ELECTROHYDRODYNAMIC PRINTER WITH FLUIDIC EXTRACTOR

An electrohydrodynamic printer has a fluidic extractor. A stream of liquid or carrier fluid at a different electrical potential than the printing fluid passes by an extraction opening to extract printing fluid from the extraction opening. The stream of liquid can be a continuous stream, a uniform stream of droplets, or a non-uniform stream of droplets. The extracted printing fluid can merge with the extraction fluid to be carried to a printing surface for deposition. The stream of extraction fluid can be intermittently charged to intermittently extract printing fluid such that selective portions of the stream do not extract printing fluid.

TECHNICAL FIELD

The present disclosure relates generally to printing and, more particularly, to electrohydrodynamic printing.

BACKGROUND

Electrohydrodynamic printing, also known as e-jet printing, is a printing technique that relies on an electric field to extract a charged or polarized printing fluid from a printing nozzle for deposition on a printing surface. E-jet printing is capable of very high-resolution printing compared to other drop-on-demand or stream printing methods with droplet size and spatial accuracy on a sub-micron or nanometer scale. Early e-jet printing was limited to electrically conductive printing surfaces because the printing surface was one of the electrodes between which the electric field was produced. Consistency with the electric field was also problematic due to the deposited ink causing interference with the field as printing progressed. U.S. Pat. No. 9,415,590 to Barton, et al. addressed these and other problems via clever ink extraction and directing techniques that did not rely on a conductive printing surface.

SUMMARY

In accordance with various embodiments, an electrohydrodynamic printer has a fluidic extractor.

In various embodiments, the extractor is a stream of carrier fluid that merges with extracted printing fluid and carries the printing fluid toward a printing surface.

In various embodiments, the extractor is a stream of liquid at a different electrical potential than a printing fluid extracted from an extraction opening of a printing fluid source.

In various embodiments, the extractor is a continuous stream of liquid.

In various embodiments, the extractor is a uniform stream of droplets.

In various embodiments, each of a first portion of droplets extracts a droplet of the printing fluid and each of a second portion of droplets does not extract a droplet of the printing fluid.

In various embodiments, a first portion of droplets carries extracted printing fluid and is directed to a printing surface, and a second portion of droplets is not directed to the printing surface.

In various embodiments, the extractor is a non-uniform stream of droplets.

In accordance with various embodiments, a printer includes a first nozzle and a second nozzle. The first nozzle is configured to direct a stream of carrier fluid toward a printing surface, and the second nozzle is configured to provide a printing fluid at an extraction opening. The stream of carrier fluid passes by the extraction opening when flowing toward the printing surface. A difference in electrical potential between the carrier fluid and the printing fluid causes the printing fluid to be extracted from the second nozzle.

In various embodiments, extracted printing fluid merges with the stream of carrier fluid to be carried toward the printing surface.

In various embodiments, the carrier fluid is uniformly pressurized in the first nozzle so that the stream of carrier fluid is a continuous stream.

In various embodiments, a pressure of the carrier fluid in the first nozzle varies at a constant frequency so that the stream of carrier fluid is a uniform stream of droplets.

In various embodiments, the printer includes a piezoelectric element configured to deform at a constant frequency to vary the pressure of the carrier fluid in the first nozzle.

In various embodiments, the printer includes an electrode located external to the first nozzle. The stream of carrier fluid is charged by the electrode to provide at least a portion of the difference in electrical potential.

In various embodiments, the printer includes an electrode configured to charge only a portion of the stream of carrier fluid so that the stream of carrier fluid extracts printing fluid when a portion of the stream of carrier fluid passes by the extraction opening and does not extract printing fluid when an uncharged portion of the stream of carrier fluid passes by the extraction opening.

In various embodiments, a portion of the stream of carrier fluid passes by the extraction opening without extracting printing fluid and is collected and returned to a carrier fluid source that supplies the first nozzle with the carrier fluid.

In various embodiments, the printer is a drop-on-demand printer, and the stream of carrier fluid is a stream of droplets. Each droplet of carrier fluid extracts a droplet of printing fluid from the extraction opening and carries the respective droplets of printing fluid to the printing surface.

In various embodiments, the carrier fluid has a viscosity that is less than 10 centipoise, and the printing fluid has a viscosity that is greater than 30 centipoise.

In various embodiments, the difference in electrical potential is at least 500V, and the stream of carrier fluid has a velocity sufficiently high to maintain a gap between the stream of carrier fluid and the extraction opening of the second nozzle.

In various embodiments, the printing fluid is soluble in the carrier fluid, and the difference in electrical potential attracts the stream of carrier fluid onto the first nozzle in a cleaning mode of the printer.

It is contemplated that any number of the individual features of the above-described embodiments and of any other embodiments depicted in the drawings or description below can be combined in any combination to define an invention, except where features are incompatible.

DESCRIPTION OF EMBODIMENTS

FIG.1schematically illustrates a portion of an electrohydrodynamic (or e-jet) printer10equipped with a fluidic extractor12. The fluidic extractor12is itself a jet or stream of carrier fluid14that is at a different electrical potential than a printing fluid16provided at an extraction opening18of an ink nozzle20. When the stream of fluid passes by the extraction opening18with a sufficient combination of difference in electrical potential (V1−V2) and distance (D), printing fluid16is extracted from the ink nozzle20and merges with the extraction stream12to be carried toward a printing surface22, such as a surface of a substrate24or a previously deposited layer of printed material. Employment of the fluidic extractor12enjoys the benefits of a solid-state extractor, such as those detailed by Barton et al. in the aforementioned U.S. Patent, while additionally addressing certain problems that can arise with solid-state extractors, such as the potential for electrical arcing between the ink nozzle and extractor, ink build-up on the extractor, and a relatively limited throw distance (H) between the ink extraction point and the printing surface22. The fluidic extractor enables printing of high viscosity fluids with a throw distance normally associated with industrial continuous inkjet (CI) printers.

In the example ofFIG.1, the printer10includes a first nozzle26containing the carrier fluid14and a second nozzle20(i.e., the ink nozzle) containing the printing fluid16. As used herein, an ink or printing fluid is any fluid that flows under pressure. Some printing fluids can be solidified after deposition. Solidification can be via various mechanisms, such as solvent evaporation, chemical reaction, cooling, or sintering. In some cases, the printing fluid is a functional ink, which is a printing fluid that provides a function other than coloration once solidified on the surface on which it is printed. Examples of such functions include electrical conductivity, dielectric properties, physical structure (e.g., stiffness, elasticity, or abrasion resistance), electromagnetic shielding or filtering, optical properties, electroluminescence, bioactivity, etc. Some other printing fluids, such as a lubricant, are not intended to be solidified after deposition.

While not explicitly illustrated, the nozzles20,26may be part of a print head of the printer10, the print head being configured to move relative to the printing surface22. The print head may for example include a housing or other structure that supports the nozzles20,26and/or includes one or more connections configured to provide pressure on the fluids14,16in the nozzles and voltage to the nozzles and/or their contained fluids. The printer10may also include other non-illustrated components, such as a base, a movement mechanism for moving the print head and printing surface22relative to each other, multiple ink nozzles20or carrier fluid nozzles26, directionality field generators, on-board ink sources, means for pressurizing the fluids14,16in the nozzles, pneumatic or other gas connectors, pressure controllers, or one or more power supplies and associated controllers to selectively control the extraction field generated between the extractor12and the extraction opening18, to name a few examples.

The carrier fluid nozzle26is configured to direct the stream12of carrier fluid toward the printing surface22, and the ink nozzle20is configured to provide the printing fluid16at the extraction opening18. The relative orientation of the nozzles20,26is such that the stream12of carrier fluid passes by the extraction opening18when flowing toward the printing surface22. In the illustrated example, the central longitudinal axes A1, A2of the respective nozzles26,20intersect in an x-z plane. The first nozzle axis A1is vertical and perpendicular to the printing surface22, and the second nozzle axis A2is horizontal and parallel with the printing surface inFIG.1. These nozzle orientations are not required, however, as the nozzle axes may intersect each other and/or the printing surface at oblique angles.

The carrier fluid14may be a relatively volatile liquid solvent (e.g., an organic solvent) with a relatively low viscosity, such as 10 centipoise (cps) or less. In some embodiments, the carrier fluid14includes a solvent or liquid that is also included in the printing fluid16—e.g., a liquid in which a solid component of the printing fluid is dissolved, suspended, or emulsified. With a sufficiently high pressure P1applied to the fluid14in the nozzle26, a high velocity stream12of carrier fluid is produced at a discharge opening28of the nozzle and directed toward the printing surface22. The discharge opening28may be in a range from 1 μm to 100 μm, from 20 μm to 100 μm, or from 20 μm to 70 μm. The pressure P1may be in a range from 5 psi to 500 psi (34 kPa to 3.4 MPa). The pressure P1may be considerably higher than conventional low resolution CU ink pressures, which are typically below 50 psi. The high pressure P1on the carrier fluid14enables higher resolution printing when the stream of carrier fluid is a stream of droplets, as discussed further below.

The relatively high velocity (v) of the stream of carrier fluid may be both necessary and advantageous. Higher velocity may translate to higher-speed printing. But below a threshold velocity, the stream of carrier fluid will flow onto the ink nozzle20due to the voltage potential difference and the resulting electrical attraction. The threshold velocity is dependent on several factors, including the voltage potential (V1−V2), the distance (D) between the extractor12and the extraction opening18, the viscosity of the printing fluid16, the size of the extraction opening, and the electric conductivity of the fluids14,16. In one non-limiting example in which the voltage potential between the fluids14,16is about 2000V, the threshold velocity is in a range from about 6 m/s to about 11 m/s. The printer10is capable of producing a stream of carrier fluid with a velocity (v) rivaling that of CIJ printers, such as in a range from 20 m/s to 50 m/s.

The carrier fluid14may be electrically conductive in some cases, which allows the carrier fluid to more readily accept a charge from the applied voltage (V1). One specific example of a conductive carrier fluid is SIGNASPRAY® (Parker Laboratories, Inc., Fairfield, NJ, USA), which has an electrical conductivity greater than 20,000 μS/cm. Another example of a conductive carrier fluid14is a solvent with a suspension of metallic (e.g., silver) particles, such as nanoparticles. Of course, any solids content of the carrier fluid14will be present in the deposited ink. In other cases, the carrier fluid14is non-conductive. One example of a suitable non-conductive carrier fluid is isopropyl alcohol (IPA), which has an electrical conductivity of about 0.06 μS/cm. A non-conductive carrier fluid can increase the arcing threshold and allow use of higher voltages, which in turn enables a higher printing fluid extraction rate and a faster printing process. As noted above, the carrier fluid may include or may be a solvent that is also part of the printing fluid16. In some cases, solvent that evaporates from the printing fluid16during travel from the extraction opening18to the printing surface is replenished by the carrier fluid so that the deposited fluid maintains the desired solvent content.

The printing fluid16may have a high viscosity relative to the carrier fluid14. The viscosity of the printing fluid may for example be in a range from 1 cps to 300,000 cps. In various embodiments the viscosity of the printing fluid is 300,000 cps or less while also being greater than 10 cps, greater than 30 cps, greater than 100 cps, greater than 1000 cps, greater than 10,000 cps, or greater than 100,000 cps. Many functional inks have high viscosities due to the high solids content and/or particle size. The back pressure P2on the printing fluid16in the ink nozzle20may be low in comparison to the pressure P1in the other nozzle26, such as between 0.5 psi and 200 psi (3.4 kPa to 1.4 MPa). The extraction opening18may be in a range from 1 μm to 200 μm. In one embodiment, the extraction opening18is in a range from 20 μm to 100 μm. Higher resolution printing typically requires a smaller extraction opening18such as a 1 μm to 2 μm opening.

The difference in electrical potential between the carrier fluid14and the printing fluid16before they merge along the fluidic extraction stream12may be in a range from 500V to 5000V, or 1000V to 5000V. Various combinations of applied voltages (V1, V2) are possible, and the voltages may be applied in various manners. For example, one or both of the nozzles20,26may be formed from a conductive material, such as a metallic material (e.g., stainless steel), and the voltages are applied to the nozzles with the fluids14,16in contact with the interior of the nozzles. In another example, each nozzle20,26has a conductive portion with the voltages being applied to that portion of the nozzle. For example, the nozzles20,26can be formed from a non-conductive material (e.g., plastic) with a metal layer plated or deposited on an internal surface, or the nozzles may include a conductive tip that includes corresponding extraction opening18or discharge opening28. In other embodiments, each voltage is applied to an electrode that is at least partly immersed in the fluid in the nozzle or in a reservoir that supplies the nozzle.

In one example, the voltage on the printing fluid16is greater than the voltage on the carrier fluid14(V2>V1). For instance, a high voltage (500-5000V) may be applied to the printing fluid16while the carrier fluid14is grounded or floating with no voltage potential applied. This arrangement is particularly suitable when using a conductive carrier fluid. This is analogous to the favored arrangement with solid-state extractors, where the extractor is grounded and high-voltage pulses are applied to the printing fluid to cause the printing fluid to be attracted toward the extractor and, thereby, extracted from the ink nozzle. In this arrangement, the charge density at the extraction opening18is very high with a sharp nozzle tip, making it likely that the arcing threshold is higher than the extraction threshold, allowing extraction of the printing fluid16without arcing concerns. This arrangement may be limited by the fact that the printing surface22may be at the same electrical potential as the carrier fluid14(i.e., zero applied voltage or ground). This means that the high voltage printing fluid16can attracted to both the stream12of carrier fluid and the printing surface22—i.e., the proximity of the printing surface22to the ink nozzle20can affect the trajectory of extracted printing fluid. This can be problematic, for example, when printing onto a polymeric substrate at close proximity and/or using a non-conductive carrier fluid. When appropriate, use of a conductive carrier fluid in this arrangement can help alleviate such problems by making the carrier fluid a more dominant element in the electric field near the extraction opening18.

In another example, the voltage on the printing fluid16is less than the voltage on the carrier fluid14(V2<V1). For instance, the high voltage may be applied to the carrier fluid14while the printing fluid16is grounded or floating with no voltage potential applied. In this arrangement, there is not a high charge density at the extraction opening18of the ink nozzle20. It may therefore not be possible to extract some types of printing fluids with this arrangement. But for fluids that can be e-jet printed with a relatively low charge density, this arrangement will avoid substrate interference because the printing fluid16and substrate24are at the same electrical potential. The stream12of carrier fluid is the only attractive feature for the printing fluid in the entire system. Even if the printing surface22has some residual static charge, the magnitude of the charge on the stream of carrier fluid easily overcomes any attraction of the extracted printing fluid to the substrate. In some cases, it may be beneficial to not ground the printing fluid (i.e., to allow the printing fluid to have an electrically floating potential) to effectively limit the current flow in the event of arcing. The amount of charge that can pass from the carrier fluid to the printing fluid is limited, as there is no pathway for the charge to leave the printing fluid. Limiting the charge passing through the ink nozzle20can help reduce heat generated by arcing current, therefore reducing the chance of nozzle clogging with heat-curable printing fluids.

In other examples, non-zero voltages with opposite polarities are applied to the carrier fluid14and to the printing fluid16. For example, a moderate voltage (e.g., V2=500V to 1500V) may be applied to the printing fluid16in the ink nozzle20with a high magnitude negative voltage (e.g., V1=−2000V to −5000V) applied to the carrier fluid14so that the extraction stream12is at a lower potential than the printing fluid16and the printing surface22. In this arrangement, the potential difference between the ink nozzle and the substrate24and other nearby components is insufficient to extract printing fluid16from the nozzle, but the positive voltage supplied to the printing fluid is enough to impart some level of charge density charge density at the ink nozzle20. The effect is that the negatively charged extractor stream12is the only feature that provides a difference in electrical potential that is sufficient to extract printing fluid when passing by the extraction opening18. This reduces the probability that extracted printing fluid will be attracted to anything other than the stream of carrier fluid, with which the printing fluid merges to continue toward the printing surface.

In a specific embodiment, a 1000V charge is applied at the ink nozzle20and a −2000V charge is applied to the carrier fluid14. These voltage levels are sufficient for the extracted printing fluid16to effectively differentiate between the stream12of carrier fluid and the substrate so that the extracted ink is more attracted to the extraction stream12and merges with the carrier fluid without significant competition from the substrate or other uncharged components. This is true even when the carrier fluid14is substantially non-conductive (e.g., IPA).

The throw distance (H) of the printing fluid may be in a range from 5 mm to 15 mm and is determined largely by the characteristics of the stream12of carrier fluid, which can be a continuous stream, a uniform stream of droplets, or a non-uniform stream of droplets (e.g., drop-on-demand). The rate of extraction of the printing fluid is determined by the voltage potential, back pressure (P2), distance (D) between the extraction opening18and the stream of carrier fluid, extraction opening size, and characteristics of the printing fluid16(e.g., conductivity, viscosity, etc.). The example ofFIG.1depicts a continuous stream12of carrier fluid. When in the form of a continuous stream, the carrier fluid is not broken into individual droplets and is able to extract the printing fluid16in a continuous stream so that the two streams merge and continue toward the printing surface generally in the direction of the stream of carrier fluid.

FIG.2schematically illustrates an example of the electrohydrodynamic printer10in which the extractor12is a uniform stream of droplets30of carrier fluid. As used here, “uniform” means that the droplets30of the stream12are evenly spaced in the direction of travel and the same size as one another. The delivery and formation of the stream of carrier fluid illustrated inFIG.2is analogous to the manner in which the jet of ink is produced in CU printing. In this case, however, it is the carrier fluid14and not the printing fluid16that is broken into droplets30. The carrier fluid14in the nozzle26is pressurized at a pressure P1. But unlike the continuous stream of carrier fluid ofFIG.1, to which a constant pressure is applied in the nozzle26, the pressure P1applied to the carrier fluid14in the nozzle26is varied at a constant frequency. One manner of varying the pressure at a constant frequency is via a piezoelectric element32. The piezoelectric element32mechanically deflects when a voltage is applied across it. The element32is arranged to increase the pressure in the nozzle26when it deflects—i.e., by slightly decreasing a volume of the carrier fluid14in the nozzle. The voltage to the piezoelectric element32can be applied at a very high frequency, such as an ultrasonic frequency (i.e., greater than 20 kHz), to break the stream of carrier fluid into the uniform stream of droplets30upon exiting the nozzle26.

The stream of carrier fluid then passes by a charging element34that imparts an electrical charge to a portion of the droplets. In this example, the charging element34is a charging ring through which the stream of carrier fluid passes. The voltage V1is applied to the charging element34intermittently to selectively charge a portion of the passing droplets30. In particular, only the droplets30intended to extract a droplet of printing fluid16from the ink nozzle20and continue to the printing surface22are charged. The charging element34may also be referred to as an electrode.

When passing by the extraction opening18of the ink nozzle20, each droplet30of a first portion of the droplets of carrier fluid—that is, the charged droplets—extract a droplet of printing fluid16, which merges with the respective droplet of carrier fluid to be carried toward the printing surface22. A second portion of the droplets30—i.e., the uncharged droplets—do not extract a droplet of printing fluid and merely continue in the original direction of the stream of carrier fluid.

After passing by the ink nozzle20, the stream of carrier fluid then passes through a directionality unit36. In this case, the directionality unit36includes a pair of oppositely charged plates. The second portion of uncharged droplets of carrier fluid is unaffected by the directionality unit36and continues along the original direction of the stream12and into a collector38, where the carrier fluid is returned to a carrier fluid source40that supplies the nozzle26or stores the clean carrier fluid for reuse. The first portion of charged droplets30′, each now merged with a droplet of printing fluid, is directed away from the collector38by the directionality unit36and toward the printing surface22to be deposited in the desired location as part of a printed pattern42.

InFIG.2, the substrate24and printing surface22are schematically shown in plan view to illustrate the printed pattern42in the same figure as the print head. The magnitude of the charge applied to each charged droplet30′ can be the same and the voltage applied across the opposing faces of the directionality unit can be constant, with the print head and/or the substrate24moving relative to one another to produce the desired pattern42. In such an arrangement, each charged droplet30′ carrying printing fluid is laterally deflected away from the axis A1of the stream of carrier fluid by the same amount, and relative substrate-to-print head movement is relied on for forming the desired pattern42of printed material.

In some embodiments, the charge applied to each charged droplet varies so that the effect of the directionality unit varies. In other words, more highly charged droplets are more affected by the directionality unit and are laterally deflected by a greater amount. Alternatively or additionally, the voltage across the directionality unit can be varied with a similar effect. In this manner, relative movement between the print head and the substrate24can be simpler. For instance, a plurality of differently charged droplets carrying printing fluid can be sequentially deposited on the printing surface22as a row of droplets in the x-direction with the print head not moving relative to the substrate24in the x-direction, then the substrate and/or print head can be indexed in the y-direction to begin another row of droplet deposition. The length of a row of droplets without print head or substrate movement in the direction of the row is of course limited to the total amount of deflection the directionality unit36is capable of. In some embodiments, the directionality unit36is configured to deflect charged droplets in more than one direction, such as the x-direction, the y-direction, and any combination of the x- and y-directions.

FIG.3schematically illustrates an example of the electrohydrodynamic printer10in which the extractor12is a non-uniform stream of droplets30of carrier fluid. In a non-uniform stream of droplets, the spacing between individual droplets varies from droplet to droplet. This configuration can perform as a drop-on-demand printer with droplets30of carrier fluid produced only as needed to extract a corresponding droplet of printing fluid16to carry to the printing surface22. The delivery and formation of the stream of carrier fluid illustrated inFIG.3is analogous to the manner in which the jet of ink is produced in non-industrial inkjet printers. The carrier fluid14in the nozzle26is subjected to a pressure pulse when a droplet of carrier fluid is desired. In this case, each pressure pulse is provided by a piezoelectric element32deflecting in a direction that causes a small decrease in the working volume of carrier fluid14. A corresponding volume of the carrier fluid14is released through the discharge opening28with each pressure pulse. The pressure pulses can be generated in other ways, such as via thermal energy (e.g., bubble jet). In another example, consistent with the drop-on-demand embodiment ofFIG.3, the carrier fluid14in the nozzle26is pressurized in a range from 5 psi to 150 psi, and a droplet release valve operated by a solenoid or piezoelectric element is used to create the stream of carrier fluid.

Each droplet30of the stream12of carrier fluid is charged, and each droplet therefore extracts a droplet of printing fluid16as it passes by the extraction opening18of the ink nozzle20. Each extracted droplet of printing fluid merges with the a corresponding droplet of carrier fluid and is deposited on the printing surface. The printed pattern is controlled by relative movement of the print head and printing surface22in the x- and y-directions and by the timing of the pressure pulses and corresponding droplet formation. In this example, the carrier fluid is charged by the application of the voltage (V1) via an electrode44in contact with the carrier fluid14in the nozzle26. In other embodiments, the droplets of carrier fluid may pass by an electrode external to the nozzle26to be charged, as inFIG.2. Because all of the droplets30of the stream of droplets are charged to extract printing fluid, no directionality field is required to direct charged droplets to the printing surface and uncharged droplets away from the printing surface. However, a directionality field can optionally be used for additional control over droplet trajectory, and the amount of charge on each droplet can be varied by varying the electrode or charging element voltage (V1).

A high viscosity printing fluid16has been successfully printed using a stream of carrier fluid as the extractor. In a working example, the printing fluid16is a silver nano paste that is typically only printable by screen printing (DGP-NO, ANP Materials, Milpitas, CA, USA, www.anapro.com). This printing fluid has a viscosity between 50,000 and 150,000 cps and includes 70-80 wt % silver nanoparticles. The ink nozzle20has a 70 μm extraction opening18spaced from the extraction stream12by a distance (D) of 200 μm. The voltage potential between the carrier fluid14and the printing fluid16is 2000V. With the stream12of carrier fluid14at a velocity between 6 m/s and 11 m/s, the printing fluid16was extracted from the ink nozzle20and merged with the stream of carrier fluid to be carried to the printing surface.

FIGS.4-7illustrate different manners in which extracted printing fluid16merges with the stream of carrier fluid14.FIG.4is an enlarged view of a portion of the continuous stream of carrier fluid14ofFIG.1. In this example, the printing fluid16is immiscible in the carrier fluid14. The two fluids14,16thus do not mix as they travel toward the printing surface. This may be a desirable condition to prevent the carrier fluid14from diluting the printing fluid16, which would otherwise have its solvent-to-solute ratio changed if the carrier fluid and printing fluid are miscible. The carrier fluid14can be selected to have a very low boiling point (e.g., acetone) so that the carrier fluid at least partly evaporates on its way toward the printing surface so that the printing fluid16is deposited with minimal carrier fluid content. The same concepts apply to the stream of carrier fluid when in the form of uniform or non-uniform droplets30, as shown inFIG.5, which is an enlarged view of a portion of the stream of carrier fluid ofFIG.3.

FIG.6is an alternative version ofFIG.4in which the printing fluid16is miscible with the carrier fluid14. The two fluids14,16mix when they merge and/or as they travel toward the printing surface. This may be a desirable condition when the printing fluid16is formulated with a low boiling point solvent that partly evaporates during travel from the ink nozzle20to the printing surface. The carrier fluid14can include or can be the same solvent that evaporates from the printing fluid16and thereby replenish the evaporated solvent so that the printing fluid is deposited with a solvent-to-solute ratio very close to that of the printing fluid in the nozzle or at some different ratio. The same concepts apply to the stream of carrier fluid when in the form of uniform or non-uniform droplets30, as shown inFIG.7, which is an alternative version ofFIG.5in which the printing fluid16is miscible with the carrier fluid14.

FIG.8illustrates a cleaning mode of the printer as applied to the configuration ofFIG.1. When the printing fluid16is soluble in or otherwise compatible with the carrier fluid14, the carrier fluid can be used as a cleaning agent or an anti-clogging agent for the ink nozzle20. When the printer is not printing, the back pressure can be removed from the printing fluid16and the pressure P1on the carrier fluid14can be reduced such that the velocity of the stream12of carrier fluid exiting the nozzle26is not high enough for the stream of carrier fluid to pass by the ink nozzle20when the voltages are applied to the respective fluids. As a result, the stream of carrier fluid is attracted onto the nozzle20and can help maintain a clean nozzle tip and prevent the printing fluid16from drying up or otherwise clogging the extraction opening18.

As used in this specification and claims, the terms “e.g.,” “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Further, the term “electrically connected” and the variations thereof is intended to encompass both wireless electrical connections and electrical connections made via one or more wires, cables, or conductors (wired connections). Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.