherwise patterned to allow matrix addressing of individual electrodes  90   a,    90   b.    
     An alternative embodiment for providing marking material metering is shown in FIG.  14 . This embodiment consists of one or more pass regions  136 , extending roughly parallel to the direction of propellant flow in channel  46 . Each pass region  136  is formed between body  26  (or suitable upper layer) and layer  138 , with layer  140  serving as a spacing layer therebetween. Each layer may be a suitable, thick, etched photoresist, machine plastic or metal, or other material as may be dictated by the specific application of the present invention. Pass region  136  may be up to 100 μm or greater in length (in the direction of marking material travel). Facing each other, and formed in pass region  136  on the surface of body  26  and layer  138 , are roughly parallel plate electrodes  142  and  144 , respectively. 
     In the case of an array of such openings, the various electrodes are addressed by either a row or column line, allowing matrix addressing schemes to be used. The electrodes form one embodiment of an electrostatic gate for metering marking material. 
     In general, and specifically in the case of parallel plate electrodes such as are illustrated in FIG. 14, the marking material used may be uncharged or charged. In the case of uncharged marking material, the marking material should have a permitivity considerably higher than both air and the propellant. In such a case, the electrode pairs are provided with opposite (+/−) charge. The uncharged marking material is polarized by the field between the parallel plate electrodes, which act together to essentially form a capacitor. With a field thus established between electrodes, the marking material preferentially stays in that field (i.e., the energetically more favorable location is between the electrodes). Marking material is thus blocked from traveling through the port. When no charge is provided to the electrodes, marking material is allowed to travel through the port and into the propellant stream, typically by way of back pressure, pressure burst, etc. An alternating current may be applied to the electrodes to avoid the buildup of marking material. 
     In the case of charged marking material, when in the “on” state, one of the electrodes attacks the marking material (the other repels it), preventing the material from entering into the propellant stream. When in the “off” state, the electrodes allow marking material to pass by and into the propellant stream, for example by way of back pressure, pressure burst or a third electrode, such as electrode  54  provided with an charge polarity opposite that of the marking material. Either polarity charge (positive or negative) on the marking material can be accommodated. 
     According to another embodiment of the present invention, liquid marking material may be metered into the propellant stream by ejecting it from a source, for example by an acoustic ink ejector, into the propellant stream. FIG. 15 shows an abbreviated illustration of this embodiment. According to the embodiment  154  shown in FIG. 15, channel  46  is located above as a top surface of a pool of marking material  156 , for example a liquid marking material such as liquid ink. Embodiment  154  comprises a planar piezoelectric transducer  158 , such as a thin film ZnO transducer, which is deposited on or otherwise bonded to the rear face of a suitable acoustically conductive substrate, such as an acoustically flat plate of quartz, glass, silicon, etc. The opposite, or front face of substrate  160  has formed thereon or therein a concentric phase profile of Fresnel lenses, a spherical acoustic lens, or other focusing means  162 . By applying an rf voltage across transducer  158 , an acoustic beam is generated and focussed at the surface of pool  156 , thereby ejecting a droplet  164  from the pool into the propellant stream. The amount of marking material injected into the propellant stream, for the purpose of greyscale control, may be controlled by controlling the size of droplet  164  (by controlling the intensity of the acoustic beam), the number of droplets injected in short succession, etc. For a more detailed description of an acoustic ink print head of the type of that may be employed by this embodiment, see U.S. Pat. No. 5,041,849, which is hereby incorporated by reference. 
     In yet another embodiment  166  for metering a liquid marking material into the propellant stream, an ink jet apparatus such as a TIJ apparatus  168  is employed. FIG. 16 shows an abbreviated illustration of this embodiment. According to embodiment  166 , TIJ ejector  168  is located proximate channel  46  such that ejection of marking material  170  from ejector  168  aligns with a port  172  located in channel  46 . Marking material  170  is, again, a liquid material such as liquid ink, retained in a cavity  174 . Marking material  170  is brought into contact with a heating element  176 . When heated, the heating element generates a bubble  177  which is forced out of a channel  179  located within the TIJ apparatus  168 . The motion of bubble  177  causes a controlled amount of marking material to be forced out of the channel (as otherwise well known) and into the propellant stream in the form of a droplet  181  of marking material. A plurality of such TIJ ejectors may be employed in conjunction with a single ballistic aerosol marking channel according to the present invention to provide a device and method for marking a substrate with improved speed, greyscale, and other advantages over the prior art. For a more detailed description of a TIJ apparatus of the type that may be employed by this embodiment, see U.S. Pat. No. 4,490,728, which is hereby incorporated by reference. 
     While there are many other possible embodiments for the ejection of liquid marking materials (such as pressurized injections, mechanical valving, etc.), it should be appreciated that previously described embodiments may also function well for such marking materials. For example, the apparatus shown in FIG. 3 may function well, with the ports  42  sized as a function of the viscosity of the marking material such that a liquid meniscus forms with the ports  42 . This meniscus and the corresponding electrode  54  essentially form plates of a parallel capacitor. Given the proper charge on electrode  54 , a droplet from the meniscus may be pulled into the channel  46 . This approach works well for conducting (and to a certain degree non-conducting) liquids such as inks, substrate pre-treatment and post-treatment materials, etc. This is similar to the technology known as tone jet, which technology may also be employed as a metering device and method according to the present invention. 
     As a further enhancement to the embodiments described herein, it may be desirable to provide a burst of pressure to urge or even force marking material out of cavities  28  and inject same into the propellant stream. This pressure burst may be provided by one of a variety of devices, such as piezo-electric transducer/diaphragms  68 ′,  68 ″,  68 ′″, and  68 ″″ (generically referred to as transducer/diaphragm  68 ) located within each cavity  28 , as shown in FIG.  17 . One or more of transducer/diaphragm  68  may be separately addressable, either in conjunction with an adjunct metering device or independently, by addressing means  69 ′,  69 ″,  69 ′″, and  69 ″″ (generically addressing means  69 ). Various alternatives may be employed, including gated pressure from the propellant source, etc. 
     Still other mechanisms may be employed for metering marking material into the propellant stream according to the present invention. For example, the technique previously referred to as a toner jet may be employed, such technique being described for example in laid open patent application WO 97 27 058 (A1), incorporated herein by reference. Alternatively, a micromist apparatus may be employed, of the type described in U.S. Pat. No. 4,019,188, which is incorporated herein by reference. 
     In numerous of the embodiments for the metering of the marking material according to the present invention, no moving parts are involved. Metering may thus operate at very high switching rates, for example greater than 10 kHz. Additionally, the metering system is made more reliable by the avoidance of mechanical moving parts. 
     One of many simple addressing schemes may be employed to control the metering system of choice. One such scheme is illustrated in FIG. 18, according to which, each “row” of an array  200  of metering devices  202  for metering marking material into channels  46  are interconnected via a common line  206 , for example connected to ground. Each “column” comprises metering devices  202 , which together control the introduction of marking material into a single channel  46 . Each metering device of each column is individually addressed, for example by way of lines  208  connecting an associated metering device to a control mechanism, such as a multiplexer  210 . It should be noted that each “column” is for example on the order of 84 μm wide, providing ample area to form lines  208 , which may for example be on the order of 5 μm wide. An alternative embodiment is shown in FIG. 19, in which common line  206  is replaced by individual addressing of each “row” of metering devices  202 , for example by multiplexer  212 , to allow for pure matrix addressing of the metering devices. 
     Several mechanisms may prove advantageous or necessary for realization of certain embodiments of the present invention. For example, returning to FIG. 3 there is a need to provide a smooth flow of marking material from cavities  28  into channel  46 , and a need to avoid clogging of ports  42 . These needs may be addressed by diverting a small amount of the propellant into the cavities  28 . This may be accomplished by balancing the pressure in the channel and the pressure in the cavity such that the pressure in the cavity is just below that of the channel. FIG. 20 illustrates one arrangement for accomplishing pressure balance. One embodiment  214  of a cavity  28  is illustrated in FIG. 20, having an associated port  42  located in one wall thereof which is in communication with channel  46  so as to allow marking material contained in cavity  214  to enter channel  46  (under control of a metering device not shown). In one wall of cavity  214 , an opening is provided with a filter  220  of a coarseness sufficient to prevent marking material from passing therethrough. Filter  220  is connected via piping  222  to a valve  224  which is controlled by circuitry  226 . Also connected to circuitry  226  is a pressure sensor  228 , located in cavity  214 , and a pressure sensor  230  located within the channel  46 , for example just before the converging region thereof (not shown). Pressure within cavity  214  is monitored by pressure sensor  228 , and compared with the pressure in the channel monitored by pressure sensor  230 . At system start-up, valve  224  is closed while the pressure in channel  46  increases. Upon reaching steady-state operating pressure, valve  224  is then controllably opened. Circuitry  226  maintains the pressure in cavity  214  just below that of the channel  46  by controllably modulating valve  224 . This pressure differential results in an amount of propellant being diverted from the channel into the cavity. 
     Returning to FIG. 3, the propellant entering the cavities  28  through ports  42  as described above (or by other means) causes a local disruption of the marking material near ports  42 . When employing a marking material having an appropriately sized and shaped particle, with a proper plasticity, packing density, magnetization, etc., the frictional and other binding forces between the particles may be sufficiently reduced by the disruption (i.e., due to the propellant passing through marking material) such that the marking material takes on certain fluid-like properties in the area of disruption. (See Fuchs, “The Mechanics of Aerosols”, §58, pp. 367-373 (Pergamon Press, 1964), incorporated herein by reference, for specifics on the parameters for creating fluidization.) Under these conditions, regions  86 ′,  86 ″,  86 ′″, and  86 ″″ of fluidized marking material may be generated (generically, they are referred to as fluidized beds  86 ). By providing a fluidized bed  86  in the manner described herein, the marking material is made to flow evenly both by creating a fluid-like material with reduced viscosity and by effectively continuously cleaning ports  42  with the propellant diverted therethrough. Accurate spot size, position, color, etc., are thereby obtained. 
     With reference now to FIG. 21, line  240  represents a plot of pressure versus time at a point in the channel  46  proximate the port  42  of FIG.  20 . Line  242  represents the pressure (P 230 ) at sensor  230  of FIG. 20 (i.e., pressure prior to the nozzle portion of channel  46 ). Line  244  represents the set point (P set ) at which the pressure within cavity  214  is maintained. Since it takes some period of time to reach steady-state pressure in the channel, and hence the desired pressure balance between channel  46  and cavity  214 , it may be desirable to accelerate the pressure balancing to avoid clogging, leaking of marking material, etc. This may be accomplished by introducing pressurized propellant into the cavity (or otherwise pressurizing cavity  214 ), for example from the propellant source by way of an opening  232  located in cavity  214  shown in FIG.  20 . 
     An alternative arrangement  260  for the provision of a fluidized bed is illustrated in FIG.  22 . In this embodiment, a system of electrodes and voltages are employed to provide not only a fluidized bed, but also a metering function. Conceptually, this embodiment may be divided into three separate and complementary functions: marking material “bouncing”, marking material metering, and marking material “projection”. A marking material carrier  262  such as a donor roll, belt, drum or the like (which is fed with marking material by a conventional magnetic brush  283 ) is held a small distance away from one embodiment  264  of cavity  28  formed in body  266 . Port  268  is formed in the base of body  266  for example as a cylindrical opening communicatively coupling cavity  264  and channel  46 . Body  266  may be a monolithic structure or a laminated structure, for example formed of a semiconducting layer  272  (such as silicon) and an insulating layer  274  (such as Plexiglas). The walls of cavity  264  may optionally be coated with a dielectric (such as Teflon) to provide a moderately smooth insulating boundary. Of course, this coating may also be applied to any of the other embodiments described herein. 
     Formed at the cavity-side of port  268  is first electrode  276 , which may be a continuous metal layer deposited within the structure, or may be patterned to correspond to each port  268  of an array of such ports. Formed at the channel-side of port  268  is second electrode  278 , which will typically be patterned into an annular planform, concentric with port  268 . An optional supplemental electrode  54  may be formed within the channel to assist with extraction of marking material from the cavity  264 . 
     By properly selecting the voltages at each of several points in arrangement  260 , the desired three functions can be achieved. For example, Table 2 illustrates one possible choice of voltages for negatively charged marking material. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Reference Point 
                 Voltage 
                 Example values 
               
               
                   
               
             
            
               
                 V U   
                 0 (ground) 
                   0 v 
               
               
                 V L  (off) 
                 V off  (“off”) 
                 −300 v 
               
               
                 V L  (on) 
                 V on  (“on”) 
                 +100 v 
               
               
                 V DC   
                 V DC   
                  −40 v 
               
               
                 V AC   
                 V AC   
                  500 v 
               
               
                 V D   
                 V DC  + V AC sin2πft 
                 varies 
               
               
                 AC frequency 
                 n/a 
                 2 kHz 
               
               
                 V P   
                 V P   
                 +170 v 
               
               
                   
               
            
           
         
       
     
     In arrangement  260 , the marking material  282  is charged, for example by trib-charging or ion charging, and is thereby retained by carrier  262 . The AC voltage within cavity  264  causes the charged toner to “bounce” between the carrier and first electrode  276 . The DC bias is the voltage difference maintained between the carrier  262  and upper electrode  276  to maintain a continuous marking material supply from marking material sump  287 . For marking material with narrow size and charge-diameter ratio (Q/d) distributions, the bounce is synchronized with the AC frequency. The optimal AC frequency is determined by the transit time of the marking material between the carrier  262  and the first electrode  276 . Specifically, the period T should be twice the transit time τ. 
     The gating voltage acts to open (turn “on”) and close (turn “off”) port  268 . For the “on” condition, the polarity of the voltage is directly opposite to the polarity of the charged marking material, thus attracting the marking material into the field between the first and second electrodes  276  and  278 , respectively. Finally, a projection voltage may be established by supplemental electrode  54  to further attract the charged marking material particles into the channel  46  where the propellant stream causes them to travel toward a substrate. 
     Material Transport 
     It may be desirable to controllably move marking material towards ports  42 , especially with speed, precision, and correct timing. This process is referred to as marking material transport, and may be accomplished by one of a variety of techniques. 
     One such technique uses an electrostatic travelling wave to move individual marking material particles. With reference to FIG. 23, according to this technique, a phased DC high voltage waveform is applied to a grid  148  of equally spaced electrodes  88  that are formed proximate each port  42 . Grid  148  may be photolithographically formed of aluminum inside the cavities, or may be formed on a lift-off carrier which may be applied within the cavities. Grid  148 , and the methods of operating same, are discussed in further detail in patent application Ser. No. 09/163,839, which is incorporated by reference herein. 
     FIG. 24 illustrates an embodiment in which electrodes  88  for an electrostatic travelling wave are provided in conjunction with electrodes  142  (not shown) and  144  for metering the marketing material. However, it will be understood that various other transport and metering combinations are within the scope of the present invention. 
     A protection and relaxation layer may be deposited over electrodes  88  to protect their surface and also to provide rapid charge dissipation at a known time constant to move the marking material along grid  148 . Also, a proper coating will assist with controlling the direction of the marking material movement, reduce marking material being trapped between electrodes, minimize oxidation and corrosion of the electrodes, and reduce arcing between electrodes. Such a coating is described in patent application Ser. No. 09/163,664 and application Ser. No. 09/163,518, each of which are incorporated by reference herein. 
     It should be appreciated that the transport and metering functions taught herein may be performed by a single device, and combined into a single step. However, separate or combined, the transport and/or meeting of marking material according to the present invention addresses many of the problems identified with the prior art. For example, marking material is available for injection into the propellant stream almost instantaneously. This solves the problem of needing to wait for a channel to refill as common in ink jet systems. Furthermore, the rate at which marking material may be moved into the propellant stream and thereafter deposited onto a substrate is significantly higher than available from the prior art; indeed, in some embodiments it may be continuously provided. 
     In certain embodiments, it is possible that despite generating a fluidized bed within the cavity, marking material may tend to congregate in stagnant regions within the cavity, such as the corners thereof, starving the fluidized bed and negatively affecting the injection of marking material into the channel. An example of this is illustrated in FIG.  25 A. To address this problem, and further assist with the transport of marking material within the cavity, the bulk marking material within the cavity may be agitated. FIG. 25B illustrates one embodiment  250  for creating such agitation. On at least one wall  254  forming cavity  28  is a piezo-electric material  256 , which causes mechanical and pressure agitation within cavity  28 . This agitation maintains marking material located in cavity  28  in a dynamic state, avoiding stagnation points within cavity  252 . 
     Mixing of Marking Material 
     In a multiple marking material regime, such as a full color printer, two or more marking materials may be mixed in-channel prior to deposition on the substrate (again, the following discussion is intended to encompass treatment materials). In such a case, each of the marking materials are individually metered into a channel. This requires independent control of the metering of each marking material, and imposes limits on the throughput rates by the required addressing and other aspects of metering. For example, with regard to FIG. 26, there is shown therein a system in which each channel  46  may be provided with one or more marking materials. To control the flow of marking material into a channel  46 , a metering device  104 , for example of a type previously described, is addressed in a matrix fashion via column address leads  106  and row address leads  108  in a manner also previously discussed. The RC time constant associated with an 8-inch long set of passively addressed column address leads  106  will limit the minimum achievable signal rise times on these lines to a few microseconds—we will assume 2 μs at 500 kHz. The minimum metering device “on” time is thus on the order of about 5 μs. For n-bit greyscale printing, full coverage for each color takes 5×2 n  μs per spot. It therefore takes 11 inches×600 spi×(5×2 n ) μs/spot, or about 33×2 n  ms to print a full coverage 600 spi page. This corresponds to about 1800×2 −n  pages per minute. 
     Other addressing schemes are known which permit faster addressing and hence faster possible marking. For example, by employing a parallel addressing scheme (i.e., no column addressing lines), the signal rise time may be shortened by an order of magnitude. 
     Marking Material Placement And Spot Size 
     The ability to accurately control the placement of a spot of marking material is in part a function of the velocity of the propellant. The spot size and shape are also a function of this velocity. In turn, selecting the propellant velocity is in part a function of the size and mass of the marking material particles. In addition, spot position, size and shape are a function of how well (i.e., over how many exit orifice diameters) the fully expanded propellant stays collimated. FIG. 28 shows an idealized case of a propellant/substrate interaction, viewed roughly perpendicular to the substrate. The streamlines  110  show that the cylindrical propellant streams form a flow pattern at the substrate surface away from the circular disk of marking material spot  112 . 
     Typically, the marking material particles are deposited onto the substrate due to their inertia (normal momentum) imparted by the propellant. However, their position on the substrate is diverted from the centroid by the lateral hydrodynamic force components that occur at the propellant/substrate interface, illustrated in FIG.  29 . The smaller the mass of the particles (in relation to propellant velocity), and the further such particles are from the center of the propellant stream, the further they are diverted from the spot centroid. The result is a spot with a Gaussian density distribution  114 , also illustrated in FIG.  29 . 
     With reference to FIG. 30, as an example of a worst case estimate of marking material particle deviation due to propellant/substrate interface effects (namely, lateral drag at the substrate surface), assume that a particle  116  with a density ρ p  is directed at perfectly flat substrate  38  with a velocity v normal to the substrate and in a propellant stream  118  of width L/2 (i.e., exit orifice  56  shown in FIG. 3 is of width L/2). Assume that at the surface of the substrate there is a lateral propellant flow  120  of thickness L, also with a velocity v caused by the propellant striking the substrate. That is, the worst case assumption that the propellant velocity is entirely converted to lateral flow upon interacting with the substrate. 
     The lateral deviation x of the marking material particle  116  due to the lateral drag force is calculated for different particle diameters D. From the Reynolds number equation,        Re   =           ρ   g     ·   v   ·   D       μ   g       =     7.65   ×       10   4     ·   v   ·   D                         
     where ρ g =1.3 kg/m 3 , and μ g =1.7×10 −3  kg−s/m 2 . For a particle size of 3 μm and a flow velocity of v=300 m/s, the Reynolds number is 70. This corresponds to a drag coefficient (CD) of 2.8. See for example Fuchs “The Mechanics of Aerosols,” p. 79 (Pergamon Press Ltd., 1964), which is incorporated by reference herein. The drag force FD is then given by        FD   =       CD   ·       ρ   g     2     ·   v   ·     g   2     ·   A     =     1.4          v   2     ·     D   2                           
     This lateral drag force deflects the normal incident trajectory of the particle  116  and sends it on a trajectory with radius of curvature R, determined from the equation for inertial centripetal force F i            F   i     =         ρ   P     ·   V   ·     v   2       R           where         V   =       π   ·     D   3       6                     
     giving R as        R   =         ρ   g     ·   D     CD           where         A   =       π   ·     D   2       4                     
     The resulting deviation x is given by 
     
       
           x=R·[ 1−cos(arcsin) L/R ))] 
       
     
     Or, if the normal propellant stream diameter L/2 is chosen to be one-half the array pitch, 
     
       
           x=R·[ 1−cos(arcsin( pitch/R ))] 
       
     
     For a flow velocity v, a particle size D, a given array density, and a particle density of 1000 kg/m 3 , the resulting deviation x is shown in Table 4 for various conditions. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Array density 
                 Flow velocity (v) 
                 Particle size (D) 
                 Deviation (x) 
               
               
                   
               
             
            
               
                 600 spi 
                 300 m/s 
                 1 μm 
                 2.5 μm 
               
               
                 660 spi 
                 500 m/s 
                 2 μm 
                 0.6 μm 
               
               
                 600 spi 
                 300 m/s 
                 1 μm 
                 2.5 μm 
               
               
                 600 spi 
                 100 m/s 
                 1 μm 
                 5.0 μm 
               
               
                 900 spi 
                 300 m/s 
                 1 μm 
                 1.1 μm 
               
               
                 900 spi 
                 100 m/s 
                 1 μm 
                 2.2 μm 
               
               
                   
               
            
           
         
       
     
     Thus, for a worst case scenario of a 300 m/s flow velocity, a 1 μm marking material particle size, and 600 spi resolution, a propellant stream (i.e., exit orifice size) of 21 μm would produce a spot of size 
     
       
         21  μm+ (2×2.5  μm )=26  μm,    
       
     
     the spot size expansion due to lateral drag at the propellant stream/substrate interface. Note that this corresponds to a worst case scenario for every condition, i.e., (1) no stagnation point, and fully developed cross flow, (2) cross flow velocity equal to full propellant stream velocity, thus ignoring frictional loss and substrate topology, (3) the full drag force is applied abruptly and two jet diameters away from the substrate. It should also be noted that the Reynolds number is very low due to the scale of the characteristic lengths and that turbulence cannot develop, per microfluidic flow theory. Finally, it should be noted that as particle size decreases, R increases such that at some point R approaches the lateral propellant flow of thickness 2L. When this happens, the marking material particles are significantly deflected from the spot centroid, and at the extreme never contact the substrate. It can be shown from the above that this occurs (based on the assumptions made herein) for marking material particle sizes in the range of around 100 nm or less. 
     This demonstrates not only acceptable spot size and position control, but illustrates that under the assumed conditions, no special mechanism is required to extract the marking material particle from the propellant stream and deposit it on the substrate. 
     However, in the event that it is desirable to further increase the extraction of the marking material particle from the propellant stream at the substrate surface (e.g., at low flow velocities/particle sizes, etc.) electrostaticly enhanced particle extraction may be employed. By charging the substrate or the platen (where employed) opposite the charge of the marking material particle, the attraction between particle and substrate/platen enhances the particle extraction. Such an embodiment  178  is illustrated in FIG. 32, in which body  26  is located proximate a platen  180  capable of accepting and retaining a net charge. The charge on platen  180  may be applied by a donor roller  182  moved in conjunction with platen  180  by a belt  184  or other means, or by other methods known in the art (such as by a tribo-brush, piezo-electric coating, etc.) 
     In one example, platen  180  is provided a net positive charge by donor roller  182 . Marking material particles  188  may be given a net negative charge, for example by the corona illustrated in FIG. 3, or by other means. A mark-receiving substrate (e.g., paper) is placed between the marking material source and the platen, proximate the platen. The attraction between the marking material  188  and the platen accelerates the marking material toward the platen, and if such attraction is sufficiently strong, especially in embodiments having a relatively slow propellant velocity, it can overcome the tendency of the propellant to be deviated from the spot centroid by lateral drag of the propellant. In addition, this attraction may help eliminate the problem of marking materials bouncing off of the substrate and either coming to rest at an unintended position on the substrate or coming to rest in a position off of the substrate prior to post-ejection modification (e.g., fusing by a heat and/or pressure roller  186 ), a problem referred to as “bounce back”. This is especially beneficial when kinetic fusing (discussed below) cannot be employed. 
     Post-Ejection Modification 
     Once the marking material has been delivered to the substrate, it must be adhered, or fused, to the substrate. While there are multiple approaches for fusing according to the present invention, one simple approach is to employ the kinetic energy of the marking material particle. For this approach, the marking material particle must have a velocity v c  at impact with the substrate sufficient to kinetically melt the particle by plastic deformation from the collision with the substrate (assuming the substrate is infinitely stiff). Following melting (complete transition to liquid or glass phase, or similar reversible temporary phase transition), the particle resolidifies (or otherwise returns to its original phase) and is thereby fused to the substrate. 
     To accomplish kinetic fusing, it is required that: (1) the kinetic energy of the particle be large enough to bring the particle beyond its elasticity limit; and (2) the kinetic energy is larger than the heat required to bring the particle beyond its softening temperature to cause a phase change. FIG. 34 is a plot  190  of the number of marking material particles versus kinetic energy for a typical embodiment of the present invention. Below a certain kinetic energy value, the particles have insufficient energy to fuse to a substrate, while above this certain kinetic energy value the particles will have sufficient kinetic energy to fuse. That certain kinetic energy value is referred to as the kinetic fusing energy threshold, and is illustrated by the boundary  192  shown in FIG.  34 . Essentially, particles whose kinetic energy falls into region  194  will not fuse due to insufficient heating, whereas particles with energies in region  196  will fuse. There are essentially two ways to increase the percentage of fused marking material particles. First, the kinetic fusing energy threshold may be shifted down. This is essentially a function of the qualities of the marking material. Second, the average kinetic energy of the particles may be shifted by, for example, increasing the propellant velocity. 
     The kinetic energy E k  of a spherical particle with velocity v, density ρ, and diameter d is given by          E   k     =       π   ·   ρ   ·     d   3     ·     v   2       12                     
     The energy E m  required to heat a spherical particle with diameter d, heat capacity C p , and density ρ from room temperature T 0  to beyond its softening temperature T s  is given by          E   m     =       π   ·   ρ   ·     d   3     ·     C   p     ·     (       T   s     -     T   0       )       6                     
     The energy E p  required to deform a particle with diameter d and Young&#39;s modulus E beyond its elasticity limit σ e  and into the plastic deformation regime is given by          E   p     =         d   3     ·     σ   e   2         2      E                       
     The critical velocity v cp  for obtain plastic deformation is then given by          v   cp     =         6     π   ·   ρ   ·   E         ·     σ   e                       
     Finally, the critical velocity v cm  to obtain kinetic melt is given by 
     
       
           v   cm ={square root over (2· C   p +L ·( T   s-   T   0 +L ))} 
       
     
     For a thermoplastic with C p =1000 J/kg K, T s =60° C., and T 0 =20° C., the critical velocity required to achieve kinetic melt is 280 m/s. This is consistent with the assumptions made above. It should be noted that this result is independent of particle size and density. 
     Attaining such a propellant flow of 280 m/s or greater may be accomplished in several ways. One method is to provide propellant at a relatively high pressure, depending on the device geometry (e.g., on the order of several atmospheres in one example), to the converging region of a channel having converging region  48  and diverging region  50 , for example a so-called de Laval nozzle, illustrated in FIG. 4, converting the propellant pressure to velocity. In one example, the propellant is subsonic (e.g., less than 331 m/s) in all regions of the channel. In another example, the propellant will be subsonic in converging region  48 , supersonic in diverging region  50 , and at or very near the speed of sound at the throat  53  between the converging and diverging regions. 
     FIG. 35 is an illustration of propellant velocity v at exit orifice  56  versus propellant pressure for a channel  46  of square cross-section 84 μm on each side (corresponding to about 300 spots per inch). As can be seen, 280 m/s is readily attainable at moderate pressures for channels both with and without a nozzle. 
     The above has assumed that the substrate is infinitely stiff, which in most cases it is not. The effect of elasticity of the substrate is to decrease the apparent E-modulus of the material without reducing its yield strength (i.e., more energy is required to attain the yield stress in the material, more energy is required to achieve plastic deformation, and v cp  increases). That is, even though the kinetic energy may be larger than the energy required to melt the particle, the collision will be elastic, causing bounce of the particle and potentially insufficient heating. Thus, in some systems (depending on the elasticity of the substrate) marking material particles must attain a higher pre-impact velocity, or fusing assistance must be provided by the system. 
     In the event that fusing assistance is required (i.e., elastic substrate, low marking material particle velocity, etc.), a number of approaches may be employed. For example, one or more heated filaments  122  may be provided proximate the ejection port  56  (shown in FIG.  4 ), which either reduces the kinetic energy needed to melt the marking material particle or in fact at least partly melts the marking material particle in flight. Alternatively, or in addition to filament  122 , a heated filament  124  may be located proximate substrate  38  (also shown in FIG. 4) to have a similar effects. 
     Still another approach to assisting the fusing process is to pass the marking material particle through an intense, collimated beam of light, such as a laser beams, thereby imparting energy to the particle sufficient either to reduce the kinetic energy needed to melt the marking material particle or at least partially melt the particle in flight. This embodiment is shown in FIG. 36, wherein a stream  130  of particles of marking material pass through an intense, collimated light source  132 , such as a laser beam generated by a laser  134 , on their way toward substrate  38 . Of course a light source other than laser  134  may provide similar results. 
     Assume that a particle with density ρ, mass m, diameter d, heat capacity C p , and softening temperature T s , travels with velocity v through a laser beam with a width L 1  and a height L 2 , as shown in FIG.  31 . The temperature change ΔT for such a particle for a give heat input ΔQ is given by          Δ                 T     =         Δ                 Q       m   ·     C   p         =       6                 Δ                 Q         C   p     ·   ρ   ·   π   ·     d   3                           
     where m=ρ·volume=ρ·πd 3 /6 
     The laser power density p is given by the laser power P divided by the area of the ellipse as        p   =     P     π   ·     L   1     ·       L   2     /   4                         
     The energy absorbed by the particle per unit of time is given by the laser power density multiplied by the projected area of the particle (πd 2 /4) multiplied by the absorption fraction α           Δ                 Q       Δ                 t       =       α   ·       4   ·   P       π   ·     L   1     ·     L   2         ·       π   ·     d   2       4       =     α   ·       P   ·     d   2           L   1     ·     L   2                             
     The energy absorbed by the particle during its travel through the beam is thus given by 
     
       
         Δt=L 2 /v  
       
     
     
       
         
           
             
               Δ 
                
               
                   
               
                
               Q 
             
             = 
             
               α 
               · 
               
                 
                   P 
                   · 
                   
                     d 
                     2 
                   
                 
                 
                   
                     L 
                     1 
                   
                   · 
                   v 
                 
               
             
           
         
         
         
             
         
       
     
     The temperature change is thus given by          Δ                 T     =       6   ·   α   ·   P       π   ·   ρ   ·     C   p     ·   d   ·     L   1     ·   v                       
     When the initial temperature of the particle is T 0 , the laser power required to heat the particle beyond its glass transition temperature is hence given by        P   =       π   ·   ρ   ·     C   p     ·   d   ·     L   1     ·   v   ·     (       T   s     -     T   0       )         6   ·   α                       
     As an example, we assume the following values: 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
             
            
               
                 α 
                 0.7 
                 absorption fraction 
               
               
                 ρ 
                 900 kg/m 3   
                 marking material particle density 
               
               
                 C p   
                 1200 J/kgK 
                 marking material particle heat capacity 
               
               
                 d 
                 1.0 × 10 −6  m 
                 marking material particle diameter 
               
               
                 L 1   
                 0.2 × 10 −3  m 
                 laser beam width 
               
               
                 v 
                 300 m/s 
                 marking material particle velocity 
               
               
                 T s   
                 60° C. 
                 marking material particle softening temperature 
               
               
                 T 0   
                 20° C. 
                 initial marking material particle temperature 
               
               
                   
               
            
           
         
       
     
     Accordingly, the laser power required to melt the marking material particle of this example is 1.9 watts. This is well within the range of commercially available laser systems, such as continuous beam, fiber-coupled laser diode arrays produced by Spectra Diode Labs (Mountain View, Calif.). 
     FIG. 37 is a plot of the light source power required for particle melt versus particle size for various particle velocities, and indicates that in-flight melting with, e.g., laser diodes should be feasible for the particle sizes and velocities of interest. The advantage provided by in-flight melting is that no bulk material is heated (neither the bulk marking material nor the substrate). Therefore, in-flight melt can accommodate a wide variety of marking material delivery packages (e.g., both fixedly mounted and removable marking material reservoirs, etc.), and can serve a wide variety of substrates due to low marking material heat content despite a relatively high particle temperature (i.e., low thermal mass). 
     Finally, other systems for assisting the fusing process may be employed, depending on the particular application of the present invention. For example, the propellant itself may be heated. While this may be undesirable in the event that the heat of the propellant melts the marking material particles, since this may lead to contamination and clogging of the channels, sufficient heat energy may be imparted to the particles short of melting to reduce the kinetic energy required for impact fusing. The substrate (or substrate carrier such as a platen) may be heated sufficiently to assist with the kinetic fusing or in fact sufficiently to melt the marking material particles. Or, fusing may take place at a separate station of the device, by heat, pressure or a combination of the two, similar to the fusing process employed in modern xerographic equipment. UV curable materials used as a marking material may be fused or cured by application of UV radiation, either in flight or to the material-bearing substrate. 
     It should be appreciated, however, that an important aspect of the present invention is the ability to provide phase change and fusing on a pixel-by-pixel basis. That is, much of the prior art has been limited to liquid phase bulk printing material, such as liquid ink or toner in a liquid carrier. Thus, the present invention can enable significant resolution improvements and pixel level multiple-material marking. 
     Closure Structure 
     During operation of one embodiment of the marking apparatus of the present invention, propellant may continuously flow through the channel(s). This serves several purposes, including maximizing the speed at which the system can mark a substrate (a constant ready state), continuously purging the channels of accumulations of marking material, and preventing the entry of contaminants (such as paper fibers, dust, moisture from the ambient humidity, etc.) into the channels. 
     In a non-operative state, such as a system power off, no propellant flows through the channels. To avoid entry of contaminants in this state, a closure structure  146 , illustrated in FIG. 38, may be brought into contact with a face of the print head  34 , specifically at exit orifices  56 . Closure structure  146  may be a rubber plate, or other material capable of impermeably sealing off the channel from the environment. As an alternative, in the case where print head  34  is movable within the marking system, it may be moved into a maintenance station within the marking system as is commonly employed in TIJ and other printing systems. As another alternative, in the case where the marking system is designed to mark to sheet media supported by a platen, roller or the like, and in addition, where the platen, roller, etc. is formed of a suitable material such as rubber, print head  34  may be moved into contact with the platen, roller, etc. to seal off the channels. Alternatively, the platen, roller, etc. may be moved into contact with print head  34 , as illustrated in FIG.  39 . 
     Cleaning of the ports  42  and any associated openings  136  and electrodes  142 ,  144  may be accomplished by the propellant flow used to establish the fluidized bed, discussed above, or by otherwise controlling the pressure balance between the channel and marking material cavities such that, when marking material is not being injected into the channel, there is a flow of propellant through said ports et al. 
     An alternative embodiment  320  is illustrated in FIG.  41 . In embodiment  320 , print head  322  is essentially inverted. Much of the description herein applies equally to this embodiment, with the exception that a fluidized bed  324  is established by an appropriate gas, such as propellant from propellant source  33  under control of valve  326 , or similar means. An aerosol region  328  is established over the fluidized bed  324 , again by the gas or other means creating fluidized bed  324 . Marking material from the aerosol region  328  may then be metered into the propellant stream. 
     It will now be appreciated that various embodiments of a ballistic aerosol marking apparatus, and components thereof have been disclosed herein. These embodiments encompass large scale systems, which may include integrated reservoirs and compressors for providing pressurized propellant, refillable or even remote marking material reservoirs, high propellant speed (even supersonic) for kinetic fusing, designed for very high throughput or rapid very large area marking for marking on one or more of a wide variety of substrates, to small scale systems (e.g., desk-top, home office, etc.) with replaceable cartridges bearing both marking material and propellant, designed for improved quality and throughput printing (color or monochrome) on paper. The embodiments described and alluded to herein are capable of applying a single marking material, one-pass multiple marking materials, applying a material not visible to the unaided eye, applying a pre-marking treatment material, a post-marking treatment material, etc., with the ability to mix virtually any marking material within the channel of the device prior to application of the marking material to a substrate, or on a substrate without re-registration. However, it should also be appreciated that the description herein is merely illustrative, and should not be read to limit the scope of the invention nor the claims hereof.