GB2482873A - Droplet generator for dispensing multiple streams of uniform liquid droplets - Google Patents

Abstract

A droplet generator for dispensing multiple streams of uniform liquid droplets simultaneously has an elongate body 10, with a wall or walls 11 defining internally a tubular channel 12. The channel has an inlet 13, 14 for delivering a flow of liquid under pressure along the channel 12. At least one part of the or a wall 11 of the body 10 has a plurality of grooves 15 transverse to the longitudinal axis of the body 10 and a plurality of orifices (17, Fig.1B) of substantially uniform dimensions disposed along the wall 11, to enable uniform streams of liquid to be dispensed. Each orifice (17, Fig.1B) is disposed in alignment with a corresponding groove 15. One or more piezoelectric actuators 19 are mounted on the body 10 in vibration-transmitting relation to the body 10, and is/are actuatable to stimulate pressure fluctuations in the streams of liquid issuing from the orifices (17, Fig.1B) to cause the multiple streams of liquid to be broken into droplets of substantially uniform diameter.

Description

DROPLET GENERATOR

The invention relates to a droplet generator for dispensing multiple streams of uniform liquid droplets simultaneously, in particular to provide streams of substantially uniform droplets, particularly of refractory liquids that are either aggressive, sensitive to contamination or otherwise difficult to handle.

In many droplet generator applications, it is desirable to produce droplets having a narrow range of sizes and speeds. Example application s include the production of near-monodisperse particulate solids by in-flight solvent evaporation from droplets containing dissolved solids, in continuous ink-jet (CIJ) equipment, for the removal by droplet impact of solid or liquid contaminants upon the surface of substrates and for the delivery of pharmaceutical drugs in aerosol form by inhalation. Some of these applications require the generation of droplets of liquid suspensions, for example pigment inks.

Prior art droplet generators (for example EP-A-0943436) typically take the form of a liquid manifold having orifices on one or more sides, the liquid being ejected under pressure through the orifices.

In many further applications of droplet generators, it is desirable to form droplets of liquids that are chemically, electrochemically, or physically aggressive. Examples include acids and suspensions of inorganic materials.

The liquid manifold of prior art droplet generators typically employs a metallic orifice-bearing face, formed for example of electroformed nickel, since such forms allow orifices to be made having precise dimensions, and consequently allow droplets of substantially-uniform size to be produced. Such materials, however, are not robust for long-term use with liquids that are chemically, electrochemically, or physically aggressive.

JP Utility Model 7-7164 of 22/2/95 discloses a nozzle head ofan ink jet printerwhich has a notched part formed in a part of an outer circumferential wall of a circular tubular substrate body in the axial direction, with a plurality of orifices for jetting ink formed in a row through the notched part. The notched part provides a thinned wall section for the orifices. In use, ink fills the tubular substrate body. A vi brat ing actuator for vibrating the ink is fixed to the outer circumferential wall of the tubular substrate body on a part other than the notched part.

In some applications, where orifice diameter is required to be very small and the volume of liquid to be sprayed from the orifices is high, pressures as high as 4 x 106 to 10 x 106 Pa (40 to 100 bar) may be necessary to produce the required droplet size and rate of flow. Such pressure, whilst being containable in a tubular body of appropriate wall thickness my not be containable by thin-walled sections of tubular bodies such as that referred to above. In many applications, where the liquid is relatively viscous the orifices are required to be short in length to enable the required flow rates.

There is a need therefore for a device which will provide uniform droplets so that the variation in size is reduced and also such that other problems like those referred to above may be overcome.

According to the present invention therefore, there is provided a droplet generator for dispensing multiple streams of uniform liquid droplets simultaneously, the droplet generator comprising an elongate body, having a wall or walls defining internally of the body a tubular channel, and including an inlet to the channel, for delivering a flow of liquid under pressure along the channel in use, at least one part of the or a wall of the body including a plurality of grooves transverse to the longitudinal axis of the body, and a plurality of orifices of substantially uniform dimensions disposed along the wall, to enable uniform streams of liquid to be dispensed therethrough, each orifice being disposed in alignment with a corresponding groove; and one or more piezoelectric actuators mounted on the body in vibration-transmitting relation to the body, and actuatable to stimulate pressure fluctuations in the streams of liquid issuing from the orifices, in use, to cause the multiple streams of liquid to be broken into droplets of substantially uniform diameter.

Such a stimulated pressurised liquid droplet generator may be constructed of a broad range of materials, including polymers, glasses, speciality metals (such as Monel) or ceramics suitable for handling refractory liquids. A range of distribution of fixed diameter apertures or orifices may be provided depending on the desired use.

A device of this form provi des a structure which can withstand high pressures required in certain applications, particularly with relatively viscous liquids required to be sprayed in small droplets.

Additionally, the grooves may be designed to direct airflow inwardly towards the liquid jets, meeting at the liquid jets and thereafter flowing parallel to those jets. The purposes of such localised air-jets are (i) to keep the liquid jets from impinging on the sidewalls of the grooves, and (ii) in some applications, to prevent liquid droplets bouncing-back' from the substrate onto the head, thereby interfering with the liquid jet production.

The grooves also allow the use of a tube which is stronger than prior art devices, so allowing higher pressures and hence smaller, faster droplets. JP Utility Model 7-7164 describes circular patches, which produce thin wall sections extending as long axially as do their on-average thicker sections transverse to the axis whereas the groove form of the present invention serves to minimise the axial length of the thinned section, so allowing higher pressures.

By making the axial dimension of the thinned section as short as possible (and remembering that in the transverse direction the wall thickens), the present design also increases the frequency of the first and all higher modes of vibration of the thinned wall section. Since this is the very region responsible for perturbing the jets so that they break-up well, this attribute is important to allow the tube to be operated reliably at higher frequencies than possible in prior art.

The grooves may be formed by a cutting process or by an etching process in which material forming the wall of the elongate body is selectively removed over a given length of the body, but, alternatively, the webs which separate one groove from its neighbour may be built up using a deposition process in which the webs are constructed in plural layers. Other methods of forming the grooves or the webs are feasible.

In the preferred circular cross-sectioned form of the channel, a device of this type solves the problem of the nozzles being blocked by micro particulates which would inevitably be harboured in the corners of a non-continuously curved channel cross-section and more particularly in the joints of a device manufactured from various parts.

This construction lends itself to be formed from ceramic materials. An additional benefit that can be afforded by the use of such "lossy" materials is the provision of a broad-spectrum device with a flat resonance across a range of frequencies (rather than the typical CIJ head which is fine-tuned to a precise frequency). This allows the device to be used in different ways. When spraying ultra-pure water on semiconductors, it can be used as an initial general clean and then as a more specific high-spec cleaner (later in the process). The other main application is in spraying biological materials which could also benefit from broad-spectrum characteristics.

The construction also lends itself to be formed from materials such a quartz, whose absence of grain structure allows the fabrication of highly-perfect nozzles of very small and uniform size and thereby the reliable production of droplets of very small and uniform size. In this case, the value of the groove' construction above in allowing increased frequency of the modes of vibration of the thinned wall section is particularly marked, since this greatly eases practical use of such non-lossy materials.

A generator of this type exploits classical Rayleigh droplet break-up to produce droplets, but does not require the jets to break up uniformly within a narrow range of distances from the emitting nozzles (as in CIJ printers) and hence can be constructed of a wider range of materials chosen better to suit the needs of the application or to conform with established industry specific approved materials.

Furthermore, the construction (potentially monolithic) can be such as to minimise the number of different materials in contact with the liquid.

Such a droplet generator has the advantage that the operating point for a given generator can be variable within a wide range of pressures (restricted only by the material strength and geometry), stimulation frequencies and amplitudes. Since the break-up length is not required to be minimal, the stimulation amplitude can be smaller than in conventional CIJ systems and this permits the use of lossy exotic generator materials and non-resonant modes of operation. This opens the way to the mono-dispersed spraying of liquids that are beyond the scope of conventional CIJ-type heads, such as ultra-pure water or highly corrosive acids or alkali solutions.

Lossy materials imply low-Q, low amplitude, flat responses which implies broader operating frequency windows.

Furthermore, since the break-up distance is not required to be uniform across all the jets, the generator can have a greater variety of shapes and carry very large numbers of nozzles.

The variability in the operating point (break-up distance) means that a given generator can produce drops of selected diameters within a range determined by the orifice diameter, the liquid pressure and the stimulation frequency. The lower limit has the form of a cut-off that occurs at a certain frequency determined by the jet velocity and diameter. The upper limit, although theoretically large relative to the orifice diameter, depends on an indeterminate combination of factors such as liquid viscosity, liquid surface tension, stimulation amplitude, orifice quality and noise levels.

It has been found that if the substrate or workpiece is placed at a relatively large distance from the orifices, much greater than the break-up length, then the emergent droplets have settled into a spherical shape of fixed size and spacing independent of the details of the break-up. At this large throw distance, possible satellites have been removed either by an airflow or have coalesced with the main drops and hence the droplet stream at the working distance is highly monodisperse and uniform.

A preferred elongate form as described below in relation to Figure 1 can be made in very long lengths, provided only that a minimal level of stimulation is applied everywhere along its length and the liquid supply along the channel is adequate.

Such a tubular embodiment may be provided in parallel multiples to increase fluid throughput.

Examples of droplet generators according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1A shows a longitudinally sectioned view of a first droplet generator to reveal internal features; Figure 1 B shows a side view of the droplet generator; Figure 2A shows an isometric view of a second droplet generator; Figure 2B shows a further isometric view of the second droplet generator, but as a line drawing to show internal features; Figure 20 shows a plan view of the droplet generator of Figures 2A and 2B, again with internal features shown; Figure 3A shows an isometric view of a third droplet generator from the underside; Figure 3B shows a section through the third droplet generator; Figure 30 shows a partial sectional view of the third droplet generator; Figure 4 shows an isometric view of a fourth droplet generator; and Figure 5 shows a cross-section through a further modification of the type of device shown in Figures lAand lB.

A monolithic or one-piece design is shown in Figures 1A & 1 B. This example has a generally tubular form with a body 10 formed of a ceramic material such as zirconium, alumina or quartz having a one-piece wall 11 defining a tubular channel 12 of circular internal cross-section with enlarged end portions 13, 14 to facilitate connection of liquid supply and removal tubes (not shown). As best seen in Figure 1A, one side of the exterior of the channel 11 has a series of grooves 15 cut into its surface to a depth such as to provide corresponding thinner portions 16 of the wall 11 (at the bottom of the grooves) through which orifices 17 are formed. The grooves 15 are separated by narrow webs 18. This example is designed to work with small orifices (of around 10 microns) which necessitates wall sections which are very thin (100 microns or less) in order to reduce the nozzle length to around 100 microns as required for high pressure, high flow rate generation of droplets of relatively viscous liquids. Using small orifices 17 of relatively short length means relatively high pressures acting on the channel wall, and the webs 18 between the grooves 15 act like beams to provide support for the thin wall portions 16 between them. A plurality of piezoelectric actuators or drivers 19 are mounted on the side of the body opposite the orifices to vibrate the body in use and cause the break-up of liquid streams ejected from the orifices into droplets of desired size.

Figures 2A to 20 show a droplet generator in the form of a "thin" body plate 20 (a few millimetres thick) of arbitrary lateral size and formed of a polymeric or other non-metallic material such as zirconium, alumina or quartz with an internal tubular serpentine channel 22 that opens below onto a foil 21 with a matching array of orifices 27, the foil being provided with grooves (for clarity not shown) across its surface, but arranged as in the example of Figures 1A & lB and aligned with the orifices 27. The serpentine channel 22 is open at both ends to allow through flow as indicated by the arrows at each side. A set of piezo actuators or drivers 29 is bonded on the back of the plate 10 in positions corresponding to the pattern of the orifices 27 opening from the serpentine channel. The cross-section of the tubular channel 11 is circular or oval, but this is not shown in detail in Figures 2A to 20. The body plate need only be thick enough, say around 3 to 5 mm, so that pressure drop down channel does not significantly attenuate jet velocities from the orifices.

The thinness of the plate 20 and the proximity of the piezo drivers 29 ensures that sufficient stimulation reaches the orifices 27 below the piezo drivers 29. Stimulation need only be sufficient to ensure jet break up at distances from the orifices 27 which can be large (for example around 50 times the diameter of the nozzles) in comparison with those in droplet generators of the continuous ink jet type (CIJ) A further type of droplet generator is shown in Figure 3A to 30, the device having rotational symmetry as shown in the figures. The device shown in Figures 3A to 30 has a generally circular 30, formed in two opposed halves 30', 30" with an annular internal channel 32 of circular cross-section formed between the two halves, and a single piezo driver 39 centrally disposed on the upper (as seen in Figure 3B) half 30".

Fluid inlet and outlets 33, 34 are provided respectively on opposite lateral sides of the channel 32 to permit the flow of pressurised fluid into, through, and out of the annular channel 32. The bottom half 30' (the top half as seen in Figure 3A) of the body half 30' carries orifices 37 disposed in radial grooves 35 with separating sections 38 which serve to reinforce the wall of the body 30 each side of each groove, where the wall is thinned by the presence of the groove 35. Figure 30 shows an enlarged sectional view of a portion of the body half 30' in the region of an orifice 37, showing the thinned wall portions 36.

In a modified version, the inlet 33 and outlet 34 may be disposed axially and extend into/out from the channel 32 on the side opposite the groves 35 and orifices 37.

Figure 4 shows a further form of construction as a flexible plate 40 with parallel channels 42 closed by an apertured, grooved foil which has been removed for clarity.

Since the degree of stimulation needed is quite small, a flexible polymer may be used as long as it is thin enough -say around 150 to 200 microns. The piezoelectric actuators closely match the orifice distribution and are narrow and (whilst not shown) are mounted on the opposite side from the orifices.

Figure 5 shows, in cross-section, a further modification of the type of device shown in Figures 1A and 1 B, where the elongate body 50 is surrounded by a shroud 60 which has an elongate opening 61 aligned with the row of orifices 57 and having an air or gas inlet 62. In use a flow of air or gas is provided between the shroud and the elongate body 50 and directed inwardly towards the liquid streams along the grooves 65, meeting at the liquid streams and thereafter flowing parallel to those streams.

The localised air-jets keep the liquid streams from impinging on the sidewalls of the grooves, and in some applications may prevent liquid droplets bouncing-back' from the substrate onto the head and thus prevent interference with the liquid stream production.

Claims (7)

1. CLAIMS1. A droplet generator for dispensing multiple streams of uniform liquid droplets simultaneously, the droplet generator comprising an elongate body, having a wall or walls defining internally of the body a tubular channel, and including an inlet to the channel, for delivering a flow of liquid under pressure along the channel in use, at least one part of the or a wall of the body including a plurality of grooves transverse to the longitudinal axis of the body, and a plurality of orifices of substantially uniform dimensions disposed along the wall, to enable uniform streams of liquid to be dispensed therethrough, each orifice being disposed in alignment with a corresponding groove; and one or more piezoelectric actuators mounted on the body in vibration-transmitting relation to the body, and actuatable to stimulate pressure fluctuations in the streams of liquid issuing from the orifices, in use, to cause the multiple streams of liquid to be broken into droplets of substantially uniform diameter.
2. 2. A droplet generator according to claim 1, wherein the channel has a continuously curved cross-section.
3. 3. A droplet generator according to claim 2, wherein the channel has circular cross-section.
4. 4. A droplet generator according to any of claims 1 to 3, wherein the elongate body is annular and the grooves are radially disposed, centred on the centre of radius of the annular body.
5. 5. A droplet generator according to any of claims 1 to 4, which includes a plurality of elongate bodies disposed parallel to one another, each including a wall or walls defining internally of the body a tubular channel, and including an inlet to the channel, for delivering a flow of liquid under pressure along the channel in use, at least one part of the or a wall of the body including a plurality of grooves transverse to the longitudinal axis of the body, and a plurality of orifices of substantially uniform dimensions disposed along the wall, to enable uniform streams of liquid to be dispensed therethrough, each orifice being disposed in alignment with a corresponding groove; and one or more piezoelectric actuators mounted on each body in vibration-transmitting relation to the body, and actuatable to stimulate pressure fluctuations in the streams of liquid issuing from the orifices, in use.
6. 6. A droplet generator according to claim 5, wherein the plural elongate bodies are disposed in a plate and formed integrally with one another within the plate.
7. 7. A droplet generator according to claim 1, further including a shroud surrounding the elongate body over at least the portion of the length of the body which contains the grooves and orifices, the shroud including an elongate opening aligned with the plural orifices and having an air or gas inlet disposed on the opposite side of the elongate body, whereby, in use, a flow of air or gas may be provided between the shroud and the elongate body and directed inwardly towards the liquid streams, meeting at the liquid streams and thereafter flowing parallel to those streams.