Droplet deposition apparatus

Abstract

A pulsed droplet ink jet printer has relatively long thin ink channels extending in parallel between an ink manifold 13, and a nozzle plate 5 providing a nozzle 6 for each channel. Side walls 11 may be formed substantially entirely of piezo-electric material so as to be displaceable transversely into a selected channel on the application of an electric field. This transverse displacement produces an acoustic wave in the channel which results in the ejection of an ink droplet. The side walls may deflect in shear mode to a cross-section of chevron formation. Usefully, it is arranged that both side walls adjoining the selected channel are displaced inwardly of the channel to cooperate in droplet ejection. Under this arrangement, the channels are assigned alternately to first and second groups of channels, only one group of channels being capable of actuation at any one instant. The nozzles associated with the respective groups of channels may be offset so as to compensate for the time delay in actuation of channels in the first and second groups.

We claim: 
     
       1. A high density multi-channel array, electrically pulsed droplet deposition apparatus, comprising a multiplicity of parallel droplet liquid channels, mutually spaced in an array direction normal to the length of the channels, each of said channels extending in a lengthwise direction and being separated from a like channel by a side wall which is transversely displaceable in opposite senses and which extends in the lengthwise direction of the channels, and in a direction which is both normal to said lengthwise direction and normal to the array direction, respective nozzles opening into said channels for ejection therefrom of droplets of liquid, connection means for connecting said channels to a source of droplet deposition liquid and electrically actuable means located in relation to said channels for effecting in each channel selected for actuation, transverse displacement generally parallel to said array direction of said transversely displaceable side wall of said selected channel to cause change of pressure in said selected channel to effect droplet ejection from the nozzle opening thereinto, .Iadd.wherein said electrically actuable means comprises for each said side wall a uniform body of piezoelectric material forming part of the side wall and adjoining each of said channels separated by said side wall and electrode means for applying a field to the piezoelectric material..Iaddend..[.2. Apparatus as claimed in claim 1, wherein said electrically actuable means comprises piezo-electric material forming part at least of a wall adjoining each of said channels and electrode means for applying a field to the piezo-electric material..]..[.3. Apparatus as claimed in claim 1, wherein said electrically actuable means comprises piezo-electric material forming at least part of each side wall and electrode means for applying a 
     
     
        field to the piezo-electric material..].4. Apparatus according to claim .[.3.]. .Iadd.1.Iaddend., wherein said piezo-electric material is 
     
     
        displaceable under the action of the applied field in shear mode. 5. Apparatus according to claim 1, wherein substantially every channel side wall is displaceable in respective opposite senses and is common to 
     
     
        two adjacent channels. 6. Apparatus according to claim 1, wherein .[.the.]. .Iadd.a.Iaddend. compliance of the side walls is such that .[.the.]. .Iadd.a.Iaddend. magnitude of the pressure changes arising in neighbouring channels as a result of side wall compliance on actuation of a selected channel represents a significant proportion of the magnitude of 
     
     
        the pressure change in the selected channel. 7. Apparatus according to claim 1, wherein each electrically actuable means serves on selected actuation of any channel to effect transverse displacement of at least 
     
     
        part of both side walls of the channel one toward the other. 8. Apparatus according to claim 7, wherein said electrically actuable means comprises piezo-electric material forming at least part of each channel side wall and common electrodes are provided one for each channel for applying a 
     
     
        field to the piezo-electric material of the side wall. 9. Apparatus according to claim 8, wherein each said common electrode comprises an electrode layer covering substantially all internal surfaces of the 
     
     
        corresponding channel. 10. Apparatus according to claim 4, wherein said piezo-electric material is disposed in two regions coextensive longitudinally of the channel and mutually spaced normal to said array direction, the direction of poling with respect to the applied electric field in each region being such that the said wall part undergoes 
     
     
        deformation generally to chevron form. 11. Apparatus according to claim 
     
     
        10, wherein said regions are substantially contiguous. 12. Apparatus according to claim 10, wherein said regions are connected through an 
     
     
        inactive wall part. 13. Apparatus according to claim 1, wherein the length of each channel is at least 30 times greater than the mean dimension of 
     
     
        the channel in the array direction. 14. Apparatus according to claim 1, wherein the length of each channel is at least about 100 times greater 
     
     
        than the mean dimension of the channel in the array direction. 15. Apparatus as claimed in claim 1, wherein, in .[.the.]. .Iadd.a.Iaddend. cross section of said channels, the extent of said transversely displaceable side walls in the direction normal to said array direction is substantially greater than the mean dimension of said 
     
     
        channels in said array direction. 16. Apparatus according to claim 15, wherein said extent of said transversely displaceable side walls is from 3 
     
     
        to 30 times greater than said dimension of the channels. 17. Apparatus according to claim 1, wherein, in the cross section of said channels, the extent of said side walls in the direction normal to said array direction is substantially greater than the mean dimension of said side walls in 
     
     
        said array direction. 18. Apparatus according to claim 17, wherein said extent of the side walls is from 3 to 30 times greater than said dimension 
     
     
        of side walls. 19. Apparatus according to claim 17, wherein each sidewall is shaped to reduce the mean displacement thereof in the array direction in response to pressure difference between the channels adjacent the side wall, compared with a rectangular cylindrical side wall of the same means 
     
     
        dimension in the array direction. 20. Apparatus according to claim 19, wherein the dimension of each sidewall in the array direction reduces 
     
     
        inwardly of the channel cross section. 21. Apparatus according to claim 19, wherein said side walls are sinuous in a plane containing both the 
     
     
        channel lengths and said array direction. 22. Apparatus according to claim 17, wherein each sidewall is provided with means .[.to reduce.]. .Iadd.for reducing.Iaddend. the mean displacement thereof in the array direction in response to pressure difference between the channels adjacent the side wall, compared with a rectangular cylindrical side wall of the same mean 
     
     
        dimension in the array direction. 23. Apparatus according to claim 4, including means comprising a surface layer provided on the piezo-electric material of a material stiffer than the piezo-electric material .[.to reduce.]. .Iadd.for reducing.Iaddend. the compliance of the piezo-electric material to pressure in the channel without substantially affecting the 
     
     
        compliance of the piezo-electric material to shear. 24. Apparatus according to claim 23, wherein said surface layer comprises insulating 
     
     
        material applied over said electrodes. 25. Apparatus according to claim 23, wherein said electrodes are made of a thickness greater than .[.that.]. .Iadd.the thickness.Iaddend. required for electrical 
     
     
        functioning thereof. 26. Apparatus according to claim 1, wherein said channel side walls extend between top and bottom walls common to the 
     
     
        array. 27. Apparatus according to claim 26, wherein said side walls are rigidly connected to said top and bottom walls to inhibit rotational 
     
     
        movement of the side walls relative to the top and bottom walls. 28. Apparatus according to claim 26, wherein said electrically actuable means comprises piezo-electrical material extending substantially from the 
     
     
        top to the bottom wall over said part of the said wall. 29. Apparatus according to claim 28, wherein said top and bottom walls are formed of 
     
     
        electrically insulating material. 30. Apparatus according to claim 26, wherein each channel is formed with a communicating channel extension in 
     
     
        either or both of the top and bottom walls. 31. Apparatus according to claim 30, wherein substantially all channel extensions are formed in the 
     
     
        same one of the top and bottom walls. 32. Apparatus according to claim 30, wherein the channel extensions successive channels are formed alternately 
     
     
        in the top and bottom walls. 33. Apparatus according to claim 1, wherein said nozzles communicate substantially directly with the respective 
     
     
        channels. 34. Apparatus according to claim 1, wherein each channel contains .[.in a quiescent state.]. a volume .[.of liquid.]. V and wherein for each channel there are provided connecting means for connecting the channel with the respective nozzle, the internal .[.liquid.]. volume 
     
     
        defined by each said connecting means being less than 0.1 V. 35. Apparatus according to claim 33, wherein said transversely displaceable side wall part extends from the location in each channel at which the channel 
     
     
        communicates with the corresponding nozzle. 36. A high density multi-channel array, electrically pulsed droplet deposition apparatus for depositing droplets upon a surface moving relatively to the array, comprising a multiplicity of parallel droplet liquid channels arranged in pairs with the two channels of each pair being assigned respectively to a first and a second group of said channels, nozzles opening respectively into the channels, a longitudinal side wall provided for each pair of channels which is transversely in opposite senses and serves to divide the channels of the pair; electrically actuable means .[.adapted.]. .Iadd.for, .Iaddend.in respective time alternating first and second operating modes, upon selection of any channel in respectively the first or second group of channels, .[.to effect.]. .Iadd.effecting .Iaddend.transverse displacement .[.in the appropriate sense.]. of the side wall associated with the pair of channels including the selected channel, so as to cause a change of pressure in the selected channel to effect droplet ejection from the nozzle opening thereinto.Iadd., wherein the nozzles opening respectively into the two channels of each pair are mutually offset in the direction of relative movement of said surface by an amount commensurate with the time 
     
     
        spacing between said first and second operating modes.Iaddend.. 37. Apparatus according to claim 36, wherein each channel of a channel pair is separated from the adjacent channel of the next succeeding pair by 
     
     
        a fixed longitudinal wall. 38. Apparatus according to claim 36, wherein each channel of a channel pair is separated from the adjacent channel of the next pair by a displaceable longitudinal side wall, the electrically actuable means .[.being adapted.]., upon selection of a channel .[.to effect.]..Iadd., effecting .Iaddend.transverse displacement mutually 
     
     
        toward one another of opposite side walls of the selected channel. 39. Apparatus according to claim 36, wherein each channel communicates with a respective channel extension projecting transversely from the channel and providing a volume not bounded by the corresponding side wall. 
     
     
        0. Apparatus according to claim 38, wherein each channel communicates with a respective channel extension with the channel extensions of the first and second groups of channels projecting in respective opposite 
     
     
        directions. 41. Apparatus according to claim 40, wherein the channel extensions of each group of channels project through a common substrate and wherein portions of the substrate defined between adjacent channel extensions of each group are displaceable to effect pressure transfer 
     
     
        between said adjacent channel extensions. 42. Apparatus according to claim 40, wherein the channel extensions associated with each group of channels extend within a common substrate and define cantilever substrate portions 
     
     
        lying between adjacent channel extensions of the group. 43. Apparatus according to claim 42, wherein the two substrate portions bounding the channel extension of any channel are adapted to deflect under the action of a pressure change in said channel to compensate in the group neighbouring channels of said channel for pressure changes arising from 
     
     
        compliant sidewall deformation. 44. Apparatus according to claim 39, wherein the volume of each channel extension is greater than the volume of 
     
     
        the corresponding channel. 45. Apparatus according to claim 39, wherein each channel extension has a bounding surface which is generally coplanar 
     
     
        with a longitudinal side wall surface of the corresponding channel. 46. A high density multi-channel array, electrically pulsed droplet deposition apparatus for depositing droplets upon a surface, comprising a multiplicity of parallel droplet liquid channels with successive channels being assigned alternately to a first and a second group of said channels, nozzles opening respectively into the channels, longitudinal side walls each serving to divide one channel from the next and each transversely displaceable in opposite senses; electrically actuable means .[.adapted.]. .Iadd.for, .Iaddend.in respective time alternating first and second operating modes, upon selection of any channel in respectively the first or second group of channels, .[.to effect.]. .Iadd.effecting .Iaddend.transversely displacement .[.in the appropriate senses.]. of both side walls associated with the selected channel, so as to cause a change of pressure in the selected channel to effect droplet ejection from the nozzle opening thereinto.Iadd., wherein successive channels are offset alternately in opposite senses along a direction normal both to the length of the channels and the direction in which the channels are 
     
     
        spaced.Iaddend.. 47. Apparatus according to claim 46, wherein the nozzles communicating with the channels of the first group of channels are offset with respect to the nozzles communicating with the channels of the second group, by an amount commensurate with the time .[.spacing.]. .Iadd.difference .Iaddend.between said first and second operating modes. .[.48. Apparatus according to claim 46, wherein successive channels are offset alternately in opposite senses along a direction normal both to the length of the channels and the direction in which the channels are 
     
     
        spaced..].49. Apparatus according to claim .[.48.]. .Iadd.46.Iaddend., wherein the channels are formed in a body and the body portions bounded by any channel and neighbouring channels of the same group as said channel are adapted to deflect under the action of a pressure change in said channel to compensate in said neighbouring channels for pressure changes 
     
     
        arising through compliant deformation of side walls. 50. A high density multi-channel array, electrically pulsed, droplet deposition apparatus, comprising a top wall, a bottom wall, side walls extending between and normal to said top and bottom walls to define therewith a multiplicity of parallel droplet liquid channels having respective longitudinal axes thereof disposed in a plane, respective nozzles opening into corresponding points of said channels for ejection of droplets of liquid from said channels and connection means for connecting said channels to a liquid source for affording replenishment of droplets ejected from said channels, wherein at least some of said side walls are formed substantially wholly from piezo-electric material and have respective wall parts adjacent said top and bottom walls with electrodes disposed on opposite surfaces of each of said wall parts extending parallel with said channels and normal to said plane to afford an electric field normal to said surfaces thereby to effect shear mode deflection of said wall parts in respective opposite senses transversely to .[.that channel.]. .Iadd.said channels and .Iaddend.generally parallel to said plane, thereby to effect droplet ejection from .[.that channel.]. .Iadd.at least one of said 
     
     
        channels.Iaddend.. 51. Apparatus as claimed in claim 50, wherein substantially every side wall is transversely displaceable in respective opposite senses and said electrodes are adapted to be energized in a first mode of operation to effect transverse displacement mutually towards one another of opposite side walls of selected channels of a first series of channels to cause droplet ejection from said selected channels of said first series of channels while in a second mode of operation transverse displacement mutually towards one another is effected of opposite side walls of selected channels of a second series of channels, respective channels of which alternate with the channels of said first series, to cause droplet ejection from said selected channels of said second series. 
     
     
         2. Apparatus according to claim 51, wherein the nozzles of said first series of channels have their axes parallel and disposed in a first plane and the nozzles of said second series have their axes parallel and disposed in a second plane parallel with and spaced from said first plane by an amount to compensate for the time difference in droplet ejection from said first and second series of channels so that deposited droplets 
     
     
        are disposed in predetermined manner. 53. A multi-channel array, electrically pulsed, droplet deposition apparatus comprising a multiplicity of parallel channels having longitudinal axes disposed in a plane and respective cross-sections extending normal to said plane and of rectangular form, respective nozzles connected with said channels for droplet ejection therefrom, and connection means for connecting said channels with a source of droplet deposition liquid, said channels being further characterised in that respective walls of piezo-electric material form sides of said channels extending normal to said plane of said channel axes and are poled in the direction parallel to said plane and electrodes are disposed on each of said walls of piezo-electric material to provide for an electric field therein normal to said direction of poling to cause deflection of said wall of piezo-electric material transversely to the axis of the channel of which .[.it.]. .Iadd.said wall .Iaddend.forms a 
     
     
        side to effect droplet ejection therefrom. 54. Apparatus as claimed in claim 53, characterised in that said channels are arranged in successive pairs and between the channels of each pair is a wall of piezo-electric material which is poled in the direction parallel to the plane of the channel axes and provides a common side wall of the corresponding pair of channels which extends normal to the plane of the axes of the channels and said electrodes are disposed in relation to each of said walls of piezo-electric material to effect transverse deflection of said wall into one of the channels of which the wall is part in a first mode of operation and transverse deflection of said wall in a second mode of operation into 
     
     
        the other of the channels of which said wall forms part. 55. Apparatus as claimed in claim 53, characterized in that all side walls of said channels which extend normal to said plane at least partly comprise piezo-electric material extending throughout the wall length and poled in a direction parallel with said plane and transversely to said channel axes, said electrodes are disposed on each of said side walls to provide for an electric field therein normal to said direction of poling, and means for energizing said electrodes are provided which in a first mode of operation effect transverse deflection of opposite side walls of channels of a first series of channels with the deflected side walls of said channels of said series of channels moving mutually towards one another to cause droplet ejection from said channels of said first series of channels whose opposite side walls are deflected and in a second mode of operation transverse deflection is effected of opposite side walls of channels of a second series of channels respective channels of which alternate with the channels of said first series with the deflected side walls of said second series of channels moving mutually towards one another to cause droplet ejection from said channels of said second series whose side walls are 
     
     
        deflected. 56. Apparatus as claimed in claim 55, characterised in that all of said side walls which extend normal to said plane comprise a central inactive wall part and outer wall parts of piezo-electric material respectively poled in directions parallel with said plane and transversely 
     
     
        to said channel axes. 57. A multi-channel array, electrically pulsed, droplet deposition apparatus, comprising a multiplicity of parallel channels formed by parallel side walls and pairs of strips of piezo-electric material extending the length of said side walls, each pair of strips being sandwiched between successive of said side walls and spaced apart to form therewith a channel of rectangular cross-section, said multiplicity of channels so formed having their longitudinal axes disposed in a plane, respective nozzles communicating with said channels at corresponding ends thereof and respective connection means for connecting said channels to a source of droplet deposition liquid, each of said strips being poled in the same sense and in a direction parallel with said plane, there being provided electrodes at faces of said strips which are opposed to said side walls to provide in each of said strips an electric field in the poling direction thereof and electrode energising means which in a first mode of operation effect displacement of strips of some at least of said pairs of strips of a first series of channels to effect transverse movement in opposite directions of the side walls engaging the displaced strips and so cause droplet ejection from the channels of the side walls so moved, and, in a second mode of operation to effect displacement of the strips of some at least of said pairs of strips of a second series of channels of which the channels alternate with the channels of the first series to effect transverse movement in opposite directions of the side walls engaging the displaced strips and so cause droplet ejection from the channels of the side walls so moved, the electrode .[.energising.]. .Iadd.energizing .Iaddend.means .[.being adapted in each of said modes of operation to effect.]. .Iadd.for, in each of said modes of operation, effecting .Iaddend.displacement of each of the side walls of each channel from which a droplet is ejected by the same amount and in the same direction as the side walls forming with the side walls of each channel from which a droplet is ejected the channels next to and on opposite sides of said channel from which droplet ejection takes 
     
     
        place. .[.58.  A multi-channel array, droplet deposition apparatus, comprising a multiplicity of parallel channels formed by bi-morph side walls and pairs of strips of inactive material each of which pairs is sandwiched between successive of said side walls to form therewith a channel of rectangular section, said multiplicity of channels so formed having their longitudinal axes disposed in a plane, and each of said bi-morph walls comprising at least one layer of piezo-electric material extending transversely to and poled in a direction parallel with said plane and provided with an electrode, respective nozzles communicating at corresponding locations with said channels and respective connection means for connecting said channels to a source of droplet deposition liquid, there being provided electrode energising means which in a first mode of operation effect transverse flexural displacement in respective opposite directions of some at least of opposite side walls of channels of a first series of channels to cause droplet ejection from the channels of the side walls so moved, and, in a second mode of operation effect transverse flexural displacement in respective opposite directions of some at least of opposite side walls of channels of a second series of channels of which the channels alternate with the channels of the first series to cause 
     
     
        droplet ejection from the channels of the side walls so moved..].59. A multi-channel array, electrically pulsed droplet deposition apparatus comprising a multiplicity of channels formed by parallel flexible side walls and spaced top and bottom planar walls with which said side walls engage at opposite ends thereof, said top and bottom walls being provided by respective layers of piezo-electric material poled in opposite senses normal thereto and formed by wall segments of rectangular cross-section of thicknesses in the direction between segments corresponding with the side wall thickness and channel width between successive of said side walls, said top and bottom walls being disposed with said segments thereof in line with opposite ends of said side walls and said channels, respective nozzles communicating with said channels at corresponding ends thereof and respective connection means for connecting said channels to a source of droplet deposition liquid, electrodes provided respectively between facing sides of said segments and electrode energising means which in a first mode of operation through shear mode deflection of segments of said top and bottom walls effects flexure in mutually opposite directions of opposed side walls of channels in a first series of channels to cause droplet ejection from the channels of the side walls so flexed, and, in a second mode of operation through shear mode deflection of segments of said top and bottom walls effects flexure in mutually opposite direction of opposed side walls of channels of a second series of channels of which the channels alternate with the channels of said first series to cause droplet 
     
     
        ejection from the channels of the side walls so flexed. 60. A high density multi-channel array, electrically pulsed droplet deposition apparatus, comprising a multiplicity of parallel droplet liquid channels organized in groups and with corresponding channels of repeated sequences of said multiplicity of channels assigned respectively to said groups, nozzles opening respectively into said channels, longitudinal side walls each serving to divide one channel from the next and each transversely displaceable at least in one sense, electrically actuable means .[.adapted.]. .Iadd.comprising piezoelectric material and electrode means for applying a field to the piezoelectric material, under the action of which field the piezoelectric material is deflected in sear mode for .Iaddend.in respective operating modes equal in number to the number of groups, upon selection of any channel in one of said groups .[.to effect.]..Iadd., effecting .Iaddend.transverse displacement in said at least one sense of one of the longitudinal walls of said selected channel so as to cause a change of pressure therein thereby to effect droplet 
     
     
        ejection from the nozzle opening into said selected channel. 61. Apparatus as claimed in claim 60, wherein each longitudinal side wall is transversely displaceable in opposite senses and said electrically actuable means are adapted in said respective operating modes, upon selection of any channel in one of said groups to effect transverse displacement in appropriate senses of both the longitudinal side walls of said selected channel so as to cause a change of pressure in the selected channel to effect droplet ejection from the nozzle opening into said 
     
     
        selected channel. .[.62.  Apparatus according to claim 36, wherein the nozzles opening into the channels of the first group of channels are offset in the direction of relative movement of said surface on which droplets are to be deposited, with respect to the nozzles opening into the channels of the second group of channels, by an amount commensurate with the time spacing between said first and second operating modes..]..Iadd.63. Apparatus according to claim 60, wherein the nozzles opening into the channels of the respective groups of channels are mutually offset in the direction of relative movement of said surface on which droplets are to be deposited, by an amount commensurate with the time spacing between said operating modes. .Iaddend..Iadd.64. A high density multi-channel array, electrically pulsed droplet deposition apparatus, comprising a multiplicity of parallel droplet liquid channels organised in groups and with corresponding channels of repeated sequences of said multiplicity of channels assigned respectively to said groups, nozzles opening respectively into said channels, longitudinal side walls each serving to divide one channel from the next and each transversely displaceable at least in one sense, electrically actuable means for, in respective operating modes equal in number to the number of groups, upon selection of any channel in one of said groups effecting transverse displacement in said at least one sense of one of the longitudinal walls of said selected channel so as to cause a change of pressure therein thereby to effect droplet ejection from the nozzle opening into said selected channel, wherein each channel communicates with a respective channel extension projecting transversely from the channel and providing a volume not bounded by the corresponding side wall. .Iaddend..Iadd.65. Apparatus as claimed in claim 64, wherein each longitudinal side wall is transversely displaceable in opposite senses and said electrically actuable means are adapted in said respective operating modes, upon selection of any channel in one of said groups to effect transverse displacement in appropriate senses of both the longitudinal side walls of said selected channel so as to cause a change of pressure in the selected channel to effect droplet ejection from the nozzle opening into said 
     
     
        selected channel. .Iaddend..Iadd.66.  A high density multi-channel array, electrically pulsed droplet deposition apparatus, comprising a multiplicity of parallel droplet liquid channels organised in groups and with corresponding channels of repeated sequences of said multiplicity of channels assigned respectively to said groups, nozzles opening respectively into said channels, longitudinal side walls each serving to divide one channel from the next and each transversely displaceable at least in one sense, electrically actuable means for, in respective operating modes equal in number to the number of groups, upon selection of any channel in one of said groups, effecting transverse displacement in said at least one sense of one of the longitudinal walls of said selected channel so as to cause a change of pressure therein thereby to effect droplet ejection from the nozzle opening into said selected channel, wherein nozzles communicating with the channels of the respective groups of channels are mutually offset, by an amount commensurate with the time 
     
     
        spacing between said operating modes. .Iaddend..Iadd.67.  Apparatus as claimed in claim 66, wherein each longitudinal side wall is transversely displaceable in opposite senses and said electrically actuable means are adapted in said respective operating modes, upon selection of any channel in one of said groups to effect transverse displacement in appropriate senses of both the longitudinal side walls of said selected channel so as to cause a change of pressure in the selected channel to effect droplet ejection from the nozzle opening into said selected channel. 
     
     
        .Iaddend..Iadd.68.  A high density multi-channel array, electrically pulsed droplet deposition apparatus, comprising a multiplicity of parallel droplet liquid channels, mutually spaced in an array direction normal to the length of the channels, each of said channels extending in a lengthwise direction and being separated from a like channel by a side wall which is transversely displaceable in opposite senses and which extends in the lengthwise direction of the channels and in a direction which is both normal to said lengthwise direction and normal to the array direction, said side wall being formed at least in part of piezoelectric material, respective nozzles opening into said channels for ejection therefrom of droplets of liquid, connection means for connecting said channels to a source of droplet deposition liquid and an electrode in each channel establishing electrical contact with piezoelectric material in both side walls abutting said channel such that application of a voltage to the electrode of a selected channel affords an electric field in piezoelectric material in both said side walls abutting said channel to effect transverse displacement of both said side walls generally parallel to said array direction to cause change of pressure in said selected channel to effect droplet ejection from the nozzle opening thereinto. 
     
     
        .Iaddend..Iadd.69.  Apparatus according to claim 68, wherein said channels are assigned alternately to a first and second group of said channels, said first and second groups of channels being enabled in respective time alternating first and second operating modes for the selection of channels 
     
     
        therein for droplet ejection. .Iaddend..Iadd.70.  Apparatus according to claim 68, wherein said piezoelectric material is displaceable under the action of the applied field in shear mode. .Iaddend.