WO1997043026A1

WO1997043026A1 - Apparatus and method for manipulating particles in a liquid medium by ultrasonic waves

Info

Publication number: WO1997043026A1
Application number: PCT/GB1997/001239
Authority: WO
Inventors: Cornelius John Schram
Original assignee: British Technology Group Limited
Priority date: 1996-05-10
Filing date: 1997-05-08
Publication date: 1997-11-20
Also published as: EP0914184A1; JP2001502225A

Abstract

The invention relates to an apparatus and a method for munipulating particles in a liquid medium by ultrasonic waves. A vessel is provided for receiving the particle-carrying liquid, as well as means, such as an ultrasonic transducer for generating an ultrasonic standing wave in the vessel such that particles are attracted to nodal fronts of the standing wave. The standing wave is intermittently suppressed, whilst the particle-carrying liquid is oscillated relative to the transducer. If the oscillation and the intermittency are carried out in synchronisation, the particles can be shepherded from one nodal front to another in a prescribed direction to concentrate them for separation from the liquid medium. The invention has application to separation or concentration of inorganic or organic particulate matter in laboratory or industrial processes.

Description

Apparatus and Method for Manipulating Particles in a Liquid Medium by Ultrasonic Waves

The present invention relates to an apparatus and a method for manipulating particles in a liquid medium by ultrasonic waves. It has particular application in the collection of fine particles, of the order of up to say 100 microns in diameter, from a liquid medium.

Conventional separation and concentration techniques, such as mechanical filtering or centrifuging, have a number of disadvantages. They are generally slow and inefficient and can prove to be very costly, even when applied on a large scale. It has also been known for some time that particulate material can be manipulated by making use of the acoustic forces in an ultrasonic standing wave acting on the suspended particles. When the acoustic field is established, individual particles are attracted to the nodes or the antinodes of the standing wave, leaving the intermediate regions free of particles. For the purposes of the present invention it is immaterial whether the attraction is to the nodes or to the antinodes and to simplify the following description these zones of attraction will be referred to only as the nodes, the nodal fronts, or the nodal planes. Since a similar mechanism is operative in situations where attraction may be towards the antinodes, this alternative is accordingly included within the scope of the invention. The acoustic separation technique is energy- efficient, fast and effective, but since the wavelength of sound in water at appropriate frequencies is of the order of a miliimetre and the distance between adjacent nodes is thus half of this, the separation achieved is, in itself, of little value. Developments in this technology have focused on using the acoustic forces to separate, concentrate and fractionate particles on a useful scale and at useful speed, particularly in a continuous process. Applicant's European Patent Application EP-A- 147032 describes how two axially opposed transducers can be used to establish a standing wave to control the movement of particles in a coaxial column of liquid inteφosed between the transducers and how, by displacing the standing wave along its axis, it is possible to move the particles along the column under the influence of the moving standing wave. One disadvantage of this method is that it is very difficult to operate in a resonant acoustic field, that is, a field in which the standing wave space, which must be equal to an even number of quarter wavelengths in length, is resonant at that frequency.

Applicant s later European Patent Application EP-A-380194 provides an alternative method of manipulating particulate material in a liquid medium in which an ultrasonic standing wave is established in a flow of said liquid medium with its nodal fronts extending obliquely to the direction of flow of the liquid so as to bring particles on the nodal fronts towards a boundary along which the flow runs. By using a channel provided with alternative outlets for streams of the flow running nearer to and further away from the selected boundary wall, separate outlet streams may be established respectively enriched or depleted in the particles. The method can be operated in a fully resonant acoustic field.

One of the problems with flow-through continuous separators or concentrators, such as. for example, the type described in EP-A-380194, is that particles tend to aggregate in certain zones of the nodal fronts. This phenomenon is due to the inevitable non-uniformity of the acoustic field in practical situations and these zones of aggregation, hereinafter referred to as 'hotspots', correspond to the positions of highest acoustic energy. Particles in or near the nodal fronts move up the acoustic energy gradient and thus aggregate at the hotspots, rapidly becoming more firmly anchored there as the particulate aggregate grows in size. As a result, the aggregate effect will tend to block the smooth flow of particles, whilst the more minor gravity force may also have an undesirable effect on the particulate. The ultimate effect of this phenomenon in a flow-through continuous separator or concentrator is that controlled manipulation of the particles is impaired, and the effectiveness of acoustic particle separation processes can be considerably reduced.

It is an object of the present invention, amongst other things, to limit the effect of the above-mentioned drawbacks and to improve the effectiveness of acoustic separation or collection methods and devices.

According to a first aspect of the invention, there is provided an apparatus for manipulating particles in a liquid medium by ultrasonic waves comprising a vessel for receiving the particle-carrying liquid, means for generating an ultrasonic standing wave in the vessel such that particles are attracted to nodal fronts of the standing wave, means for intermittently suppressing the standing wave, and means for oscillating the particle-carrying liquid relative to the standing wave. This is preferably done by way of mechanical

-1. oscillation of the vessel, but it may alternatively be done in other ways, such as by controlled pumping from the outlet(s) of the vessel in a flow-through arrangement.

In a preferred form of the invention, at least a component of the oscillation motion is in the direction of propagation of the standing wave, and said component preferably has an amplitude at least as great as the internodal separation of the standing wave.

Preferably, means are provided for carrying out the oscillation in synchronisation with the intermittent suppression of the standing wave.

Means may be provided for providing a flow of the particle-carrying liquid through said vessel to afford relative movement between the liquid and the standing wave, at least a major component of said movement being in a direction peφendicular to the direction of propagation of the standing wave, such that particles attracted to the one or more nodal fronts of the standing wave are moved along the front or fronts. The standing wave suppression means thus allows particles trapped at hotspots to be released and carried along their respective nodal fronts by movement of the liquid, whilst the reestablishment of the standing wave prevents particles from dispersing too far. This means that particles can move more steadily along their nodal fronts than otherwise possible. The degree of concentration of particles can be optimised for particular applications by controlling the degree and frequency of the modulation.

The means for generating the standing wave may be arranged such that the standing wave has an axis passing through a boundary wall of said vessel, said wall extending obliquely to the axis of the standing wave, such that the relative movement between the liquid and the standing wave brings particles attracted to nodal fronts of the standing wave towards said boundary wall. In this form, it is this oblique arrangement which provides the lateral component of liquid flow relative to the orientation of the standing wave. Preferably, the angle of intersection between the nodal fronts and the wall is substantially less than 45°.

The vessel may be provided with a flow inlet means and a flow outlet means for passing the particle-carrying liquid through the vessel, the flow inlet and outlet means being mutually arranged to produce a component of the relative movement between the liquid and the standing wave in the direction of propagation of the standing wave. Flow control means for controllably suppressing the component of the relative movement between the liquid and the standing wave in the direction of propagation of the standing wave can be incoφorated. said flow control means being operable in synchronisation with said means for intermittently suppressing the standing wave.

One form of device provides an apparatus including a plurality of flow outlets mutually spaced in the direction of propagation of the standing wave, the said flow control means comprising individual flow rate control means associated with the flows through each of said flow outlets (eg. variable pumping rates from the different outlets). In this way, separate flow streams can be obtained, respectively enriched or depleted in the particles. The means for oscillating the vessel may comprise a motor device arranged and operated to rotate the vessel in a reciprocating manner.

Preferably, the amplitude of oscillation of the particle-carrying liquid relative to the generation means in the direction of propogation of the standing wave is approximately equal to an integer multiple of the internodal separation of the standing wave. The means for intermittently suppressing the standing wave may comprise a square wave modulation means to successively reduce and re-establish the intensity of the ultrasonic standing wave in a regular manner.

In one form of the invention, the vessel comprises two vessel portions mutually spaced along the direction of propagation of the standing wave, the two vessel portions arranged to oscillate in like opposed manner. A resilient sealing means may serve to sealingly interconnect the two vessel portions to enable them to displace relative to one another while retaining said particle-carrying liquid.

Means for generating a second ultrasonic standing wave in the vessel may provided, the two standing waves being mutually inclined, and preferably mutually orthogonal. According to a second aspect of the invention, there is provided a method for manipulating particles by ultrasonic waves in a liquid medium within a vessel, comprising generating an ultrasonic standing wave in the liquid and intermittently suppressing said standing wave whilst mechanically oscillating the particle-carrying liquid relative to the standing wave. Preferably, the mechanical oscillation is carried out in synchronisation with the intermittent suppression of the standing wave. In a preferred mode of operation, during one half of the oscillation cycle the acoustic field holds the particles to the nodes despite the forces provided by the relative movement of the liquid. During the other half of the cycle the acoustic field is reduced or extinguished so that particles are no longer fixed at loci determined by the standing wave, but are free to move with the liquid in the moving vessel. Particles can thus be moved from node to node in the direction of propagation of the standing wave.

This last mentioned technique is generally referred to in this specification as 'rastering' of the particles, as it allows particles to be regularly stepped from one node to the next. By way of example only, the invention will be described in more detail with reference to the accompanying drawings, in which;

Fig. 1 illustrates one form of a flow-through apparatus for concentrating particulate material in a liquid medium;

Fig. 2 illustrates an embodiment of a flow-through apparatus according to the invention; and

Fig. 3 represents a mode of modulation of the operation of the apparatus of Fig. 2.

Fig. 4 shows diagrammatically in section a general arrangement of a batch- wise end particle concentrator;

Fig. 5 shows in section a centre particle concentrator, also in diagrammatic form; and

Fig. 6 shows an apparatus for providing two standing waves which cross one another at an angle of 90°.

The apparatus shown in Fig. 1 is broadly similar to that described in EP-A-380194, the contents of which are included herein by reference. A flow-through separator or concentrator 10 comprises a water-filled, acoustically transparent duct 20 of rectangular cross-section arranged between an upper acoustic coupling block 30 and a lower reflecting block 31. The mutually opposed faces 32 and 33 of these blocks are parallel and inclined at an angle, such as 5°, to the axial direction of the duct 20. Disposed on the top surface of the acoustic coupling block 30 is an ultrasonic source 34 comprising a lead-zirconium-titanate ultrasonic transducer to output acoustic energy to be transferred through the coupling block 30 in a direction normal to the inclined face 32. and then reflected from the reflecting surface 33 of the reflecting block 31. The orthogonally projected areas of the faces 32 and 33 are substantially coincident with one another, and the faces are separated by a distance equal to an integral number of half waves of the radiation frequency, so that a standing wave is set up between the faces with nodal planes 25 extending parallel to the surfaces and thus at a small angle to the axis of the duct 20.

The acoustic coupling block 30 and the reflecting block 31 are shown in Fig. 1 as having continuous parallel straight faces, although they may be provided instead with a series of stepped parallel faces as described in EP-A-380194, in order to reduce the overall separation of the faces for a given size of duct 20.

In an apparatus tested by the inventor, the acoustic coupling block 30 and the reflector block 31 were fabricated from aluminium and a reflecting surface 33 was provided on the upper face of the reflector block, the reflecting surface made from a thin plate of tungsten. The duct 20 was provided with acoustically transparent windows fabricated from Mylar (Trademark). An appropriate acoustic coupling liquid may be used to fill the voids 40 between the faces 32 and 33 and the walls of the duct, and seals 41 are provided to contain this liquid. The apparatus included means (not shown) for fine adjustment of the separation of the faces 32 and 33, such that the cavity therebetween can be tuned to the operating frequency to create fully resonant conditions. In general operation, then, liquid carrying particulate material enters the duct 20 from the right as seen in Fig. 1. Suspended particles approaching the duct section where the acoustic field is present are moved to and held at the nodal planes 25 of the standing wave. The influence of the continuous flow moves the particles along the planes in a direction oblique to the axial direction of the liquid flow, i.e. towards the bottom boundary wall 20a of the duct. When particles reach the boundary wall the flow forces will detach them from their respective nodal planes and carry them along wall 20a. The net effect is therefore to concentrate the particles towards wall 20a as they flow along the duct with the liquid medium. At the exit end (the left hand end in Fig. 1) of the duct flow is separated into a lower stream enriched with particles and an upper stream depleted of particles. Exit passages 21 , 22 draw off these separate streams.

A problem associated with this device has been found to be the occurrence of so- called 'hotspots' in the nodal planes, which tend to lead to the rapid aggregation of particles as mentioned above. The hotspots do not of course move with the liquid flow and therefore an aggregation tends to block the movement of other particles in that nodal plane, those particles adding themselves to the aggregate. This significantly reduces the desired effect of the standing wave, considerably impairing the effectiveness of the separation/concentration process.

To alleviate this effect the acoustic field can be modulated by the application of an intermittent reduction in intensity. By periodically reducing the energy density in the standing wave, if need be right down to zero, the particles are released from the loci of hotspots and have the chance to separate from their aggregation and move downstream with the liquid flow. If the full field is re-established quickly enough then the particles will be attracted back to the same nodal plane at a position sufficiently far downstream from the hotspot they previously occupied to avoid their being pulled back into that position. By establishing an intermittent standing wave in this way, more effective particle management in a somewhat irregular acoustic field is possible. The degree of particle aggregation in the nodal planes is controlled by balancing the primary acoustic forces with the Stokes' forces providing dispersion in the liquid flow.

The degree of concentration of the particles can be controlled by adjusting the parameters of the signal (eg. a squarewave) controlling the ultrasound field. The precise form and degree of the intermittency may be varied as appropriate. As mentioned above, the acoustic signal need not be reduced to zero, so long as the field is reduced to an intensity at which at least some of the particles may be released from the loci of the hotspots. A square wave modulation has been tested using polystyrene microsphere particles in water, although other waveforms may be employed as appropriate. The frequency of the modulation may also be varied as appropriate for the particular application and the conditions encountered. In general, a high density of field hotspots will demand a high frequency of modulation.

In experimentation, the apparatus of Fig. 1 was shown to produce a marked increase in concentration of particles in lower exit passage 21 than in upper exit passage 22 when subjected to full modulation at a frequency of 1 Hz.

Fig. 2 illustrates in diagrammatic form a flow-through cell concentrator according to the invention. A similar system of reference numbers as used in Fig. 1 has been used in respect of this embodiment, with each number increased by 100.

The general concept of this device is similar to that described with respect to Fig. 1, but the longitudinal axis and walls of the flow-through duct are arranged substantially parallel to the opposed faces 132 and 133 (ie. substantially peφendicular to the direction of propagation of the standing wave). The vessel 120 is disposed with its longitudinal axis vertical. Once again, the acoustic wave is generated by a planar ultrasonic transducer 134 operating at about 2MHz, attached to an aluminium coupling block 130, with a plane propagation face 132 parallel to the plane propagation face 133 of an aluminium block 13 serving as an ultrasonic reflector. The length of the coupling block 130 and the length of the intermediate cavity 145 between the faces 132 and 133, as measured in both cases in the direction of wave propagation, are tuned to the operating frequency to provide a fully resonant cavity. An acoustically transparent working vessel of rectangular section 120 is located in the resonant cavity 145 such that the nodal planes 125 when established lie parallel to the walls of the vessel. At the top of the vessel 120 an entry port is provided in the form of a slot 123 arranged parallel to the vessel walls on the extreme left hand side of the vessel (as shown), to allow entry of a particle suspension. At the base of the vessel, three planar flow dividers provide four equally-sized outlet slots arranged parallel to the nodal planes 125 which lead to four outlet ports 121, which connect to four flexible outlet tubes 126 leading to a multichannel peristaltic pump 150. This arrangement affords equal rates of liquid pumping from the base of the vessel to provide four separate effluent streams marked A, B, C and D, which can therefore be separately analysed for particulate content. The vessel 120 is pivoted about a pivot point 151, the pivot axis arranged peφendicular to both the direction of the nodal planes and the direction of propagation of the ultrasonic wave. A stepping motor 152 is mounted to rotate the vessel 120 about this pivot point, which is at a sufficient distance from the vessel that oscillation of the motor shaft by a small angular displacement translates the vessel in the direction of wave propagation in a to-and-fro motion within the cavity 145. The coupling block 130, reflector block 131 and the motor are mounted to a fixed support frame in which the pivot point 151 is provided, whilst means are provided (not shown) to fill the cavity outside the vessel 120 with a suitable acoustically transparent liquid. The motor 152 is controlled by the operation of a controller 153 which outputs a bipolar signal thereby to drive the motor output in an oscillating manner. The waveform, the amplitude and the frequency may all be varied by the controller 153. Controller 153 also controls the operation of the ultrasonic transducer 134 and the operation of the peristaltic pump 150.

For operating the apparatus of Fig. 2, vessel 120 is filled with liquid using pump 150 whilst inlet slot 123 is connected to a source of the liquid. The pump is then stopped, inlet slot 123 is connected to a source of the particle feedstock, and the pump is then restarted to draw in the particle suspension which exits the vessel by outlet ports 121 and flexible tubes 126. The acoustic field is activated, the cavity 145 is tuned by adjusting the acoustic path length between block 130 and reflector 131, and the pump 150 is set by means of the controller to draw in the particle feedstock at a velocity which maintains streamline flow. Particles move down the nodal planes 125 disposed immediately below entry slot 123 (Le. on the left hand side of the vessel) and proceed down these nodes due to the flow of liquid through the vessel. Since liquid enters the top of the vessel at the extreme left hand side but is pumped uniformly from the base, it flows laterally from left to right during its passage down through the vessel. This lateral component to the liquid flow results in particles being progressively more concentrated at the left hand side of the vessel at levels progressively towards the bottom of the vessel, due to the fact that the particles are constrained to remain on their respective nodal planes and therefore do not participate in the lateral movement. Particle concentration in effluent stream A is thus higher than in the other effluent streams, with concentration decreasing through effluent streams B, C and D.

As previously explained, the ideal operation of such an apparatus is precluded by the non-uniform nature of the acoustic field which results in acoustic hotspots. Particles collect on the hotspots and thus interfere with the movement of particles down the nodal planes.

By modulating the intensity of the acoustic field at a low frequency (for example, 0.5-1.0 Hz) using controller 153, particles may be released from the hotspots by the flow of liquid and are therefore free to resume their passage down the nodal planes when the acoustic field is returned to full strength. If the modulation frequency is too high, particles will be recaptured by their hotspots despite the downward flow, and if the frequency is too low. the particles released from the hotspot may not be retained by that node due to the lateral component of the flow. In addition to this effect, the modulation of the field also helps release particles from the boundary of the standing wave in the region of the outlet ports 121. The overall effect of the field modulation is to significantly increase the effectiveness of the process, as measured by the resulting particle concentration in the respective effluent streams.

According to the invention the effectiveness of the process is improved by oscillating vessel 120 in the acoustic field, using motor 152 in synchronisation with the modulation of the acoustic field. For example, controller 153 can provide a sinusoidal signal to the motor of an amplitude selected such that the vessel is reciprocated in simple harmonic motion with an amplitude equivalent to at least one internodal distance.

The acoustic field is modulated in synchronisation with the oscillation by the controller 153. When the vessel is moving from left to right, the standing wave is fully operational, and the particles are therefore maintained in their nodal planes but carried further to the left hand side relative to the position of the vessel itself. When the vessel reaches its furthest position the standing wave is reduced or extinguished for the vessel's travel from right to left, during which time the particles, no longer subject to the influence of the acoustic field, will move with the liquid. The field is then re-established to 'fix' the particles at their new nearest nodal plane. This cycle has two effects. Firstly, it greatly reduces the problem of hotspots, and secondly it results in many or most of the particles being shifted one (or more) nodes to the left and can be constantly repeated to more effectively concentrate the particles to the left of the vessel and further increase the particle concentration in effluent stream A. Clearly the pivoting motion of the vessel will not provide a true displacement in the direction parallel to the acoustic axis of the standing wave, but if the amplitude is such that all pans of the vessel 120 move by a distance equal to at least one internodal spacing, then the overall effect will be to move the particles at least to the next adjacent nodal plane to the left. For example, at an acoustic frequency of 1 MHz, taking the speed of sound in water at 20°C to be 1480 ms^"1, the wavelength λ is 1.48 mm. The internodal distance is therefore 0.74 mm, and the oscillation amplitude of the vessel at its uppermost point can be selected to be approximately this value, thus allowing the stepwise movement of particles leftwise to successive nodal planes. The speed of oscillation can be suφrisingly high, since acoustic forces holding the particles at the nodes are substantial. Generally, it has been determined that modulation frequencies from 0.1-10 Hz are appropriate for reducing the problem of hotspots, and these frequencies correspond to appropriate speeds of vessel oscillation.

In addition or as an alternative to the mechanical oscillation of the vessel, the modulation of the acoustic field can be synchronised with a modulation of the pumping from outlet ports B, C and D. This is readily achieved by means of the controller 153 selectively driving peristaltic pump 150 to selectively control the flow rates in effluent streams A, B. C and D in synchronisation with the modulation of transducer 134. The effect of this is to reduce the lateral flow component during the short period when the constraining action of the standing wave is reduced or eliminated.

Fig. 3 illustrates the modulation described above. The acoustic field modulation is shown in Fig. 3a, with the modulation voltage V_τ on the vertical axis against time on the horizontal axis. The modulation selected here is square wave full modulation, meaning that the field intensity is periodically switched from 100% to zero, although the field intensity need not be reduced to zero and other modulation waveforms can be used (such as a sinusoidal waveform). The modulation waveform in Fig. 3a is shown as a regular squarewave, but alternatively the periods when the field is switched off may be considerably shorter than the periods when the full field is established, to give an occasional periodic release of particles which may have gathered at field hotspots. Fig. 3b shows the pump operation as applied to effluent streams B, C and D, P_BCD on the vertical scale denoting pumping power in this stream. It can be seen that, in synchronisation with the field reduction, P_BCD is switched to zero (or alternatively may be reduced). Fig. 3c shows the angular displacement θ of the shaft of motor 152, the motor being arranged to produce simple harmonic motion of vessel 120. The left-to-right half of the waveform corresponds to the full amplitude of V_τ, whilst the right-to-left half corresponds to the suppressed period of V_τ. Clearly the sinusoidal waveform may be replaced by alternative forms of oscillation, and in some applications a generally saw-toothed waveform may be preferable, the slower periods of vessel motion corresponding to the full acoustic field to avoid particles being swept off the nodes as they are displaced relative to the fluid in the vessel.

In an experimental test a vessel 120 9.6 mm wide, 6 mm deep and 40 mm long, fabricated from Perspex with Mylar windows, with an entry slot 123 1 mm wide and four equal outlet slots 121 each 2.4 mm wide, was filled with water and immersed in a water- filled acoustic resonance cavity 145 in which was established a standing wave at a frequency of 2.2 MHz, generated by controller 153 feeding an electrical signal to transducer 134. The electric power delivered was 10 watts. A feedstock source of 7 micron polystyrene microspheres was connected to inlet slot 123 and liquid was pumped by pump 150 at a rate of 1.3 ml/min.

The concentration of particles in effluent stream A was analysed and found to be twice that of the feedstock entering the vessel at inlet slot 125.

The vessel was then oscillated at 0.5 Hz about point 151 in simple harmonic motion such that the midpoint of the vessel had an amplitude of 0.5 mm. The oscillation was carried out synchronously with the switching of the acoustic field, such that the standing wave was generated only when the vessel was moving towards the right. When the vessel was moved to the left, pumping from effluent streams B, C and D was interrupted to temporarily reduce the lateral component of the flow. These measures produced a significant increase in particle concentration in stream A. Whilst the apparatus and method of the invention have been described above in application of ultrasonic particle manipulation to a continuous, flow-through process, the invention can also be applied to batch processes, i.e. those in which there is little or no non- periodic relative movement between the liquid and the standing wave. Figures 4 and 5 illustrate embodiments of apparatus of this type. For convenience, similar elements and features to those illustrated in Fig. 2 are designated here generally by the same reference numbers, increased by 100 in the case of Fig. 4 and by 200 in the case of Figure 5.

Fig. 4 shows a rectangular section working vessel 220, having acoustic end windows 220a, positioned in a water-filled acoustic resonant cavity 245 formed by a metal acoustic coupling block 230, to which is attached an ultrasonic transducer 234, and by a reflector 231 , placed with its planar reflecting surface 233 parallel to the surface 232 of the block 230 and adjustable in axial position to allow the tuning of the cavity 245. Access to the vessel 220 is provided by four like slit ports 223a, 223b, 223c and 223d, each of which extend over the whole width of the acoustic windows to which they are inclined at a small angle. Vessel 220 is supported by rigid support member 260 which itself is supported and hinged by flexible member 261 at one end to a fixed mounting 262, and is arranged to be oscillated by an eccentric or cam 263 at the other. Flexible member 261, which may for example be a thin metal strip, therefore acts as a resilient hinge about which support member 260 can be rotated to cause movement of the vessel 220 through an arc, which approximates to axial movement over a short distance. Cam 263 has attached a further cam 264, operating a micro switch 265, which controls a signal from signal generator 266, to RF amplifier 267, which powers the transducer 234.

Slit ports 223a and 223b are linked hydraulically by way of flexible tubes 226a and 226b to a pump arrangement as shown, such that when liquid enters the vessel via port 223a an equal volume of liquid can be pumped simultaneously from port 223 b. This is achieved using two hypodermic syringes 250a, 250b operating back to back having pistons 255a, 255b rigidly linked by member 256. Alternatively, an appropriate peristaltic pump may be employed operating two similar channels arranged such that whilst one is pumping into the vessel, the other is pumping an equal quantity of fluid out of the vessel. The operation of the pump arrangement will be described below. In operation, the vessel 220 is completely filled with particle-bearing liquid via any of the ports, and the cavity 245 is filled with water. When a standing wave is established, and the reflector 231 is adjusted to produce a highly resonant acoustic field, particles in the vessel move to the nodes 225 of the standing wave. The particles are then 'rastered' along the axis of the vessel by the synchronous modulation of the acoustic field and the oscillation over a small section of an arc by the vessel 220 in a manner similar to that described in relation to Fig. 2. Fig. 4 shows a simple method of providing such a facility using the eccentric or cam 263, linked to a second cam 264 such that at the limits of the motion induced in member 260 and thus in the location of vessel 220, cam 264 operates micro switch 265 to provide a square wave switching of the input and thus the output of the RF amplifier 267. Rastering may be in either direction, but we will assume that the particles are moved towards the ports 223a and 223b, from right to left of the vessel in Figure 4.

When all the particles are constrained by the acoustic window at the left hand end of the vessel 220, a small volume of feedstock contained in syringe 250a is pumped through the slit port 223a to sweep the particles packed on the window down towards port 223b while syringe 250b rigidly linked to syringe 250a removes an equal volume of liquid, thus promoting the clean transfer of the particle concentrate from vessel 220 into syringe 250b from which it can easily be recovered.

A device of the type illustrated in Figure 4 was constructed and tested by the inventor. The acoustic reflector used was a brass block faced with a tungsten plate to improve reflectivity). The working vessel was 22mm long and 1.5 ml in volume, with acoustic windows of 12 micron Mylar and optical windows of 3 mm methyl acrylate, to allow observation and video recording of the apparatus in operation. The inlet/outlet ports were made of stainless steel. 1.5 ml of a dilute particle suspension (feedstock) was placed in the working vessel, and a standing wave at 2.5 MHz was applied, resulting in an intemodal distance in the aqueous suspension of 0.3 mm. The amplitude of the oscillation imposed on the vessel was 0.6 mm (twice the internodal distance) and the frequency of the oscillation was 1 Hz.

The number of nodes in the vessel was 73, so that 37 cycles were required to move all the particles to one end of the vessel. Operating at a frequency of 1 Hz this took 37 seconds, and after this period it was clear that almost all of the particles were contained within a thin layer on the acoustic window and the rastering was stopped. To remove the particles required the pumping of 0.3 ml of feedstock passed into the vessel via slit port 223 a, to sweep particles down the acoustic window to be pumped out at an equal rate from the bottom port 223b. The pumping into and out of the vessel was carried out using a double 1ml syringe system as described above and illustrated in Figure 4.

The feedstock contained 10,000 particles per ml. It was estimated that 90% of the

15,000 particles in the vessel were transferred to the syringe in 0.3 ml of feedstock. The number of particles recovered was therefore 13,500 + 3,000 = 16,500 in 0.3 ml, that is, a concentration of 55,000 per ml. The particle concentration was therefore found to be increased by a factor of 5.5.

In practice, the degree of concentration which can be achieved depends on the length of the vessel and the volume of the feedstock required to transfer the particles to the syringe.

Particles at nodes move along the plane of the node towards areas where the acoustic energy density is highest, so that after a few moments of having established an acoustic field particles are all in small groups at the nodes, each group being at an acoustic hotspot. While such hotspots can create problems in flow-through rastering concentrators, in batch concentrators such as that shown in Figure 4, where all the relative motion between sound field and liquid is substantially along the acoustic axis, hotspots are less of a problem and can even be helpful, since particle groups are easier to handle than individual particles. Since particles are not being moved along nodes, uniformity of acoustic field is not required and it may be that some advantage may be obtained by expressly working with non- uniform acoustic fields, e.g., those in the near field.

It is noted that it is also possible to apply the invention to moving particles along the nodal planes of an established acoustic standing wave, in order to help release particles held at hotspots. To this end, oscillation motion can be applied in a direction substantially normal to the acoustic axis of the standing wave, synchronised with a periodic suppression of the acoustic field.

The rastering concentrator shown in Figure 4 operates by the aggregation of particles at the acoustic window. In other words, the window removes particles from a node on each cycle as the window passes through the node. During this action, substantial forces which are generated in the standing wave press particles onto the acoustic window. While similar forces promptly act in the reverse direction, it is nevertheless sometimes difficult to clear particles off the window.

Since a batch raster concentrator may use a symmetrical mechanical oscillation such as simple harmonic motion, it is possible to collect particles at a central point rather than at an end window. To this end, the vessel 220 of Figure 4 can be divided into two halves arranged back-to-back and sealingly connected together by an extensible means, such as a gasket, at the centre. The vessel is placed in an acoustic resonant cavity as in Figure 4, but the two halves of the vessel oscillate over one internodal distance mutually 180° out of phase. As the two halves of the vessel move apart by, say, 0.5 mm (as would be appropriate at a standing wave frequency of 3 MHz), the acoustic field operates, but as they converge towards one another, the wave is extinguished or significantly reduced, so to carry particles in both halves of the vessel progressively from one node to the next towards the central region. An exit port in the base of the centre of the vessel allows liquid to move in and out of the vessel at each cycle. Near the centre of the vessel the relative liquid flow changes from being axial to one in which a lateral components rapidly becomes increasingly significant. At the centre, the relative motion is essentially parallel to the nodal planes as the direction of flow is into and out of the exit port. Such a device according to the invention is further described with reference to Figure 5. Referring to Figure 5, the working vessel is divided into two vessel halves 320a and

320b, each of which has an open end, the open ends being sealingly connected together by gasket 327 designed to accommodate an axial extension of about 0.5 mm. The gasket, which may be made from a standard elastomer such as silicon rubber, is thus fixed to the walls of the vessel halves to form a fluidtight seal while allowing a slight extension and compression in the axial direction of the vessel. This allows the vessel halves to oscillate 180° out of phase while maintaining the watertight integrity of the vessel. Vessel halves 320a and 320b are connected by members 360a and 360b via a hinge mechanism 361 to two mechanically or electronically linked cams (or alternatively eccentrics or stepping motors) 363a and 363b which provide the oscillating motion. The hinge strip is supported by a rigid mounting 362. Cam 363b also operates microswitch 365 which controls an RF signal from a signal generator 366 to amplifier 367 and thus to an ultrasonic transducer 334. The vessel 320a/320b is of square section and is fitted with slit ports 223a and 223b to allow feedstock filling and removal. During operation all four ports 223a and 223b are closed. A central exit port 321 is provided, and when open this port allows liquid to move in and out of the vessel 320a/320b during the out of phase oscillating motion. The nodal array 325a in vessel half 320a moves particles from left to right while the nodal array 325b in vessel half 320b moves them from right to left. In this way, at the completion of the rastering action, all the particles are disposed in the centre of the device (and not on an acoustic window or a vessel wall) and can be pumped out of the vessel via exit port 321. The resonant cavity is provided by means of an acoustic coupling block 330 to which is attached ultrasonic transducer 334 and an acoustic reflector 331 arranged plane parallel to member 330 and axially adjustable to allow the cavity to be tuned.

It is noted with reference to the above-described embodiments (Figs. 4 and 5) that whilst the oscillation motion of the vessel in the resonant cavity is preferably substantially in the direction of propagation of the standing wave, this is not essential to the operation of the device. If a component of the oscillation motion lies in this direction, then the rastering effect from node to node can be achieved. If the device is arranged to provide a component of motion peφendicular to this direction, such a mode of operation might be used to assist in aggregating particles in one corner of a rectangular vessel, for example.

Furthermore, although methods described above with reference to Figures 4 and 5 have been presented as applying to a batchwise process, it is of course possible to provide a feedstock flow though the vessel. For example, an axial flow can be superimposed on the axial motion due to the vessel oscillation.

The oscillation of the vessel may be a symmetrical mechanical reciprocation, such as simple harmonic motion, but the invention contemplates other modes of oscillation including non-symmetrical ones, such as saw-tooth oscillation. The desired mode of oscillation may be achieved, for example with reference to the embodiments illustrated in Figures 4 and 5, by modifying the form of the cam used to move the vessel.

A further form of apparatus is shown in Fig. 6. Two equal standing waves are established, crossing one another at 90°, by means of two acoustic sources 430 and two reflectors 431 , the positions of the reflectors 431 being adjustable to provide resonant conditions for both the standing waves. The standing waves may be equal, but can alternatively be arranged to differ in intensity and/or frequency. In Fig. 6, the standing waves are shown intersecting orthogonally, but they may cross at an angle other than the 90° illustrated. An acoustically transparent vessel 420 of square section is placed within the acoustic field. In this arrangement, the two sets of nodal planes 425 intersect at an array of nodal intersections 500, and when the vessel 420 is filled with particulate-carrying liquid the particle will therefore concentrate at these nodal intersections, where the acoustic energy is greatest. Hotspots are of relatively little concern with this arrangement, since the energy gradients in the nodal planes of one standing wave resulting from the intersecting nodal planes of the other standing wave are much higher than those provided by any non- uniformities in the acoustic field. The nodal intersections therefore dominate particle management.

By oscillating vessel 420 at an amplitude equal to the distance between nodal intersections and synchronously modulating one of the standing waves, particles can be made to move along the nodal planes of the other standing wave. The field modulation may be applied to one or both of the standing waves. The oscillating movement may be along one or the other of the acoustic axes or along the bisector of the angle between them. In the latter case, if this direction is vertical, and the two standing waves are at 45 ° to the vertical and thus mutually peφendicular, then both standing waves may be pulsed with a vertical oscillation of 0.7λ. In this way, particles may be moved towards and concentrated at the bottom apex of the vessel, where harvesting is relatively straightforward.

The invention may be applied to a wide variety of inorganic or organic particulate materials, and may be used in a laboratory or industrial process, either in a batch or continuous procedure. Examples of suitable applications include the separation of biological particles such as blood, viruses, bacteria, yeasts, animal and plant tissue cells, as well as the separation of water-borne mineral particles such as fine clays.

Claims

1. An apparatus for manipulating particles in a liquid medium by ultrasonic waves comprising a vessel for receiving the particle-carrying liquid, means for generating an ultrasonic standing wave in the vessel such that particles are attracted to nodal fronts of the standing wave, means for intermittently suppressing the standing wave, and means for oscillating the particle-carrying liquid relative to the standing wave.

2. An apparatus according to Claim 1 , wherein the means for oscillating the particle- containing liquid relative to the generation means comprise means for mechanically oscillating the vessel.

3. An apparatus according to Claim 1 or Claim 2, wherein at least a component of the oscillation motion is in the direction of propagation of the standing wave.

4. An apparatus according to any preceding claim, wherein means are provided for carrying out the oscillation in synchronisation with the intermittent suppression of the standing wave.

5. An apparatus according to any preceding claim, comprising means for providing a flow of the particle-carrying liquid through said vessel to afford relative movement between the liquid and the standing wave, at least a major component of said movement being in a direction peφendicular to the direction of propagation of the standing wave, such that particles attracted to the one or more nodal fronts of the standing wave are moved along the front or fronts.

6. An apparatus according to Claim 5, wherein said means for generating the standing wave is arranged such that the standing wave has an axis passing through a boundary wall of said vessel, said wall extending obliquely to the axis of the standing wave, such that the relative movement between the liquid and the standing wave brings particles attracted to nodal fronts of the standing wave towards said boundary wall.

7. An apparatus according to Claim 6, wherein the angle of intersection between the nodal fronts and the wall is substantially less than 45°.

8. An apparatus according to Claim 5, wherein the vessel has a flow inlet means and a flow outlet means for passing the particle-carrying liquid through the vessel, the flow inlet and outlet means being mutually arranged to produce a component of the relative movement between the liquid and the standing wave in the direction of propagation of the standing wave.

9. An apparatus according to any of Claims 5 to 8, wherein flow control means for controllably suppressing the component of the relative movement between the liquid and the standing wave in the direction of propagation of the standing wave are provided, said flow control means being operable in synchronisation with said means for intermittently suppressing the standing wave.

10. An apparatus according to Claim 9 including a plurality of flow outlets mutually spaced in the direction of propagation of the standing wave, the said flow control means comprising individual flow rate control means associated with the flows through each of said flow outlets.

1 1. An apparatus according to Claim 2 or any claim dependent thereon, wherein the means for oscillating the vessel comprises a motor device arranged and operated to rotate the vessel in a reciprocating manner.

12. An apparatus according to any preceding claim, wherein the amplitude of oscillation in the direction of propogation of the standing wave is approximately equal to an integer multiple of the internodal separation of the standing wave.

13. An apparatus according to any preceding claim, wherein the means for intermittently suppressing the standing wave comprises a square wave modulation means to successively reduce and re-establish the intensity of the ultrasonic standing wave in a regular manner.

14. An apparatus according to any of Claims 2 to 4, wherein the vessel comprises two vessel portions mutually spaced along the direction of propagation of the standing wave, the two vessel portions arranged to oscillate in like opposed manner.

15. An apparatus according to Claim 14, wherein a resilient sealing means is provided to sealingly interconnect the two vessel portions to enable them to displace relative to one another while retaining said particle-carrying liquid.

16. An apparatus according to any preceding claim wherein a means for generating a second ultrasonic standing wave in the vessel is provided, the two standing waves being mutually inclined.

17. An apparatus according to Claim 16, wherein the two standing waves are mutually orthogonal.

18. A method for manipulating particles in a liquid medium by ultrasonic waves comprising generating an ultrasonic standing wave in the liquid and intermittently suppressing said standing wave whilst oscillating the particle-carrying liquid relative to the standing wave.

19. A method according to Claim 18, wherein said vessel is oscillated to provide the oscillation of the particle-carrying liquid relative to the ultrasonic standing wave.

20. A method according to Claim 18 or 19, wherein said oscillation is carried out in synchronisation with the intermittent suppression of said standing wave.