Note: Descriptions are shown in the official language in which they were submitted.
~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
WO 95/15822 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
12
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
13
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
14
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
18
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
21
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 .
