DROPLET EJECTORS

Abstract: A nozzleless droplet ejector for ejecting droplets from a free surface (16) of a pool of liquid (17), such as a pool of ink, comprises a selectively-energizeable droplet-emission controller (21) for generating a freely-propagating capillary wave on the surface of the pool to provide on/off timing control and/or ejection trajectory angle control for the ejector. The controller comprises a conductor (22) and a counter-electrode (23). The conductor is immersed in the pool, whereby a capillary surface wave is generated when a periodic voltage is applied across the conductor and the counter-electrode. In one embodiment, a ultrasonic pressure wave is focused on the free surface of the pool, and the capillary wave supplied by the controller coherently interacts with that pressure wave to provide the desired control. Separate controllers may be provided for independently controlling the ejectors of multiple ejector arrays.

Description: This invention relates to nozzleless droplet ejectors and, more particularly, to emission controllers (e. g., on/off switches and directional controllers) for such ejectors. Droplet ejectors having emission controllers embodying this invention are useful for liquid ink printing and similar applications. Ink jet printing has the inherent advantage of being a plain paper compatible, direct-marking technology. "Continuous stream" and "drop-on-demand" ink jet print heads have been developed to exploit that advantage. Unfortunately, however, the nozzles which are used in conventional ink jet print heads are expensive to manufacture and are a significant source of maintenance problems. Others have proposed nozzleless droplet ejectors for liquid ink printing. For example, US-A-4,308, 547 describes a print head in which a piezoelectric transducer having a hemispherically shaped focusing lens is submerged in a reservoir of ink to generate a spherically focused ultrasonic pressure wave for exciting the ink near the surface of the reservoir sufficiently to eject individual droplets of ink. DE-A-3 211 345 discloses a printhead having a planar piezoëlectric transducer for parametrically exciting a pool of ink to produce a capillary wave on the ink surface. JP-A-6 164 456 uses a set of interdigitated electrodes for imposing a spatially periodic field on the ink to create a surface wave. In both cases the desired droplets are pulled from the crests of the capillary waves by an external field. These mechanisms of droplet production are different from those of the present invention. As a general rule, liquid ink printing requires substantial control over the timing of the drop ejection process. The transducers of nozzleless print heads of the above-described type may be driven by amplitude-modulated r.f. signals to provide the necessary timing control, but the electronics needed to modulate a r.f. signal are expensive. Thus the preferred approach is to provide timing controllers which operate independently of the transducers. Under those circumstances, the transducer or transducers may be driven by a relatively-inexpensive r.f. signal generator to excite the ink to a sub-threshold, incipient energy level for droplet emission, thereby enabling the timing controller or controllers to destabilize the excited ink selectively so that individual droplets are ejected on command. Some liquid ink printing processes, such as matrix printing, are easier and less costly to implement if there also is provision for directionally steering the ink droplets. In recognition of that, some transducers are configured to generate focused acoustic waves having a directionally-controlled asymmetry. In accordance with the present invention, a nozzleless droplet ejector for ejecting droplets from a free surface of a pool of liquid, such as a pool of ink, comprises a selectively energizable droplet emission controller for generating a freely propagating capillary wave on the surface of the pool to provide on/off timing control and/or ejection trajectory angle control for the ejector. The controller comprises a conductor and a counter electrode which are immersed in the reservoir, whereby a capillary surface wave is generated when a periodic voltage is applied across the conductor and the counter electrode. In one embodiment, a focused ultrasonic pressure wave or the like periodically perturbs the pressure acting on the free surface of the pool, and the capillary wave supplied by the controller coherently interacts which that pressure wave to provide the desired control. Separate controllers may be provided for independently controlling the ejectors of multiple ejector arrays. The functionality of these emission controllers is dependent on the geometry of their conductors, so a few exemplary geometries are disclosed with the understanding that there are others which may be used. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partially sectioned and fragmentary, schematic elevational view of a nozzleless liquid droplet ejector array having emission controllers constructed in accordance with the present invention; Figure 2 is an enlarged simplified plan view of one of the capillary wave control switches shown in Fig. 1, and Figure 3 is an enlarged simplified plan view of a capillary controller which is similar to the switch shown in Fig. 2, except that it has a segmented conductor to provide angular trajectory control in addition to on/off control. Turning now to the drawings, and at this point especially to Fig. 1, there is an array of liquid droplet ejectors 11 a and 11 b comprising a plurality of acoustic transducers 12 a and 12 b which are submerged in a liquid-filled reservoir 13. The transducers 12 a and 12 b are laterally displaced from each other and are driven by an r.f. power supply (not shown) to launch ultrasonic pressure waves 14 a and 14 b into the reservoir 13, so that the pressure waves come to generally-circular foci on laterally-offset centers 15 a and 15 b , respectively, at or near the surface 16 (i.e., the liquid/air interface) of the reservoir 13. Known transducers may be employed periodically to perturb the pressure acting on the free surface 16 of the reservoir or pool 13, so the transducers 12 a and 12 b are illustrated schematically. Indeed, there are mechanical, electrical, thermal, pnuematic and other alternatives to the transducers 12 a and 12 b which may be employed to provide a focused (e.g., circularly-focused or linearly-focused) periodic pressure perturbance, on the free surface 16 of the reservoir 13. Furthermore, while only two ejectors 11 a and 11 b are shown, it will be understood that the number of transducers may be increased to form larger arrays. The ejector packing density is limited primarily by the transducer center-to-transducer center spacing that is required to prevent objectionable levels of "crosstalk" between adjacent ejectors, such as between the ejectors 11 a and 11 b . In a printer, of course, the reservoir 13 is filled with liquid ink 17. Moreover, a suitable record medium 18, such as plain paper, is located above the reservoir 13, with just a narrow air gap 19 separating it from the ink/air interface or surface 16. Typically, the ejectors 11 a and 11 b are assembled in a linear array, so the record medium 18 is advanced in an orthogonal cross-line direction (into or out of the plane of Fig. 1) relative to the ejectors 11 a and 11 b while a two-dimensional image is being printed. As will be appreciated, the individual picture elements or "pixels" of such an image are determined by (1) the time dependent on/off switching of the individual ejectors, such as the ejectors 11 a and 11 b , and (2) in some cases, by the time dependent steering of the individual droplets of ink. In accordance with the present invention, relatively inexpensive and easily fabricated capillary wave control devices 21 a and 21 b are provided for controlling the on/off timing of the ejectors 11 a and 11 b , respectively, and/or for steering the droplets of ink emitted thereby. The control devices 21 a and 21 b comprise electrical conductors 22 a and 22 b and counter-electrodes 23 a and 23 b , respectively,which are immersed in the liquid 17. The conductors 22 a and 22 b are located near (for example, within about 10 mm of) the focal centers 15 a and 15b of the pressure waves 14 a and 14 b , respectively. The counter-electrodes 23 a and 23 b should be nearby and preferably are concentric with the electrodes 22 a and 22 b , respectively. Typically, the counter-electrodes 23 a and 23 b are returned to a suitable reference potential (hereinafter, "ground potential"). Furthermore, a switched power supply 25 (Fig.2), which is also referenced to the ground potential, has electrically independent outputs coupled to the conductors 22 a and 22 b for applying appropriately and independently timed voltage pulses thereto. Alternatively, the controllers 21 a and 22 b could be driven by an a.c. power supply having appropriate control circuitry. Electric field gradients associated with the applied potential between the conductors 22 a and 22 b and the counter-electrodes 23 a and 23 b exert a dielectric body force on the liquid 17. This results in a disturbance at the liquid surface 16 which subsequently propagates as a free capillary wave on the surface 16. Generation of capillary waves is accomplished with moderately high voltage (e.g., 300 volts or so) pulses of brief duration (e.g., on the order of 500 µsecs) being periodically applied across the conductors 22 a and 22 b and the counter-electrodes 23 a and 23 b . The voltage and time limits, if any, of this wave generation process have not been determined, so it is noted in the interest of completeness that the foregoing examples are based on data from experiments conducted in water. However, the experimental data indicate that the emission control is most effective if the conductors 22 a and 22 b are located just below the free surface 16 of the liquid 17. For example, as shown, the conductors 22 a and 22 b may be supported on an electrical insulator 26, such as a ′Mylar′ (reg. trade mark) sheet, so that they are covered by a thin film of liquid 17. A sufficiently thin sheet 26 will allow essentially unimpeded passage of the pressure waves 14 a and 14 b . As will be understood, the capillary waves propagate radially with respect to the conductors 22 a and 22 b at the capillary surface wave velocity, ν, in the liquid 17, and they are damped as a function of time because of the viscosity of the liquid 17. Their wavelength, λ, is dependent on the dominant Fourier transform component(s) of the voltage pulses applied to the conductors and is given to a first approximation by λ ≈ ν/Δt, where Δt equals the width of the pulses applied to the conductors 22 a and 22 b . The damping of the capillary waves is an important consideration for determining the maximum permissible radial displacement of the conductors 22 a and 22 b from the pressure wave focal centers 15 a and 15 b , respectively. The radial propagation of the capillary waves and the pulse width dependency of their wavelengths, on the other hand, are relevant to optimizing the configuration of the conductors 22 a and 22 b and to selecting the phase and the width of the pulses applied thereto for the specific emission control tasks which the control devices 21 a and 21 b are intended to perform. More particularly, as best shown in Fig. 2, the conductor 22 a and its associated counter-electrode 23 a have constant radius, ring-like configurations and are generally circularly symmetric with respect to the focused pressure wave 14 a (i.e., concentric with its focal center 15 a ). Thus, a capillary wave launched by them converges, as indicated by the arrows, to a symmetrical focus at approximately the focal center 15 a of the pressure wave 14 a , thereby enabling the controller 21 a to provide axial on/off switching control for the ejector 11 a (Fig.1). The relative phase relationship of the focused capillary and pressure waves determines whether they interact constructively (additively) or destructively (subtractively). For example, the controller 21 a may be employed to "turn on" the ejector 11 a if the amplitude of the pressure wave 14 a is selected to excite the liquid 17 upon which it is focused (i.e., the liquid within the waist of the pressure wave 14 a ) to be near but below the threshold of incipient droplet formation. In this case, the ejector 11 a would be operated in a "normally-off" mode. While the circular symmetry of the conductor 22 a is well suited to the switching function of the controller 21 a , other symmetrical geometries could be employed, including equilateral polygon-shaped conductors. The symmetrical focus of the capillary wave is the key to providing axial on/off control for the ejector 11 a . Referring to Fig. 3, there is another controller 31 which is constructed in accordance with this invention to provide on/off switching and angular trajectory control for a nozzleless droplet ejector, such as the representative ejector 11a (Fig. 1). As will be seen, the controller 31 is similar to the controller 21 a (Fig. 2), except that its ring-like conductor 32 comprises a plurality of electrically independent segments 33 and 34 which are selectively addressable by a switched power supply 35. When the power supply simultaneously applies equal amplitude voltage pulses to all of the conductor segments 33 and 34, the capillary waves launched by them converge to a generally symmetrical focus at or near the focal center 15 a of the pressure wave 14 a (Fig. 1), thereby causing the controller 31 to perform essentially the same axial on/off switching function as the controller 21 a . When, however, the conductor segments 33 and 34 are differentially driven, such as if voltage pulses are applied to one of them but not the other, the capillary wave or waves come to an asymmetrical focus, thereby altering the angular trajectory of any droplets which are then being emitted by the ejector 11 a . The phase of the asymmetrically focused capillary wave may be selected to switch the ejector 11 a on, or the on/off control for the ejector 11 a may be provided by means not shown. Dividing the conductor 32 into two diametrically opposed, independently-addressable, segments 33 and 34, such as shown, allows the angular trajectory of the ejected droplets to be controlled along an axis parallel to the center line of the segments 33 and 34 over a range on the order of ±30° (at a droplet diameter of about 100µm) with respect to longitudinal axis of the ejector or, in other words, with respect to an axis normal to the plane of the record medium 18. Smaller diameter droplets are capable of being steered over even wider angles. If multiaxial trajectory control is desired, the conductor 32 may be divided into a larger number of individually-addressable segments. Furthermore, it will be understood that the conductor 32 may be composed of individually-addressable, polygonally-arranged segments, without materially altering its performance. In view of the foregoing, it will be seen that the present invention provides relatively reliable and inexpensive ejection controllers for nozzleless droplet ejectors of various types. These controllers may be design optimized to perform a variety of different control functions. For example, they can be employed not only as on/off switches and/or angular trajectory controllers as described herein, but also as droplet ejection velocity controllers. Thus, while the controllers may be used to substantial advantage in nozzleless liquid ink printers of the above-described type, it will be understood that the broader aspects of the invention are not limited to printing,

Claims: A nozzleless droplet ejector including a reservoir (13) for a pool (17) of liquid with a free surface (16), and means (12) for launching a pressure wave into the pool such that the pressure wave comes to a focus approximately at the free surface, including a droplet-emission controller (21) for generating a capillary wave on the liquid surface, the ejector including: an electrical conductor (22) and an adjacent counter-electrode (23), the conductor being adapted to be immersed at least partially in the liquid, near the surface level, and proximate to the focus of a pressure wave, and means (25) coupled across the conductor and the counter-electrode for applying a periodic voltage there across on command, to cause a freely-propagating surface wave to radiate from the conductor, which wave is intended to interact with said pressure wave to control at least one characteristic of the emitted droplets. The ejector of Claim 1, wherein the pressure wave excites the liquid upon which it is focused to an energy level which is offset from a threshold energy level for destabilizing the liquid, and wherein the capillary wave causes the energy level of the excited liquid to cross over the threshold level, whereby the droplet-emission controller provides on/off control for the ejector. The elector of Claim 1 or 2, wherein the conductor (22) is symmetrical with respect to the focus of the pressure wave and is electrically continuous, whereby the droplet-emission controller provides axial on/off timing control for the ejector. The ejector of Claim 1 or 2, wherein the conductor (22) is asymmetrical with respect to the focus of the pressure wave, whereby the controller provides angular trajectory control fort the emitted droplets. The ejector of Claim 4, wherein the conductor has two or more electrically-independent segments (33, 34), and wherein the means for applying the periodic voltage include means for selectively addressing the segments, whereby the voltage is able to be applied to the segments selectively. The ejector of Claim 1 or 2, wherein the conductor has two or more electrically-independent segments (33, 34) which are symmetrical with respect to the focus of the pressure wave, whereby an axial ejection trajectory is provided when the pulses are applied simultaneously to both or all of the segments. The ejector of Claim 6, wherein the conductor is circularly symmetrical with respect to the focus of the pressure wave. A printer having a nozzleless droplet ejector as claimed in any preceding claim.

Inventors: ELROD, SCOTT ALAN
Applicants: XEROX CORPORATION
Application year: 1987
Main classification: B41J2
IPC codes: B41J2