Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D21/00—Separation of suspended solid particles from liquids by sedimentation
  • B01D21/28—Mechanical auxiliary equipment for acceleration of sedimentation, e.g. by vibrators or the like
  • B01D21/283—Settling tanks provided with vibrators
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D43/00—Separating particles from liquids, or liquids from solids, otherwise than by sedimentation or filtration
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/08—Processes employing the direct application of electric or wave energy, or particle radiation; Apparatus therefor
  • B01J19/10—Processes employing the direct application of electric or wave energy, or particle radiation; Apparatus therefor employing sonic or ultrasonic vibrations

Definitions

• 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.
• 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 inte ⁇ osed 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.
• 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.
• the method can be operated in a fully resonant acoustic field.
• 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
• 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.
• 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.
• 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 pe ⁇ endicular 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.
• this oblique arrangement which provides the lateral component of liquid flow relative to the orientation of the standing wave.
• 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 inco ⁇ orated. 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).
• the means for oscillating the vessel may comprise a motor device arranged and operated to rotate the vessel in a reciprocating manner.
• 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.
• 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.
• 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.
• the mechanical oscillation is carried out in synchronisation with the intermittent suppression of the standing wave.
• the acoustic field holds the particles to the nodes despite the forces provided by the relative movement of the liquid.
• 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.
• 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
• 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
• Fig. 6 shows an apparatus for providing two standing waves which cross one another at an angle of 90°.
• 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.
• 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.
• 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.
• 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.
• 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.
• the acoustic field can be modulated by the application of an intermittent reduction in intensity.
• 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.
• 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.
• 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.
• Fig. 1 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 longitudinal axis and walls of the flow-through duct are arranged substantially parallel to the opposed faces 132 and 133 (ie. substantially pe ⁇ endicular to the direction of propagation of the standing wave).
• the vessel 120 is disposed with its longitudinal axis vertical.
• 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.
• 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.
• 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 pe ⁇ endicular 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.
• 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.
• the intensity of the acoustic field 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.
• a low frequency for example, 0.5-1.0 Hz
• 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.
• 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.
• 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.
• 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.
• the wavelength ⁇ 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 su ⁇ risingly high, since acoustic forces holding the particles at the nodes are substantial.
• 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.
• 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 ⁇ 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. 3 illustrates the modulation described above.
• the acoustic field modulation is shown in Fig. 3a, with the modulation voltage V ⁇ on the vertical axis against time on the horizontal axis.
• the modulation selected here is square wave full modulation, meaning that the field
• 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 ⁇ 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 ⁇ , whilst the right-to-left half corresponds to the suppressed period of V ⁇ .
• 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.
• 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.
• pumping from effluent streams B, C and D was interrupted to temporarily reduce the lateral component of the flow.
• 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.
• 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.
• the vessel 220 is completely filled with particle-bearing liquid via any of the ports, and the cavity 245 is filled with water.
• 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.
• cam 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.
• 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
• 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.
• 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.
• the window removes particles from a node on each cycle as the window passes through the node.
• 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.
• 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.
• 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.
• 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.
• 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 pe ⁇ endicular 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.
• 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.
• FIG. 6 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.
• 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.
• 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.
• 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.
• 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.

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• 1997
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