Ultrasonic device for the continuous production of particles

An ultrasonic device for the continuous production of microdroplets of uniform particle size distribution. This device comprises a vibrating surface (11) which, by its orthogonal ultrasonic vibratory mode, atomizes a material in the liquid state brought up from the interior of the device by means (20,22,24) comprising an intermediate flow-regulating and/or heat-regulating chamber (22) subjacent the vibrating surface (11).

This invention relates to an ultrasonic device for the continuous 
production of particles having as uniform a particle size distribution as 
possible, more particularly microdroplets of controlled diameter and 
sphericity. 
The problem of producing finely divided microdroplets is encountered in 
numerous fields where organic and/or mineral liquids have to be sprayed or 
atomized, for example in the pharmaceutical field for delivering a 
suspended product (medicament or the like) by means of an aerosol or a 
product coated in a matrix (slow-release medicament). The same problem is 
encountered in the field of cosmetics, for example for producing what may 
now be called liposomes. 
It is known that ultrasonic devices, such as those disclosed in DE 2 537 
772 or DE 3 036 721 for the continuous production of microdroplets 
specifically from a liquid, can be used for this purpose, these 
devices--which comprise a vibrating surface--atomizing the liquid coming 
from inside under the effect of their orthogonal, ultrasonic vibratory 
mode. However, due to the configuration of the means for delivering the 
liquid, these known devices only operate providing the liquid does not 
have an excessive viscosity. In addition, the rate at which the liquid is 
delivered can be fairly irregular, the liquid being distributed in the 
form of a sheet of more or less constant thickness. 
On the other hand, it is known that carburettors of internal combustion 
engines or injectors for boilers, such as described in EP 0 202 381, 
operate ultrasonically to atomize a liquid fuel. In view of the very 
narrow flow cross-sections involved, these atomizers can again only 
operate properly with volatile fuels. 
Now, numerous components are at present being made from fine metal 
particles or granules, particularly by sintering processes. The problem of 
particle size distribution is significant because it is desirable to 
produce batches of microbeads all having substantially the same size. 
An ultrasonic device enabling microbeads between 5 and 200 microns in 
diameter to be obtained is known in the field of molten metals. A material 
in the liquid state is allowed to fall dropwise onto the end surface of a 
vibrator oscillating at a frequency of 5 to 50 kilohertz. An ultrasonic 
vibrator comprises, for example, a piezoelectric transducer extended by a 
concentrator, i.e. an element of which the particular shape allows it to 
resonate on the end surface at a frequency higher by one order of 
magnitude than the oscillating frequency of the transducer. A vibrator of 
this type is also known as a "sonotrode" in the industrial field in 
question. This process is preferably carried out in an enclosure which 
enables the atmosphere to be controlled (either a vacuum or an inert gas). 
When a drop of liquid falls onto the ultrasonic vibrating surface, it forms 
a film of which the upper surface assumes the form of a network of 
stationary waves having a wavelength of the order of 200 microns for 
example. When the amplitude of these waves reaches a sufficient value, 
drops of liquid separate from the crest of the waves and, by cooling, form 
solid microbeads of the order of 50 microns in diameter. It has been found 
that the diameter of the microbeads thus obtained depends to a large 
extent on the frequency of vibration of the surface. The distribution of 
the diameters about a mean value depends in part on the amplitude of 
vibration, but to a greater extent on the rheological characteristics of 
the material which varies significantly between a liquid and a molten 
material. For its part, the hourly output of microparticles depends on the 
amplitude of vibration and the dynamic viscosity coefficient of the 
material. 
However, the ultrasonic device described above has parasitic effects due to 
the configuration and arrangement of the elements which give rise to a 
particle size distribution that is still unsatisfactory because the 
microbeads then have to be graded by passage through a series of screens. 
It has been found on the one hand that the wave network of the sheet of 
liquid is unstable on the vibrating surface and, on the other hand, that 
the thickness of the sheet was variable, namely thick near the place where 
the material arrives and very fine towards the edges of the surface. 
The problem addressed by the present invention was to obviate the 
above-mentioned disadvantages due in large part to the flow rate and to 
the excessively irregular distribution of the liquid on the vibrating 
surface in order to produce microdroplets having a more controlled 
diameter in an improved output. 
This problem has been solved by an ultrasonic device comprising a vibrating 
surface which, by its orthogonal ultrasonic vibratory mode, atomizes a 
material in the liquid state brought up from the interior of the device by 
means comprising an intermediate flow-regulating and/or heat-regulating 
chamber subjacent the vibrating surface. The base of the intermediate 
chamber, where the passage(s) for delivering the material in the liquid 
state open, is advantageously situated in the plane of a wave node of the 
vibratory regime. 
In a useful embodiment, the material in the liquid state is distributed 
over the vibrating surface by one or more channels of which one of the 
dimensions of the flow cross-section is submillimetric and of which the 
total flow cross-section is greater than 8 mm.sup.2, the flow of the 
material taking place by capillarity and/or by pressure gradient induced 
by the vibratory mode. 
In the context of the invention, a "material in the liquid state" is 
understood to be both a liquid per se at ambient temperatures and a molten 
material, i.e. a material which is solid at ambient temperatures and which 
is made liquid as required This material may be mineral and/or organic. By 
virtue of the presence of the intermediate chamber, it becomes possible on 
the one hand to control the viscosity of the liquid on the vibrating 
surface through its temperature and, on the other hand, to control the 
losses of pressure more effectively by making the ejection channels, which 
are now shorter, with greater precision. Thus, an initially solid material 
may be kept in the liquid state or may be liquefied by heating in the 
intermediate chamber acting as a crucible very close to the vibrating 
surface. 
In a first preferred embodiment, the internal intermediate chamber is 
cylindrical and parallel to the vibrating surface. The material in the 
liquid state is thus distributed by several channels situated at regular 
intervals along the line of the chamber closest to the surface. 
In a second preferred embodiment, the internal intermediate chamber is 
central and has a tubular shape orthogonal to the vibrating surface into 
which it opens directly at one of its ends which may optionally be 
tapered. 
In a third preferred embodiment, the tubular internal intermediate chamber 
is situated at the periphery of the vibrating device subjacent and 
orthogonal to the concave vibrating surface at the periphery of which it 
opens. 
The vibrating surface advantageously comprises a network of parallel or 
crisscross grooves or a network of circular and/or radial grooves which 
tend to stabilize the position of the undulating regime of the film. The 
grooves may have a rectangular cross-section, a trapezoidal cross-section, 
a U-shaped cross-section, etc. 
To improve the distribution of the material in the liquid state over the 
vibrating surface, the channels may be designed to open at an angle of 
25.degree. to 60.degree. in relation to the surface. Alternatively, a 
cover parallel to the surface is arranged above and close to the exit of 
the channel(s).

FIG. 1 shows the end of a vibrating element 10, in the present case the 
concentrator of a sonotrode, which terminates in a vibrating surface 11 
forming an atomizer. The partial section through the vibrating element 10 
shows an internal intermediate chamber 22 which is substantially 
cylindrical in shape and parallel in length to the atomizing surface 11 
and which is used as an internal crucible. The internal intermediate 
chamber 22 is fed with a material in the liquid state, for example a melt, 
by a feed channel 20. Since the vibrating element 10 is heated, the molten 
material remains heated inside the intermediate chamber 22. The 
intermediate chamber 22 is connected to the atomizing surface 11 by a 
series of ejection channels 24, for example 15 in number. The ejection 
channels 24 are preferably arranged at regular intervals along that line 
of the intermediate chamber which is nearest surface. The diameter of each 
of the channels is of the order of 1 millimetre. 
The vibratory mode of the sonotrode creates a difference in pressure 
between the channels 24 and the outside, causing the molten material to 
issue from the channels. In view of the narrow flow cross-section of each 
of the channels, the exit rate of the molten material is influence by 
capillary effects which are themselves dependent on the one hand on the 
ultimate shape and quality of machining of the channels and, on the other 
hand, on the rheological properties of the molten material which in turn 
are dependent on the final temperature of the material. 
As can clearly be seen from FIG. 1, the atomizing surface 11 comprises a 
series of parallel and regular striae or grooves approximately 1 
millimetre wide and approximately 0.25 millimetre deep for a spacing of 
the order of 2 millimetres. The ejection channels 24 respectively open at 
the bottom of a groove thus formed. The function of these grooves is to 
stabilize the spread film of material in its lateral position. 
To prevent the projection of material beyond the vibrating surface 11 from 
the exit of the channels, a shoe is provided on the virbating element 10, 
its end forming a cover 30 which is parallel to the atomizing surface 11 
and which is disposed above the ejection channels 24. The shape, 
dimensions and weight of the shoe and cover 30 are obviously gauged to 
avoid excessive modification of the vibratory regime of the vibrating 
element 10, particularly the concentrator. 
Alternatively, the effect of the cover 30 may be replaced by oblique 
openings of the ejection channels 24, i.e. opening onto the vibrating 
surface 11 at an angle of 15.degree. to 75.degree. in relation to that 
surface. In practice, however, the formation of angled channels such as 
these is more difficult and, in addition, can create losses of pressure 
within the channels. 
As will be appreciated, the feed rate of molten material onto the atomizing 
surface 11 is determined, on the one hand, by the flow cross-section of 
each of the channels 24 and, on the other hand, by the number of channels. 
FIG. 2 shows a second variant of the end of the sonotrode in which the 
principal components are coaxial to the cylindrical vibrating element 60. 
To this end, a blind hole 75 is first drilled at the middle of the 
vibrating element 60. An element 76 is forcibly inserted into the blind 
hole thus drilled, its lower part having a diameter corresponding to that 
of the blind hole 75 and its upper part having a restriction 74. 
The bottom of the element 76 is advantageously situated in the plane of a 
wave node, i.e. at a height of the sonotrode where the amplitude of 
vibration is minimal, thus voluntarily limiting the more or less 
controlled resonance of the element 76. 
In conjunction with the upper part of the blind hole 75, the restriction 74 
creates a cylindrical, elongate channel 72, i.e. an annular channel, which 
opens directly onto the upper atomizing surface 61. The annular channel 
acts both as an internal chamber/crucible and as an ejection channel. If 
the volume of this internal chamber has to be larger for reasons of flow 
regulation by buffer effect, the restriction 74 is reduced at its upper 
end to form a tapered ejection channel. 
A trasverse feed channel 70 intersects the channel 72. The feed channel 70 
can open into the blind hole 75 at any height, but preferably at its base 
or at the level of the wave node plane. 
As can be seen from FIG. 2, the upper atomizing surface 61 also has a 
series of parallel grooves 64 substantially identical in their dimensions 
with the grooves shown in FIG. 1. It may be necessary to stabilize the 
film of molten material in the two axes of the plane of the surface 61, 
which is done by cutting complementary transverse grooves 62. 
In addition, the element 76 is completed at its upper end by a cover 80 
projecting beyond the exit of the channel 72. In the same way as before, 
the function of the cover 80 is to direct the molten material ejected 
towards the atomizing surface 61. Alternatively, the blind hole 75 could 
have a conical opening at the level of the surface 61 with an apex angle 
of 30.degree. to 150.degree. C. Thus, instead of a cover 80, the element 
76 is conically widened at a corresponding angle. 
FIG. 3 shows a third variant of the end of the sonotrode comprising a 
cylindrical vibrating element 100, which becomes frustoconical in shape 
towards its lower end, and being formed in its upper part subjacent to the 
vibrating surface 105 with a peripheral groove 127 which forms an 
intermedate chamber 128 in conjunction with a detachable collar 110. The 
internal diameter of the collar 110 is larger than the external diameter 
of the vibrating element by 1 to 2 millimetres. The collar 110 is 
preferably fixed to the vibrating element 100 at the level of a wave node 
plane to limit possible vibrations in the manner of a bell. 
The material may be brought to the intermediate chamber 128, preferably 
near the wave node, i.e. to the base of the chamber, through internal 
channels 122, 124 and 126 and even through an external channel. For 
machining reasons, the central channel 124 may be drilled from the lower 
surface so that the inlet opening is reclosed. 
An ejection channel 130 is formed at the periphery of the vibrating surface 
105 by the small difference between the external diameter of the vibrating 
element 100 and the internal diameter of the collar 110. As can be seen 
from FIG. 3, the vibrating surface 105 is slightly convex and, in 
particular, is rounded at the exit of the ejection channel 130. This 
rounded section avoids disruption of the film of liquid material 
progressing radially from the channel towards the centre of the surface. 
If necessary, the upper edge of the collar 100 may also be slightly turned 
radially inwards to accompany the movement of the liquid over the peripery 
of the vibrating surface. 
The outer part 106 of the vibrating surface 105 also has a certain 
inclination of the order of 10.degree. to 20.degree. relative to the 
horizontal both to soften the rounded section at the exit of the ejection 
channel and to direct the majority of the microdroplets formed there 
towards the exterior of the device. Since this peripheral zone 106 is the 
most important from the point of view of the formation of microdroplets, 
it may be of advantage to cut a network of radial and circular grooves 
into this zone to regulate the distribution of the liquid material. 
As will readily be appreciated, the sonotrode with its vibrating surface 
according to the invention is installed in an enclosed space to enable the 
surrounding medium to be controlled, i.e. to enable a vacuum or an inert 
gas atmosphere to be established. 
The vibrating element 10 in FIG. 1 is advantageously inclined upwards at an 
angle of 45.degree. with the feed channel 20 also directed upwards. A 
stream or sheet of liquid or a rod of preheated metal can thus be 
introduced into the channel 20 from above, the metal then melting in the 
internal intermediate chamber 20. Alternatively, the metal is preheated in 
a main crucible which is then brought into the molten state in the entry 
channel 20. It has been found that the grooves 12 provide for lateral 
stabilization of the sheet and, to a certain extent, for an intake of 
material coming from the channels. 
The vibrating element 60 in FIG. 2 or the element 100 in FIG. 3 may be 
installed in the enclosure at an angle of 0.degree. to 180.degree. 
relative to the horizontal. However, even when the element is installed at 
an angle of greater than 90.degree., the sheet still remains on the 
vibrating surface 61 under the effect of surface tension. This phenomenon 
is further confirmed when the vibrating surface is directed downwards. The 
detached microdroplets are projected and fall directly downwards by 
gravity, cooling and forming microbeads during their descent. 
If necessary, particularly for achieving higher vibration frequencies and 
hence smaller microbead diameters, it would also be possible to use a 
vibrating plate instead of the sonotrode, in which case the intermediate 
chamber with its ejection channels would be arranged in the plate beneath 
the upper surface. 
The invention may be used in various fields where it is desirable 
continuously to produce particles having as uniform a size distribution as 
possible, including the fields mentioned earlier on, namely molten metals, 
pharmaceuticals, cosmetics and internal combustion engines. 
Numerous improvements may be made to the ultrasonic device for the 
production of particles within the scope of the present invention.