CA1292384C

CA1292384C - Acoustic lens arrays for ink printing

Info

Publication number: CA1292384C
Application number: CA000550780A
Authority: CA
Inventors: Scott Alan Elrod; Butrus T. Khuri-Yakub; Calvin F. Quate
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1986-12-19
Filing date: 1987-11-02
Publication date: 1991-11-26
Anticipated expiration: 2008-11-26
Also published as: EP0272899B1; EP0272899A3; JPS63162253A; EP0272899A2; DE3782490D1; CN87101228A; BR8706818A; US4751530A; CN1017694B; DE3782490T2; JPH0645233B2

Abstract

ABSTRACT

To facilitate the fabrication of acoustic printheads, arrays of spherical acoustic lenses are provided for bringing rf acoustic waves to essentially diffraction limited focii at or near the free surface of a pool of ink. These lenses produce focal patterns which are relatively free of localized amplitude variations, so they may be employed to fabricate acoustic printheads having relatively stable characteristics for acoustic printing.

Description

23~

ACOUSTIC LENS ARRAYS FOR INK PRINTING

FIELD OF THE INVENTION

5 This invention relates to acoustic printers and, more particularly, to printheads with integrated acoustic lens arrays for such printers.

BACKGROUND OF THE INVENTIOIU

10 Substantial effort and expense have been devoted to the development of plain paper compatible direct marking technologies. The research and development activities relating to drop on demand and continuous stream ink jet printing account for a signiflcant portion of this investment, even though conventional inl~ jets su~er from the fundamental disadvantage of 15 requiring nozzles with small ejection orifices which easily clog.
Unfortunately, the size of the ejection orifice is a critical design parameter of an ink jet beGause it determines the size of the droplets of ink that the jetejects. As a result, the size of the ejection orifice cannot be increased, without sacrificing resolution.
Acoustic printing is a potentially important, alternative direct marking technology. It is still in an early stage of development, but the available evidence indicates that it is likely to compare favorably with conventional ink jet systems for printing either on plain paper or on specialized recording 25 media, while providing significant advantages on its own merits. More particularly, acoustic printing has increased intrinsic reliability because 3t~9L

.
there are no nozzles to clog~ As will be appreciated, the elimination of the clogged nozzle failure mode is especially relevant to the reliability of large arrays of ink ejectors, such as page width arrays comprising several thousand separate ejectors. Furthermore, small ejection orifices are avoided, so acoustic printing can be performed with a greater variety of inks than conventional ink je~ printing, including inks having higher viscosities and inks containing pigments and other par~iculate components. In keeping with still another feature of the technology, the size of the indivi~ual picture elements ("pixels") printed by an acoustic printer may be controlled during operation, either by varying ~he size of the individual droplets that are ejected, or by regulating the number of droplets that are used to form the individual pixels of the printed image.

As is known/ an acoustic beam exerts a radiation pressure against objects upon which it impinges.
Consequently, if an acoustic beam impinges on a free surface (i.2. ,liquid/air interface) of a pool of liquid from beneath, the radiation pressure which the beam exerts against the ~ree sur~ace may reach a sufficiently high level to release individual droplets of liquid from the surface of the pool, despite the restraining force of surface tension. ~o accomplish that, the acoustic beam advantageously is brought to ~ocus on or near the surface of the pool, thereby intensifying its radiation pressure for a given amount of input power. These principles have been applied to ink jet and acoustic printing previously, using ultrasonic (rf) acoustic beams to release small droplsts of ink from pools of ink. For example, K. A. Krause, "Focusing Ink Jet Head", IBM Technical Disclosure Bulletin, Vol 16, No. 4, September 1973, pp. 1168 - 1170 describes an ink jet in ., ~3~3~
,, which an acoustic beam emanating from a concave surface and confined by a conical aperture is used to propel ink droplets out through a small ejection orificeO
Lovelady et al. United States Patent No. 4,308,547, 5 which issued December 29, 1981 on a "Liquid Droplet Emitter" showed that the small ejection orifice of the conventional ink jet is unnecessary. To that end, they provided spherical piezoelectric shells as transducers for supplying focused acoustic beams to eject droplets of ink from the free surface of a pool of inkO They also proposed acoustic horns driven by planar transducers to eject droplets of ink from an ink coated belt. Thereafter, to reduce the cost of acoustic printheads and to simplify the fabrication o~ multiple ejector arrays, a commonly assigned United States Patent No. ~,~97,195 of C. F. Quate et al., issued Septemb r 29, 1987, entitled "LeaXy ~ayleigh Wave ~ozzleless Liquid Droplet ~jectors" introduced a planar int~rdigitated transducer (IDT) and planar IDT arrays, ~0 Quate el al also disclosed that the droplet ejection process can be controllad/ either directly by modulating the acoustic beam or indirectly in response to supplemental bur~ts o~ power from a suitably controlled rf source.

The IDT provides an economical technology for fabricating arrays of acoustic droplet ejectors, but its hollow beam focal pattern results in a higher sensitivity to minor variations in the surface level of the ink than is desired for some applications.
Accordingly, there still is a need for a technology which permits arrays of high ejection stability acoustic droplet ejectors to be assembled at moderate cost.

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SU~M~RY OF THE INVENTION

This invention responds to that need by providing spherical acoustic lens arrays for bringing rf acoustic waves to essentially diffraction limited focii at or near the free surface of a pool of ink. These lenses produce focal patterns which are relatively free of localized amplitude variations, so they may be employed to fabricate acoustic printheads having relatively stable characteristics for acoustic printiny.
Another aspect of this invention is as follows:
An acoustic printhead for ejecting droplets of ink on demand from a free surface of a pool of ink, said ink having a predetermined acoustic velocity; said printhead comprising a solid substrate having an upper surface with a plurality of essentially identical, generally spherically shaped indentations formed therein on predetermined centers to define an arxay of acoustic l~nses, and a lower surface; said substrate being composed of a material having an acoustic velocity which is substantially higher than the acoustic velocity of said ink; and piezoelectric transducer means intimately mechznically coupled to the lower surface of said substrate for generating rf acoustic waves to illuminate said lenses, such that said lenses launch respective converging acoustic beams into said ink, with the focal lengths of said lenses being selected to cause said beams to come to focus approximately at the surface of said pool.

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~23~

ÆRIEF DESCRIPTIO~ OF ~l~E DRAWINGS

Still other features and advantages of this invention will become apparent when the following detailed description is read in conjunction with the attached drawings, in which:

Figure 1 is an isometric view of an acoustic printhead construc~ed in accordance with the present invention;
1~
Fig. Z an cross sectional view of th~ printhead shown in Fig. 1, with the printhead being submerged in a pool of ink for operation;

Fig. 3 is an isometric view of a modified printhead in which the acoustic beam is partially pre-focused by the transducer:

4a Figs 4A - 4D are schematic views illustrating some of the printer configurations to which this invention can be applied;

5 Fig. 5 is a more detailed longitudinal sectional view of an embodiment cf the present invention in which the acoustic lenses are separately illuminated for drop on demand printing;

Fig. 6 is a bottom view of the printhead shown in Fig. 5;
Figs. 7 and 8 are longitudinal sectional views of alternative embodiments of the printhead shown in Fig. 5 to illustrate that provision may be made for acoustically isolating the lenses from each other; and 15 Fig. 9 is a cross sectional view of a pianarized printhead.

Fig. 1 û is a cross sectional view of another planarized prin thead DETAILED DESCRIPTION OF THE iLLUSTRATED EMBODIMENT
While the invention is described in some detail hereinbelow with reference to certain iliustrated embodiments, it is to be understood that there is no intent to limit it to those embodiments. On the contrary, the aim is to cover all modifications, alternatives and equivalents falling within the 2~ spirit and scope of the invention as defined by the appended claims.

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Turning now to the drawings, and at this poin~ especially to Figs. 1 and 2, there is an acoustic printhead 11 comprising an array of precisely positioned spherical acoustic lenses 12a - 12i for launching a plurality of 5 converging acoustic beams 15 into a pooi of ink 16 (shown only in Fig. 2).
Each of the acoustic beams 15 converges essentially syrnmetrically relative to the center of the lens 12a ..., or 12i from which it ori3inates, and the focal lengths of the lenses 12a -12i are selected so that each of the beams 15 comes to focus at or near the free surface (i. e., the liquicl/air interface)10 17 of the pool of ink 16. Suitably, the printhead 11 is submerged in the ink 16. Alternatively, the lenses 12a - 12i may be coupled theretc> by a low acoustic loss medium, such as via a thin film of mylar or the like (not shown).

16 The acoustic lenses 12a - 12i are defined by smali, generally spherically shaped indentations which are formed in the upper surface of a solid substrate 22. A piezoelectric transducer 23 is deposited on or otherwise maintained in intimate mechanical contact with the opposite or lower surface of the substrate 22, and a suitable rf sowrce (not shown) is coupled 20 across the transducer 23 to excite it into ~scillation. The oscillation of the transducer 23 causes it to generate ultrasonic acoustic waves 24 for collectiveiy or, as subsequently described in additional detail, separately illuminating the lenses 12a - 12i. If the same acoustic wave 24 illuminates all of the lenses 12a - 1 2i, its amplitude is selected to cause the beams 15 to25 excite the free surface 17 of the ink 16 to an incipient, subthreshold energy level for droplet formation. Additionally, a suitable source of 38~
supplemental power (not shown) is provided for selectively addressing the acoustically excited focal sites, so that individual droplets of ink are ejected from them on demand. See, the aforementioned Quate et al patent ~,697,195. Also see a commonly assigned United States Patent No. 4,748,461 of S. A. Elrod, issued May 31, 1988, entitled "Capillary Wave Controllers for Nozzleless Droplet Ejectors".

As illustrated in Figs. 1 and 2 the transducer 23 has a planar profile, so it generates generally planar wavefront acoustic waves 24. However, transducers having other profiles may be employed. For example, as shown in Fig. 3, a cylindrical transducer 23' may be employed for generating partially pre focused acoustic waves 24' to illuminate a linear array of lenses 12a -12i.

In keeping with one of the more detailed aspects of this invention, to significantly reduce, if not eliminate, aberrations of the focused acoustic beams 15, the lens substrate 22 is composed of a material having an acoustic velocity, vs, (i.e., the velocity of sound in the substrate 22) which is much higher than the velocity of sound in the ink 16, vi, so v5 ~ vi. Typically, the velociky of sound in the ink 16, vi, is in the range o~
1 - 2 km/sec. Thus, the substrate 22 may be composed of any one of a wide variety of materials, such as silicon, silicon nitride, silicon carbide, alumina, sapphire, fused quartz, and certain glasses, to maintain a refractive index ratio (as determined by the ratio of the acoustic velocities, vs/vi) in excess of 38~

2.5:i at the interface between the lenses 12a - 12i and the ink 16. A 2.5:1 ratio is sufficient to ensure ~hat the aberrations of the beams 15 ar~ small. However, if the substrate 22 is composed of one of the higher acoustic velocity materials, such as silicon, silicon nitride, silicon carbide, alumina and sapphire, a re~ractive index ratio of 4:1 or higher can be easily achieved, thereby reducing the aberrations of the beams 15 to an essentially negligible level. See, C. F.
Quate, "The Acoustic Microscope" Scientific American, Vol. 241, No. 4, October 1979, pp 62 - 72 for a more detailed discussion of the principles involved.

Acoustic printing requires precise positioning of the lenses 12a - 12j with respect to each other on very closely spaced centers. Preferably, therefore, in keeping with another aspect o~ thi~ in~ention, the lenses 12a - 12i are chemically etched or molded into the substrate 22. A suitable photolithographic process for isotropically etchinq them into silicon is descri~ed by K. D. Wise et al, "Fabrication of Hemispherical Structures Using Semiconductor Technology for Use in ThermonucIear Fusion ~esearch," J Vac. Sci. Technol., Vol. 16, No. 3, May/June 1979, pp. 936-g39, and that process may be extended to fabricating the lenses 12a -12i on substrates 22 composed of other chemicallyetchable materials. Alternatively, the lenses 12a - 12i may be cast into materials such as alumina, silicon nitride and silicon carbide through the use of hot press or injection molding processes. If desired, an anti-reflecti~e coating 26 tFig. 2), composed of a ~z/4 thicklayer of impedance matching material (where ~z = the wavelength of the acoustic A

31~4 beams 15 in the coating 26), rnay be deposited on'the outer spherical sur~aces of the lens~s 12a - 12i.

Typically, the radii of the lenses 12a - 12i are greater than the depth of the 5 indentations which define them so that their focal plane is offset from the upper surface of the substra~e 22 by a distance which is approximately equal to the thickness of the overlying layer of ink 16 (plus the thickness of any intervening medium, such as any film that is used to support the ink).
Thus, if the lenses 12a - 12i are chemically etched into the substrate 22 in 10 accordance with the aforementioned teachings of Wise et al., a grinding operation, an additional chemical etch, or the like may be employed to cut the upper surface of the etched substra~e 22 back to displace it by a sufficient ~istance frl~m the ~ocal plane o~ the lenses 12a - 12i.
Additionally, the finish on the upper surface of the substrate 22 may be 15 roughened, such as by grinding, to diffusively scatter any incident acoustic :~ energy that is not collected by the ienses 1 2a - 1 2i.

Linear and two dimensional lens arrays (as used herein a "two dimensonal array" means an array having two or more rows of lenses~ for various types 20 of acoustic printing may be provided in accordance with this invention, including page width linear and two dimensional lens arrays for line printing, smaller linear arrays for multi-line raster printing, and two dimensional arrays for matrix printing. To emphasize that point, Fig. 4A
schematically illustrates a line printer 31 in which a suitable recording 2~ medium 32, such as plain paper, is advanced in a sagittal direction, as indicated by the arrow 33, relative to a tangentially aligned page width g 3~i linear lens array 34; Fig. 4B schernatically illustrates an~ther line printer 36which has a page wid~h two dimensional staggered lens array 37; Fig. 4C
schematically illustrates a multi-line raster printer 41 in which the recording mediurn 32 is advanced in the sagi~tal direction while a sagittally 5 s:)riented linear lens array 42 is being advanced in a tangential direction, as indicated by the arrows 33 and 43, respectively; and Fig. 4D schematically illustrates a matrix do~ printer 51 in which the recording medium 32 is advanced along one axis of the matrix while a two dimensional, matrix configured lens array 52 is being advanced along the orthogonal axis of the 10 matrix, as indicated by the arrows 53 and 54, respectively. These examples are not exhaustive, but they illustrate the substantial design flexibility which e~ists.

In keeping with an impo~ant feature of this invention, as shown in Figs 5 -15 8, provision can be made for selectively and individually illuminating the lenses 1Za - 12i with separate acous~ic waves 24 (Fig. 2). This permits the acoustic beams 15 (Fig. 2) to be independently modulated for spatially controlling the droplet ejection process on a lens-by-lens basis. To that end, in these rnore detailed embodiments the transducer 23 comprises a 20 thin piezoelec~ric element 61, such as thin ZnO film or a thin I iNbO3 uystal, which is sandwiched between an array of individuaily addressable eiectrodes 62a - 62i (best shown in Fig. 6) and a counter electrode 63. The electrodes 62a - 62i are placed so as to properly illuminate the lenses 12a -12i, respectively. Furthermore, the transducer 23 is intimately mechanically 25 coupled to the lower surface of the lens substrate 22. For example, the transducer counter electrode 63 may be deposited on the lower surface of .

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the substrate 22, either directly or after that surface bas been overcoated with a suitable electrical insulator 64, such as a layer of SiO2.

In operation, independently controlled rf drive voltages are applied across the electrodes 62a - 62i, respectively and the counter electrode 63, thereby locally excitincJ the piezoelectric element 61 into oscillation at spatially separated sites which are centered in the normal direction on the electrodes 62a - 62i, respectively. The l~calized oscillations of the piezoelectric element 61 generate spatially displaced acoustic waves 24 which propagate through the substrate 22 in a predetermined direction to iliuminate the lenses 12a- 12i, respectively, Accordingly, the rf drive voltages which are applied to the electrodes 62a - 62i at any given time independen~ly control the radiation pressures of the ac~ustic beams 15 that are lawnched into the ink 16 by the lenses 12a - 12i, respectively, at that particular time. Typicaliy, the transducer 23 has a relatively narrow bandwidth, so the droplet ejection process may be spatially controlled on a Iens-by-lens basis by appropriately modulating the amplitude, frequency or duration of the drive voltages applied to the electrodes 62a - 62i.

As will be appreciated, the acoustic waves 24 (Fig. 2) are diffrac~ecl as they propagate through the substrate 22. This diffraction may be ignored, as indicated in Fig. 5, if the thickness of the substrate 22 is on the order of oneRayleigh length. However, if thicker substrates 22 are employed, the lenses 1 2a - 1 2i preferably are acoustically isolated from each other, such asby providing narrow slots 66 between them which are filled with air or some other medium having an acoustic impedance which cliffers ~2~231~

significan~ly from the acous~ic impedance of the substrate 22 such that an acoustic mismatch is created.
These slots 66 may be extended upward through the lower sur~ace of the substrate 22 tFig. 7) or downward through its upper surface (Fig. 8). If the substrate 22 is composed of a chemically etchable crystalline material, such as silicon, the slots 66 may b~ anistropically etched therein. See, for example, K.~. Petersen, "Silicon as a Mechanical Material," Proceedinqs of the IEEE, VQ1~ 70, No~ 5, May 1982, pp. 421-457.

Preferably, the outer surfaces of the lenses 12a - 12i have a smooth finish and are cleaned as required to remove particulate deposits from them, such as pigment and dust particl~s that may precipitate out of ~he ink 16. Furthermore, in some embodiments, it may be desirable to transport the in~ 16 o~er the lenses 12a -12i on a thin mylar film or the like which may tend to abrade or drag against the edges of the lenses 12a -12i. Therefore, as shown in Fig. 9, the lenses 12a -12i may be planarized~ by filling the indentations whichdefine them with a suitable polymer 71, such as an epoxy resin, or similar solid material having an acoustic impedance and velocity intermediate between the acoustic impedance and the velocity of the ink 16 and the substrate 22. See a commonly assigned United States Paten$ No. 4,751,534 of Elrod et al, issued ~une 14, 1988, entitled "Planarized Droplat Ejectors for Acoustic Printing". This filler layer 71 may be flush with the upper surface of the substrate 22 (Fig. 9), or it may form a thin overcoating thereon ~Fig. 10). The anti-reflective lens coating 26 (Fig. 2) is not shown in Figs. 9 and lO to emphasize that it is optional.

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one of the more important applications of the present inYention relates to providing page width acoustic print heads for line printing, so that application will be reviewed in additional detail. As is known, the diameter of the spot or "pixel" that a droplet of ink makes when deposited on paper is approximately equal to twice the diameter of the droplet. Therefore, a page width linear array of substantially identical acoustic lenses 12a - 12i (Fig. 4A), each designed to provide a focused acoustic beam 15, is sufficient to print an essentially unbroken line of ink across the full width of the page, provided that multiple clroplets of ink are placed on each pixel as described below.
Alternatively, the same result can be achieved through the use of a page width two dimensional array comprising two or more stag~ered rows of lenses (Fig. 4B), with each of the lenses being designed to provide a focused ~ beam having a waist diameter equal to one quarter the : center-to-center spacing of the lenses. Furthermore, the center-~o-center spacings of the lenses within ~ these arrays may be increased, without impairing their ; solid line printing capability, if the duration o~ the rf drive pulses applied to the transducer drive electrodes 62a - 62i is increased (typically, the duration of the rf pulses for drop on demand printing is restricted to a range from about l~sec and lOO~sec).
If the electrodes 62a - 62i are rapidly and repeatedly pulsed to deposit up to as many as fifteen or so droplets on each pixel, the lens spacing may also be increased. These pulse width modulation and multiple droplet printing techniques may he combined to increase the size of the pixels printed by a given spherical lens-type droplet ejector by a factor of more than four, so part of the pixel size control capacity may be utilized to increase the center-to-center spacing of the 238~

lenses 12a - 12i, with the remainder being held in reserve to provid~ a gray scale representation when desired.

For example, a pixel diame~er of about 50 micr~ns i required to provide a resolution of roughly 500 spi, which is typical o~ the resolution needed for high quality printing. This suggests a center-to-center spacing of approximately 100 microns for the lenses of a dual row staggered array~ More particularly, it can be shown that a rf frequency on the order of 50MHz is sufficient to print 50 micron spots. The wavelength, ~i of the acoustic beams 15 in the ink 16 at that frequency is approximately 30 microns. Moreover, at the aforementioned acoustic velocity ratios, v5/vi of 2.5:1 : 15 and 4:1, the corresponding wavelengths, ~$~ f the acoustic waves 24 in the substrate 22 are 75 microns and 120 microns, respectively. Fortunately, it has been found that small aperture lenses 12a - 12i (lenses having apertures, A ~ ) provide sufficient focusing of the acoustic beams 15 on the free surface 17 of the ink 16 to eject individl1al droplets of ink therefrom on demand. See commonly assigned United States Patent No.
; 4,751,529 of Elrod et al, issued June 14, 1~88, entitled "~icrolenses for Acoustic Printing". It is not yet known precisely how small the lens apertures may be made while still providing sufficient focusing of the beams for drop on demand printing, but it has been experimentally verified that drop on demand operation can be achieved using le~ses having apertures as small as 1.5\s, which corresponds to a lens aperture of .~

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approximately 6~; at a 4:1 ratio between the acous~ic velocities of the substra~e 22 and the ink 16.

ONCLUSION

In view of the foregoin~, it wil1 now be understood that the present invention permits arrays of relatively stable acoustic droplet ejectors to be assembled at moderate cost. Moreover, it will be apparent that droplet ejector arrays embodying this invention may be employed for various 10 forms of acoustic printing.

Claims

1. An acoustic printhead for ejecting droplets of ink on demand from a free surface of a pool of ink, said ink having a predetermined acoustic velocity;
said printhead comprising a solid substrate having an upper surface with a plurality of essentially identical, generally spherically shaped indentations formed therein on predetermined centers to define an array of acoustic lenses, and a lower surface; said substrate being composed of a material having an acoustic velocity which is substantially higher than the acoustic velocity of said ink;
and piezoelectric transducer means intimately mechanically coupled to the lower surface of said substrate for generating rf acoustic waves to illuminate said lenses, such that said lenses launch respective converging acoustic beams into said ink, with the focal lengths of said lenses being selected to cause said beams to come to focus approximately at the surface of said pool.

2. The printhead of Claim 1 wherein said acoustic lenses are aligned to define a page width long linear array of lenses.

3. The printhead of Claim 1 wherein said acoustic lenses are aligned to define a page width long two dimensional array of staggered lenses.

4. The printhead of Claim 1 wherein said acoustic lenses are aligned to define a linear array of lenses.

5. The printhead of Claim 1 wherein said acoustic lenses are aligned to define a two dimensional array of lenses.

6. The printhead of Claim 1 wherein said transducer means supplies independently modulated rf acoustic waves for individually illuminating said lenses, whereby said lenses launch separately modulated acoustic beams into said ink, with the modulation of said acoustic beams being controlled on a lens-by-lens basis for drop on demand printing.

7. The printhead of Claim 6 wherein said substrate has acoustic impedance mismatch regions which are disposed between said lenses for acoustically isolating said lenses from each other.

8. The printhead of Claim 7 wherein said impedance mismatch regions extend upward into said substrate from its lower surface.

9. The printhead of Claim 7 wherein said impedance mismatch regions extend downward into said substrate from its upper surface.

10. The printhead of Claim 1 wherein the velocity of sound in said substrate is at least 2.5 times higher than the velocity of sound in said ink.

11. The printhead of Claim 10 wherein the velocity of sound in said substrate is at least four times higher than the velocity of sound in said ink.

12. The printhead of Claim 1 wherein said indentations are filled with a solid material having an acoustic velocity comparable to that of said ink, whereby said printhead presents a generally planar upper surface to said ink.

13. The printhead of Claim 12 wherein the velocity of sound in said substrate is at least 2.5 times higher than the velocity of sound in said ink.

14. The printhead of Claim 12 wherein the velocity of sound in said substrate is at least four times higher than the velocity of sound in said ink.

15. The printhead of Claim 1 wherein said acoustic waves have a predetermined wavelength in said substrate, and said acoustic lenses have a predetermined diameter which is less than ten times said wavelength.

16. The printhead of Claim 15 wherein the velocity of sound in said substrate is at least 2.5 times higher than the velocity of sound in said ink.

17. The printhead of Claim 16 wherein the velocity of sound in said substrate is at least four times higher than the velocity of sound in said ink.

18 18. The printhead of Claim 17 wherein said transducer means supplies independently modulated rf acoustic waves for individually illuminating said lenses, whereby said lenses launch separately modulated acoustic beams into said ink, with the modulation of said acoustic beams being controlled on a lens-by-lens basis for drop on demand printing.

19. The printhead of Claim 18 wherein said indentations are filled with a solid material having an acoustic velocity comparable to that of the ink, whereby said acoustic beams are launched into said ink from a generally planar surface of said printhead.

20. The printhead of any of Claims 1, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 or 19 wherein said substrate and said transducer means are submerged in said ink.