CA2176573A1 - Liquid spray apparatus and method - Google Patents

Abstract

A method of and apparatus for atomising a liquid are disclosed. in which a liquid is caused to pass through tapered perforations (50) in a vibrating membrane (5) in the direction from that side of the membrane (5) at which the perforations (50) have a smaller cross-sectional area to that side of the membrane (5) at which the perforations (50) have a larger cross-sectional area.

Description

~0 95/15822 : ~ 2 1 7 6 5 7 3 P~ ,., 1.'^7^~2 LIOUID SPRAY APPARATUS AND METHOD
This inYention relates to apparatus and methods for the production of sprays of liquid or of liquid emulsions or 5 suspensions (hereinafter called 'liquids') by means of an actuator .
It is known to produce fine droplet sprays by the action of high frequency mechanical oscillations upon a liquid at its sur~ace with ambient air. Prior art of possible relevance includes: EP-A-O 432 992, GB-A-2 263 076, EP-A-O 516 565, US--A-3 738 574, EP--A--O 480 615, US--A-4 533 082 &
US-A-4 605 167.
In some instances (e.g. US-A-3 738 574) the liguid is introduced as a thin film formed on a plate excited in bending oscillation by the transmission of ultrasonic vibrations from a remote piezoelectric trAn~durPr through a solid coupling medium ~L l~ Lu~
In some instances (e.g. US-A-4 533 082) the r-^hAnicAl oscillations are propagated as sonic or ultrasonic vibrational waves through the liquid towards a perforate membrane or plate (hereinafter referred to as a membrane) 25 that otherwise retains the liquid. The action of the vibrational waves in the liquid causes the liquid to be ejected as droplets through the perforations of the membrane. In these cases, it has been found advantageous to make the pores decrease in size towards the 'front' face 30 (herein defined as that face from which liquid droplets emerge) from the 'rear' face (herein defined as the face opposite the 'front' face).
In other instances (e.g. EP-A-0 516 565) which may be 35 regarded as an amalgamation of the two cases cited above, the r- An;cll oscillations pass through a thin layer of the liquid towards a perforate membrane that otherwise 2 1 7 6 5 7 3 r~, ~ . . ~ . 92 retains the liguid. In EP-A-0 516 565 there is no tea~ h;nq of any advantages or disadvantages for particular geometrical forms of perforation.
In yet other instances (e.g. GB-A-2 263 076, US-A-4 605 167 and EP-A-0 432 992) the source of mechanical oscillations is mechanically coupled to a perforate membrane that otherwise retains the liquid. The action of the oscillations causes the liquid to be ejected as droplets through the perforations of the membrane. In these cases, it again has been found advantageous to make the perforations decrease in size from the 'rear' face towards the 'front', droplet-emitting, face of the membrane.
The devices above can be classified into two types:
Spray devices of the first general type, for example as disclosed in US-A-3 378 574 and EP-A-0 516 565, transmit the vibration through the liquid to the liquid surface from which the spray is produced, but they describe no geometrical features at that surface which influence droplet size. They either have no perforate membrane to retain the liquid in the absence of oscillation (as in US-A-3 378 574) or they do possess a perforate membrane, but the perforations do not influence droplet size (as in EP-A-0 516 565, eg column 6, line 21).
Spray devices of the second general type, for example as disclosed in US-A-4 605 167, US-A-4 533 082, EP-A-0 432 992 and GB-A-2 263 076, have a perforate membrane ~o~nAin~ or defining the liquid surface at which droplets are produced and the r ` c~ne perforations do have an influence upon droplet size.
In these cases the present inventors have observed that a substantially cylindrical fluid jet emerges from the 'small-orifice' opening in the front face of .. _ . _ . _ . _ . .. .. _ . .. . _ _ ..

~vo95115822 2 ~ 76 5 73 r~ r~92 the membrane and that this jet oscillates towards and away from the membrane once per cycle of vibration.
When the excitation is sufficiently strong the end portion of the jet breaks off to form a free droplet.
This behaviour is represented in Figure l. In both cases, the droplet tl;~ r typically lies in the range l . 5 to 2 times the diameter of the small-orifice opening in the 'front' face of the Dembrane.
This relationship is also well known in ink jet printing, and has been found in many studies of the instability of liquid ~ets. The benefit to spray production of having orifices that reduce in sise towards the 'front' face is common to all these devices and is also known from ink jet technology.
See, for example, US-A-3 683 212.
The f irst type of device is relatively inef f icient in use of electrical input energy to its (piezoelectric) vibration actuator. For example a practical device of the type described in US-A-3 378 574 may atomise 2 . 5 microlitres of water for l ~oule of input energy. The i ov~ I_ of EP-A-0 516 565 is claimed to allow about 10 microlitreS to be so atomised with 1 joule, but limits liquid feed to capillary action requiring a membrane carefully separated from the actuator and a relatively complex construction.
Neither provides an apparatus in which the membrane perf orations have a substantial inf luence on droplet size .
Further, in delivery of suspension-drugs and in other applications the constraint of EP-A-0 516 565 to capillary feed and the absence of function of the perforations to define or to influence the droplet size can be disadvantageous. It is generally desirable to be free to select from a wide variety of liquid feed methodS to achieve the most appropriate method for the application.
For example, for sprays of pharmaceuticals it is desirable to provide a metered dose of liquid to the atomiser and to avoid 'hang-up' i.e. residual drug liquid left on the _ _ _ _ _ _ _ _ _ _ _ _ _ . ~ 21 76573 Wo 95115822 ` ` r~ g2 atomiser that could aid contamination of subsequent dose deliveries. For other, larger, suspensates, for example antiperspirant suspensions, the limited range of capillary-gaps could lead to blockage of the capillary feed. It is 5 also helpful for the droplet size to be AP~Prmi nPd or at least influenced by physical features of the apparatus, 50 that by maintaining manufacturing quality of the apparatus, repeatability of droplet size can be assisted.
lO Devices of the second type with perforations narrowing in the direction in which droplets are ejected generally have larger ratio of droplet size larger than orif ice exit diameter. This makes it difficult for such devices to atomise suspensions into droplets unless the solids 15 particle size is markedly smaller than the desired droplet diameter .
Secondly, devices of the second type are also poorly adapted to creation of sprays with very small droplet size.
20 For example, it i5 desirable to create sprays of suspensions or of solutions of pharmaceutical drugs in a form suitable for inhalation by patients. Typically, for p~ ry delivery of asthmatic drugs, sprays with mean droplet size in the region of 6~m are desirable to allow 25 'targeting' of the drug delivery to the optimum region within the p~ ry tract. With devices of the second type this may require perforations with exit diameter in the region of 3~um to 4~1m. Membranes of such small perforation size are difficult and expensive to manufacture 30 and may not have good repeatability of perforation size, droplet diameter and therefore of such 'targeting'. In addition, such suspension drug formulations are often most readily produced with mean solids size around 2~m. With such small orifices and with such solids particle sizes, 35 blockage or poor delivery can occur.
Thirdly, even for large droplet sizes, the flow of solids ~NO 9SIIS822 2 1 7 6 5 7 3 , ~ 92 carried in liquid suspension into a narrowing perforation can induce blocking, particularly when the solids size is comparable with the size of the channel. As one example the relatively large diameter of the perforation at the 5 rear f ace of the membrane admits particles too large to be able to pass through the relatively small perforation diameter at the front face. As a second example, the narrowing of the perforation may and in general will bring two or more solid particles into contact both with each 10 other and with the sidewalls of the perforation. These may then be unable to continue forward motion and so induce blocking .
Objects of the present invention include the provision of lS a form of spray device that is of low cost, of simple construction or which is capable of operation with a wide range of liquids, liquid suspensions and liquid feed means.
According to a f irst aspect of the present invention there 20 is provided a liquid droplet spray device comprising:
a perforate membrane;
an actuator, for vibrating the membrane; and means for supplying liquid to a surface of the membrane, 25 characterised in that perforations in the membrane have a larger cross-sectional area at that face of the membrane away from which liquid droplets emerge than at the opposite face of the membrane. Throughout this specification, the term 'membrane' includes the term 'plate'.
The actuator may be a piezoelectric actuator adapted to operate in the bending mode. Preferably the thickness of that actuator is substantially smaller than at least one - other dimension.
Preferably, means are provided to create a pressure di~ference such that the pressure exerte~ by the ambient -~ 2~`76~73 WO 95/15822 ~ 92 O

gas either directly or indirectly on the droplet . ye ---surface of the membrzne equals or exceeds the pressure of liquid contacting the opposite membrane surface, but which pressure difference is not substantially greater than that 5 ~ CaULa at which gas passes through the perforations of the membrane into said liquid. The L.LesauLa exerted by said ambient gas may be indirectly exerted, for example, when it acts on a liquid f ilm that itself is rormed upon that face of the membrane. The liquid supply means or the 10 effect of operation of the device itself to expel droplets of liquid from a closed reservoir or some other means may be used to create this ~L~saure difference.
Pref erably, the device includes a E~L e:S::~UL a bias means 15 providing a lower pressure in the liquid OPPO5; n~ the passage of the liquid through the perforations.
Advantageously, the perforations, on that face of the membrane away from which liquid droplets emerge, are not 20 touching.
The means for supplying liquid to a surface of the membrane preferably comprises a capillary feed -- ' -n;~ or a bubble-generator feed ~ni~
The device may include both normally tapered and reverse tapered perforations. The normally tapered perforations are the preferably disposed around the outside of the reverse tapered perforations. The means for supplying 30 liquid to a surface of the membrane may be adapted to supply said liquid to the face of said membrane away fro~
which liquid droplets emerge.
According to a further aspect of the invention, there is 35 provided a method of atomising a liquid in which a liquid i8 caused to pass through tapered perf orations in a vibrating membrzne in the direction from that side of the -~ ~ ~ 21 76573 ~0 95/15822 1 ~ " ~ I A,692 membrane at which the perforations have a smaller cross-sectional area to that side of the membrane at which the perforations have a larger cross-sectional area.
S It is believed by the inventors that apparatus according to the present invention operates by means of exciting capillary waves in the liquid to be atomised. Their understanding of such capillary-wave atomisation is given below .
Hereinafter, in the text and claims, perforations which have larger area at the rear face than at the front, droplet-emergent, face will be referred to as 'normally tapered' and perforations which have smaller area at the lS rear face than at the front face will be referred to as 'reverse tapered~. We COLL-7~ 1ing1Y define ' tapered' and 'normally-tapered' membranes.
The actuator, its mounting and the electronic drive circuit 20 for operating the actuator may, for example, take any of the prior art forms disclosed in WO-A-93 l09l0, EP-A-0 432 992, US-A-4 533 082, US-A-4 605 167 or other suitable forms that may be convenient. It is found generally desirable for the actuator and drive electronics to act cooperatively 25 to produce such resonant vibrational excitation.
One advantage of this arrangement is that simple and low cost apparatus may be used for production of a droplet spray of liquid suspensions wherein the ratio of mean 30 droplet size to mean sllcppn~ate particle size can be reduced over prior art apparatus.
.

A second advantage of this arrangement is that liquid and liquid suspension sprays of small droplet diameter suitable 35 for pulmonary inhalation can be produced, using membranes that are easier to manuf acture and which have reduced 1 ikPl ih-~od in use of blockage of the perforations.

W095/15822 ' ' ' ` 2 1 76573 P~_l,~,.,. ~.'. ~2 A third advantaye of this arrangement is that relatively low-velocity li~uid sprays suitable for uniform coating of surfaces can be produced.
5 Preferred embodiments of the invention will now be described by way of example only and with reference to the A~ nying drawings, in which:
Figure 1 is a schematic section of prior art apparatus showing, in sequence, successive stages of the ejection of a liquid droplet from perforations which are smaller in area at the front of the membrane (from which droplets emerge) than at the rear of the membrane;
Figure 2 shows, in section, a preferred droplet dispensation apparatus;
Figure 3 illustrates, in section, preferred forms of perforate membrane for the apparatus of figure 2;
Figure 4 is a plan and sectional view of a preferred ~mho~ t of an atoTni ~:; r~-J head;
Figure 5 shows schematic sections of alternative fluid E~ es~-lL ~ control devices that can be used with an atomising head to form droplet dispensation devices according to the invention;
Figures 6 show methods of droplet generation as understood by the inventors;
Figure 7 is a schematic section of a second droplet dispensation apparatus; and Figure 8 illustrates, in section, an alternative membrane structure (for the apparatus of figure 7);
and Figure 9 schematically illustrates in section, droplet ejection from both 'normally' tapered and 'reverse' tapered perforations.
Figure 1 shows a membrane 61 having 'normally' tapered perforations and in vibratory motion shown by arrow 58 (in ~O 9S/15822 . 2 1 7 6 5 7 3 PCT/G1194/02692 a direction substantially perpendicular to the plane of the membrane) against a liquid body 2 contacting its rear face.
Figures la to lc show, in sequence during one cycle of vibratory motion, the understood evolution of the liquid 5 meniscus 62 to create a æubstantially cylindrical jet of fluid 63 from the tapered perforations and the subsequent formation of a free droplet 64.
Figure 2 shows a droplet dispensing apparatus 1 comprising 10 an enclosure 3 directly feeding liquid 2 to the rear face 52 of a perforate membrane 5 and a vibration means or actuator 7, shown by way of example as an annular electroacoustic disc and substrate and operable by an electronic circuit 8. The circuit 8 derives electrical 15 power from a power supply 9 to vibrate the perforate membrane 5 substantially perpendicular to the plane of the membrane, so producing droplets of liquid emerging away from the front face 51 of the perforate membrane.
Perf orate membrane 5 and actuator 7 in combination are 20 hereinafter referred to as aerosol head 40.
The aerosol head 40 is held captured in a manner that does not unduly restrict its vibratory motion, for example by a grooved annular mounting formed of a soft silicone rubber 25 (not shown). Liquid storage and delivery to rear face 52 are effected, for example, by an enclosure 3 as shown in Figure 2.
Figure 3a shows cross-sectional detail of a first example 30 perforate membrane 5, which is opQrable to vibrate substantially in the direction of arrow 58 and which is suitable for use with droplet tli~:pon-:in~ apparatus 1 to produce ~ine aerosol sprays. In one ~mho~ t the membrane 5 comprises a circular layer of polymer which 35 contains a plurality of tapered conical perforations 50.
Each perforation 50 has op~ninqs 53 in the front exit face and openings 54 in the rear entry face, which perforations _ _ _ _ _ _ _ . _ _ . .. , . _ _ . _ . . _ _ _ _ _ _ _ _ _ _ _ _ WO 95/15822 2 ~ 7 6 5 7 3 r~ fi92 are laid out in a square lattice. Such perforations may be introduced into polymer membranes by, for example, laser-drilling with an excimer laser.
5 Figure 3b shows cross-sectional detail of a second example perforate membrane 205 according to the invention, which membrane is operable to vibrate substantially and suitable for use with droplet dispensing apparatus 1 in the direction of arrow 58. The membrane is formed as a 10 circular disc of diameter 8mm from electroformed nickel, and is manufactured, for example, by Stork Veco of Eerbeek, The Netherlands. Its thickness is 70 microns and is formed with a plurality of perforations shown at 2050 which, at 'front' face 2051, are of diameter shown at "a" of 120 15 microns and at 'rear' face 2052 are of diameter shown at "b" of 30 microns. The perforations are laid out in an equilateral triangular lattice of pitch 170~Lm. The profile of the perforations varies smoothly between the front and rear face diameters through the membrane thickness with 20 substantially flat 'land' regions (shown at "c"~ of smallest dimension 50~m in front face 2051.
Membranes with similar geometrical forms to those described with reference to Figures 3a,3b, fabricated in alternative 25 materials such as glass or silicon, may also be used.
Figure 4 shows a plan and a sectional view through one appropriate form of the aerosol head 40. This aerosol head consists of an electroacoustical disc ?0 comprising an 30 annulus 71 of nickel-iron alloy known as 'Invar' to which a piezoelectric ceramic annulus 72 and the circular perforate membrane 5 are bonded. The perforate membrane is as described with ref erence to Figure 3b . The nickel-iron annulus has outside diameter 2amm, ~hirl~n~ 0.2mm and 35 contains a central cO~ce;~Lric hole 73 of diameter 4.5mm.
The piezoelectric ceramic is of type P51 from Hoechst CeramTec of ~auf, Germany and has outsidQ diameter 16mm, , WO gS/15822 ~ ; ~ ' 2 ~ 73 P~~ , L'~692 internal diameter lOmm and thickness 0 . 25mm. The upper surface 74 of the ceramic has two ele~_L-~des: a drive electrode 75 and an optional sense electrode 76. The sense electrode 76 consists of a 1.5mm wide metallisation that, 5 in this example, extends radially substantially from the inner to the outer diameter. The drive electrode 75 extends over the rest of the surface and is electrically insulated from the sense electrode by a 0. 5mm air gap.
Electrical contacts are made by soldered cr~nnP~ti~ns to 10 f ine wires not shown .
In operation, the drive electrode 75 is driven using the electronic circuit 8 by a sinusoidal or square-wave signal at a frequency typically in the range 100 to 300kHz with an 15 amplitude of approximately 30V to produce a droplet spray emerging away from the front face 51 of the perforate membrane wherein the mean droplet size is typically in the region of 10 microns. The actuator head will in general have vibrational r~sr~n~ncr~c at whose frequencies droplets 20 are produced effectively. At such r~c~ln~nc~s the signal from the sense electrode 76 has a local maximum at that frequency. The drive circuit may be open-loop, not using the feedback signal from electrode 76, or may be closed-loop using that feedback. In each case the electronic 25 drive circuit can be responsive to the changing electrical behaviour of the actuator head at resonance so that actuator head and drive circuit cooperate to maintain resonant vibration of the actuator head. Closed-loop forms, for example, can ensure that the piezo actuator 30 maintains resonant vibration by maintaining a phase angle between the drive and feedback or sense ele~;~.odes that is predet~mi necl to give maximal delivery.
Figure 5a shows in sectional view, a fluid feed comprising 35 a conduit formed of an open-celled capillary foam. Such a capillary feed may be used to provide liquid pressure control. (The advantage of pr~s,,uL~ control is described WO9S/15822 ~ 2 1 765 73 P~ S2 below. ) By the action of vent 83 and c2pillary 81 liquid is contained within capillary 81 at a ~LesauL~ below that of the :>uLLuullding atmosphere. The pore size in the capillary foam can be used to control the value of this 5 pressure. SurL~lullding capillary 81 is a robust external housing 82. This aLL~ g L is particularly useful for spray delivery of dangerous, eg toxic, liquids whilst reducing the danger of other means of liquid 105s. The capillary action of material 81 has an action to contain 10 the liquid so that liquid escape is reduced or minimiced even if damage to the external housing 82 occurs.
Applications of this benefit are to retain rh~ Putical or medicinal liquids or fl; hle liquids.
15 Figure 5b shows in 6ectional view, a so-called 'bubble generator' dQvice known from the writing in_~L - ~ art that may also be used to provide liquid ~L~:YYU~ control.
The action of dispensing liquid from the perforations in the membrane causes the p~6:S_UL~ in the reservoir 90 and 20 therefore of the liquid 91 contacting the membrane to decrease below ai ~^ric pIe:s_uLe. When the pLes~uL~ is low enough f or air to be sucked in against the liquid meniscus yLes~uL~ through either the membrane perforations or, alternatively through an AIIY; 1 j ;Iry opening (or 25 openings) 92, air is ingested as bubbles until the reservoir pressure rises sufficiently for the liquid meniscus to withstand the ~ eSl.UL~: differential. In this way the liquid ~Ies:,uLe is regulated at a value below the ambient ~IesYuLe. (Opening 92 is generally selectQd to be 30 small enough that liquid does not easily leak out of the enclosure. ) Both these methods of p~as~uLe control, within the pressure range cited above, have been found capable to enhance spray delivery from the at~ iC;n~ head 40 and it is to be understood that other methods may also be suitable 35 within this invention.
Below follows a description (in relation to figures 6a to ~O 95~15822 - 2 ~ 7 6 5 73 . ~1 . ¦IA7, A~2 6g) of mQthods of operation of the invention. Also described are the droplet generation r- ' Ani e~lc provided by the invention as they are presently perceived by the inventors. These ---hAn; crC are not fully proven nor are 5 they to be understood to be limiting of this invention:
When the p.~s~u.~ difference applied to the liquid is closely zero (ie the ~L~S~UL'~ of the liquid at the atomising head is closely equal to the pressure on the 10 front face of the perforate membrane) then liquid 2 contacts the membrane with menisci 65 attached at rear face 52 of membrane 5 as shown in Figure 6a. It i8 observed that, responsively to vibrational excitation 58 of that membrane liquid flows towards the-front face Sl of membrane 15 5, as shown in an intermediate position in Figure 6b.
Most commonly, with a ~LeS-uLt: difference small compared to that needed f or air to be drawn in against the liquid -n;~C~c ples~iu.~ through the membrane perforations when 20 the membrane is not vibrated, the materials of the membrane and the cross-sectional profile of the perforations allow liquid 2 to flow out on to front face 51 of the membrane as a thin f ilm as shown in Figure 6c. On that face the vibration of membrane 5 can excite capillary waves in the 25 surface of the liquid --n;~:r~lC 67, as shown in Figure 6d.
This has been found to occur, for example when using polymer material for the membrane 5 in the aerosol head described according to Figure 3a. The location of these waves is not constrained by the sidewalls of the 30 perforations 50 or their intersection with the front face 51 that bound the op~ni n~s 53 . If the vibrational amplitude of the liquid ~ c~c 67 is large enough, droplets will be emitted, typically with a droplet tl;A~
approximately one third of the capillary wavelength (see 35 for example Rozenberg - Principles of Ultrasonic Technology) . The perf orate f orm enables ef f ective repl ~nic---nt of liquid lost as droplets from ~ n;CCllC 67.
_ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ .. . . _ WO95/15822 21 76573 r~ 92 The membrane form enables efficient vibrational excitation.
Preferably face 51 is not completely filled with perforations, but the liquid is free to spread out over an 5 area of face 51 larger than the perforation area. This feature allows a balance to be achieved between the rate of flow responsive to vibration 58 (through perforations 50) and the rate at which liquid is sprayed as droplets from capillary waves in r-n; ccllc 67 . This balance may, 10 alternatively or in combination with the above method, be achieved by use of a pressure differential (opposing the flow through perforations responsively to vibration) small enough that a thin film still forms on face 51. By means of this balance, the flow of excessive liquid onto front 15 face 51, which can inhibit the formation of a droplet spray, is prevented.
The ~le6Dur ~: differential opposing flow through perforations 50 may alternatively be selected so that bulk 20 liquid does not flow onto front face 51 of the membrane 5 but has menisci 66 that contact the membrane S at or between the front 51 and rear 52 faces of the membrane, as shown in Figure 6e. In this event the vibration of the membrane can excite vibration in each of the liquid menisci 25 66 as shown in Figure 6e. (Typically this requires a pressure differential comparable to, but not larger than that needed for air to be drawn in through the perforations against the maximum liquid -i cClle ~Lt:Sa-lL~' in the perforations when the membrane is not vibrated. ) The 30 coupling of the vibration of the membrane into the liquid is particularly efficient in this case since the ~
of the perforations complements the geometry of the fluid menisci. The induced excitation of the liquid menisci takes the form of capillary waves. Preferably an inteyer 35 number of such capillary waves 'fit' within the perforations. In this way the geometry of the perforations is a good match to that of the menisci when excited with . _ _ ... , . .. .. , .. . .. . , . . ... ,, . _ _ _ _ _ _ _ _ , .

~VO95/15822 - - 2 ~ 73 ~1 ~ L' '^.2 capillary waves and those waves are created efficiently.
Again droplet ejection is observed with ~ uyLiate frequency and amplitude of vibration.
5 In Figures 6~ and 6g are shown special cases according to Figure 6e in which the ~JL~SZ:~UL~ differential is selected so that the ~ i ccuc of liquid is retained either at or in the vicinity of the intersection of the perforations 50 with the rear face 52 (Figure 6f) or with the front face 10 51 (Figure 6g) of the perforate membrane; whilst capillary waves are formed in that ~-rl; CCllc through the action of vibration 58. Again, this enables efficient vibrational excitation of the - ; ccl~c and if the amplitude and frequency of vibration 58 are appropriate, the droplets of 15 liquid are ejected. It is found that a value of pressure differential between zero and that yLeSaULC: no~ocsi~ry to draw air (or other ambient gas) in through the membrane perforations against the action of the surface tension of the liquid contacting those perforations acts to improve 20 the effectiveness of droplet generation.
In the cases shown in figures 6e, 6f and 6g, conveniently, only a single capillary-wave (i.e. one capillary wavelength) f its within the diameter of the perforation 25 betwQen openings 53 and 54 although, if desired, higher-frequency excitation may be employed so that more than one such capillary-wave so fits. This can be ~:,.yLessed by requiring the following relation approximately to hold at the f requency of vibrational excitation:
~ _ n~c where:
= the ~ or of the tapered perforation at some point between the front and the rear face of the membrane 3 5 n = an integer Wo 95115822 ~ ` . 2 1 7 6 5 7 3 P~ 2 Ac= the wavelength of capillary waves in the liguid The relationship between the wavelength ~c of capillary 5 waves and the excitation frequency, f, is given by:
Ac~ f2 = 8~o/ p where: c = fluid surface tension (at frequency f) p = fluid density.
lO We f ind that this relation also holds approximately in the case of capillary waves bounded by the perforations as described above. Therefore, for tapered perforation of diameter ~ as defined above, it is desirable that the ~pparatus is designed and operated such that:
f 2 ,~ , 87~ o n~
p CuL~ta,uonding to the approximate nature of the relation ~
-- n)~c noted above, operation is found to be satisfactory when this relation holds in this range 51~on~ < f 2~ < 12~on~
P P
In devices where it is advantageous to ensure that only a 20 particular number p, of capillary waves can form within a tapered perforation, the ratio of the large diameter of the perforation (shown at 53) to the small diameter of the perforation should lie in the range l to (p+l) /p. This is most effective for small integer values of p.
Since capil~ary-wave droplets have tli~--t~r approximately one-third of the capillary wavelength, ~" apparatus according to the present invention allows droplets to be produced whose diameter is approximately one-third or less 30 of diameter of the exit openings 53. (When the liquid --n; ccllc is maintained at or close to openings 54 in rear face 52 of the membrane, apparatus according to the present _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~A7O 95/15822 ~ 2 1 ' ~ PCT/G1~94~02692 invention allows droplets to be produced whose ~; ~ t~r i8 approximately one-third or less of the ~iA- '~r of the smaller openings 54. Unlike prior art devices, the - dimensions of the perforations then have an influence upon 5 the droplet size and can therefore advantageously be - selected to assist in the creation of droplets of the desired diameter.) The apparatus is F-~peCiAlly useful for producing small droplets, as required for example in pulmonary drug delivery applications.
Droplet generation occurs according to the apparatus and methods described with regard to Figures 6e, 6f and 6g when using a perforate membrane described with reference to Figure 3b, an atomising head described with reference to 15 Figure 4 and a bubble generator described with reference to Figure 5b. When spraying water from such a device optimum spraying ~r~c~ at a E Les~uLe differential (opposing fluid flow out onto the front face of the membrane) of -30mbar. As the pressure differential increased, spray flow 20 rate and efficiency improved up to a pl~SsuL~ differential of -76mbar. At that pressure the perforate membrane acted as bubble generator and optimum spraying was achieved.
This behaviour is typical. Bubble generator, capillary feeds and other means for providing a pressure differential 25 opposing flow therefore give particular advantage for the present invention. Spray operation for this device was achieved with sinusoidal excitation of 30V amplitude at frequencies of 115, 137, 204 and 262kHz with c~LL~ i calculated capillary wavelengths in the range 51~Lm to 30~m.
3 0 The latter wavelength ~o- L eayonds to the minimum opening dimension of the perforations and produces droplets of approximate size 101~ microns. This device is the best F~mhO,l i - t of the invention known to the inventors f or producing droplets in the region of 10 microns.
In these various ~nho~lir ts, the use of 'reverse-tapered' perforations in the present invention helps to prevent wo sstls822 2 t 7 6 5 7 3 P . ~ n7~92 blockage when atomising liquid suspensions: firstly, unlike prior art devices, the perforations do not admit solids particles that cannot pass completely through the membrane ( but are agitated by the membrane vibration so not to 5 permanently obscure the perforation); secondly two or more solids particles arQ not induced to come into contact both with each other and with the sidewalls of the perforations and so block the perforation; thirdly to produce a given droplet size relatively large perforations can be used and lO so pass relatively large solids particles in liquid suspension without blockage. Apparatus according to this invention further enables rQlative ease of membrane manufacturing when small droplets, such as those desired for pulmonary drug delivery, as required.
There is also distinction between the relative droplet emission frequencies of the present apparatus and prior art apparatus of similar perforation size. For example, with minimum perforation diameters of 15~m, prior art apparatus 20 generally is found to operate to eject droplets at frequencies in the region of 40kHz. With the present apparatus, droplet ejection typically occurs in the region 400--700kHz .
25 Further distinction from prior art apparatus is seen from the actions of the negative liquid bias pressure referred to above. With the prior art devices, eg as shown in Figure 1, it is known to use negative bias ~L~:sauLes, Pcp~ci~lly to prevent wetting of the front face of the membrane.
30 However, such bias does not provide for the ---iccllc to be withdrawn to a new equilibrium position within the perforation - with prior art devices as soon as the bias ~r-asauL~ is sufficient to detach the edge of the -~icC--c from the intersection of the perforations with the front 35 face of the membrane, the r-n; CCIlC pulls completely away from the perforation and spray operation is prevented.
With the present invention, the pressure difference either _ ~vO 9S/1582~ ~ 2 1 7 6 5 7 3 I ~ ~ A92 is selected still to allow a wetted front face of the membrane or, (in the case where that ~l~S~,u~e difference is sufficient to pull the fluid niccl-c back within the tapered perforations) enables the fluid - i cc~lc to reach 5 a new equilibrium position within the tapered perforation - and thereby maintain stable droplet PmiRsio~. The latter is believed also enables combinations of bias ~ Sl:~ULt: and frequency to be established at which an integer number of capillary wavelengths 'fit' within the perforation and 10 efficiently eject droplets.
Pigure 7 shows a second droplet tl i Cppncing apparatus 101 with an alternative liquid feed. The liquid feed i nr~ Pc feed pipe 103, and annular plate 102 acting together with face 1051 of membrane 105 to provide a capillary liquid channel to holes 1060 in membrane 105. Membrane 105 is coupled to a vibration means or actuator 7. Actuator 7 is coupled to sealing support and mount 108, electronic circuit 8 and thence to power supply 9. Feed pipe 103 may be mounted relative to sealing support 108; this i5 not shown. Circuit 8 and power supply 9 may for example be similar to that shown in the first example apparatus.
Vibration of the perforate membrane 105 substantially perpendicular to the plane of the membrane in the direction of arrow 58 produces droplets of liquid 1010 from the front face lOS1 of the membrane. Perforate - ~ ~ne 105 and actuator 107 in combination are hereinafter referred to as aerosol head 1040.
Figure 8 shows cross-sectional detail of liquid in contact with the perforate membrane 105. The membrane 105 comprises a layer of polymer which contains a plurality each of normally-tapered and ~ se tapered conical perforations shown at 1060 and 1050. The L v~lc~ tapered perforations 1050 are positioned to be free of liquid on the f ront f ace of the membrane . The normally-tapered perforations 1060 are positioned to receive liquid from the WO 95/15822 . 2 1 7 6 5 7 3 PCT/GB94/02692 front face of the membrane and may, for example, conveniently be laid out pcripherally around the reverse-tapered perforations 1050.
5 In this second droplet dispensation apparatus the droplet generation v- '~n;~-C described above for the first example apparatus may be employed. The ~lese~ce o$ holes of type 1060 however enables a variety of liquid feeds to the front of the membrane 5 to be employed. Liquid feed is to holes 10 of type 1060 in the ~ront face o$ the membrane.
Conveniently one liquid delivery means may be capillary feed means comprising annular plate 102 acting together with face 1051 of membrane 105. In use, this second example droplet ~; CpPncation apparatus acts to trsnsmit liquid through holes type 1060 to the rear face 1052 of membrane 105 and so by liquid wetting action maintains holes type 1050 in the rear face 1052 of said membrane in contact with the liquid, enabling droplet ~;cppnc~tion from the front face of holes 1050 in a manner similar to that of 20 the first example droplet dispensation apparatus. Other details follow those for the first droplet ~l;crpncation apparatus described above.
Figure 9 illustrates a second use of membranes in which the 25 perforations are both 'normally' and 'reverse' tapered.
This allows the combination in a single device of both the conventional v ~ni ~m of droplet generation shown as understood in Figure 1 and Figure 6. The 'forward' and the ' reverse ' tapered perf orations may be of roughly similar 30 sizes or of differing sizes. ~ccordingly, such devices are capable of creating droplets by one - ~-h~nl~ at one operating frequency and by the other - ~ -ni C~ at another frequency. Similarly, such devices are capable to create droplets 1011 of relatively large size from normally 35 tapered perforations 1060 by one r ^h~ni ~m and droplets 1010 of relatively small size from 'reverse' tapered perforations 1050 by the other ~ n; ~. Further, it is . ~ ~

V095/15822 ~ 21 76573 r~l~. L~^~fig2 possible to create sprays of relatively high velocity by one m~ hAni~m and of relatively small velocity by the other rqch~ni ~m. Other combinations of droplet size, operating frequency and droplet velocity will be apparent. Finally S the droplet production r--h;~n; ~m of the 'normally' tapered perforations 1060 can also, for example in a bubble-generator enclosure design as described above, be used to create a negative ~lesau- e bias for i _u~d droplet generation from the 'reverse' tapered regions of the 10 membrane.
The best conditions and details of the atomising head of that apparatus currently known to the inventors have been described with reference to Figures 3b, 4, 5b and 6e to 6g 15 above.
Notwithstanding the drawings, it is to be understood that devices according to the present invention may be operated in a range of orientations, spraying downwards, sidewards 2 0 or upwards .

Claims (25)

1. A liquid droplet spray device comprising:
a perforate membrane;
an actuator, for vibrating the membrane; and means for supplying liquid to a surface of the membrane, characterised in that perforations in the membrane have a reverse taper, namely a larger cross-sectional area at that face of the membrane away from which liquid droplets emerge than at the opposite face of the membrane, from which opposite face liquid flows in use to replace that in the emerging droplet spray .

2. A device according to claim 1, further including a pressure bias means providing reduced pressure in the liquid contacting that face of the membrane which is opposite to the face away from which the liquid droplets emerge .

3. A device according to claim 2, wherein the reduced pressure lies in the range zero to that pressure at which air is drawn through the perforations of the membrane contacted by fluid.

4. A device according to any of claims 1 to 3, wherein the perforations, on that face of the membrane away from which liquid droplets emerge, are not touching.

5. A device according to any of claims 1 to 4, wherein the actuator is a piezoelectric actuator.

6 . A device according to claim 5, wherein the piezoelectric actuator is adapted to operate in the bending mode.

7. A device according any of claims 1 to 6, wherein the means for supplying liquid to a surface of the membrane comprises a capillary feed mechanism.

8. A device according to any of claims 1 to 6, wherein the means for supplying liquid to a surface of the membrane comprises a bubble-generator feed mechanism.

9. A device according to any of claims 1 to 8, wherein all the perforations have a reverse taper.

10. A device according to claim 9, wherein the membrane further includes normally tapered perforations.

11. A device according to claim 10, wherein the normally tapered perforations are disposed around the outside of the reverse tapered perforations.

12. A device according to claim 10 or claim 11, wherein the means for supplying liquid to a surface of the membrane is adapted to supply said liquid to the face of said membrane away from which liquid droplets emerge .

13. A device according to any of claims 1 to 11, wherein the means for supplying liquid to a surface of the membrane is adapted to supply said liquid to the face of said membrane opposite to the face away from which liquid droplets emerge.

14. A device according to any of claims 1 to 13, wherein the actuator is arranged to vibrate said membrane such that the following relation is satisfied:
where:
.PHI. = the diameter of the tapered perforation at some point between the front and the rear face of the membrane n = an integer .lambda.c= the wavelength of capillary waves in the liquid o = fluid surface tension (at frequency f) p = fluid density.

15. A device according to any of claims 1 to 14, wherein the actuator is arranged to vibrate said membrane in a frequency range of 20kHz to 7MHz.

16. A method of atomising a liquid in which a liquid is caused to pass through tapered perforations in a vibrating membrane in the direction from that side of the membrane at which the perforations have a smaller cross-sectional area to that side of the membrane at which the perforations have a larger cross-sectional area.

17. A method according to claim 16, wherein a pressure bias is provided in the liquid opposing the passage of the liquid through the perforations.

18 . A method according to claim 17, wherein the pressure bias lies in the range zero to that pressure at which air is drawn through the perforations of the membrane contacted by fluid.

19. A method according to claim 16, wherein the actuator is a piezoelectric actuator and is caused to operate in the bending mode.

20. A method according to any of claims 16 to 19, wherein the liquid is supplied to a surface of the membrane through a capillary feed mechanism.

21. A method according to any of claims 16 to 19, wherein the liquid is supplied to a surface of the membrane through a bubble-generator feed mechanism.

22. A method according to any of claims 16 to 21, wherein the liquid is supplied to the face of said membrane away from which liquid droplets emerge.

23. A method according to any of claims 16 to 21, wherein the liquid is supplied to the face of said membrane opposite that face away from which liquid droplets emerge.

24. A method according to any of claims 16 to 23, wherein the actuator causes said membrane to vibrate such that the following relation is satisfied:
where:
.PHI. = the diameter of the tapered perforation at some point between the front and the rear face of the membrane n = an integer .lambda.c= the wavelength of capillary waves in the liquid o = fluid surface tension (at frequency f) p = fluid density.

25. A method according to any of claims 16 to 24, wherein the membrane is vibrated at a frequency in the range of 20kHz to 7MHz.