APPARATUS FOR THE GENERATION OF ULTRASONIC FIELDS IN LIQUIDS

Ultrasonic propagation in liquids has been intensively investigated and is well understood. The phenomenon of very high frequency acoustic radiation forces causing particle migration and concentration at half wavelength intervals in standing waves has led to methods of biological cell manipulation.

A number of small-scale biological and industrial particulate separation methods have been published. Two aims are satisfied in these approaches: first, the concentration and detection of very low concentrations of disperse particles for various diagnostic procedures and, second, the removal of small to large concentrations of particles in order to purify the suspension.

In most cases, the methods employed to generate the high frequency (250KHz - 3MHz) sonic fields have been based on either piezoelectric or magnetostrictive transducers. As yet, the methods disclosed are very much restricted to small sample volumes because, currently, no satisfactory method has been developed to generate the high power, high frequency, acoustic forces over a wide area necessary to process large sample volumes. The poor efficiency and unsuitability of piezoelectric and magnetostrictive methods, currently favoured, has retarded the large scale development of the method. Hence, water and effluent treatment (purification) is, as yet, beyond the scope of these methods.

To produce very high energy acoustic fields requires the generation of longitudinal pressure 'P' waves in the fluid since shear propagation in fluids tends to be poor. The common method is the coupling of a mechanically displacing surface at the fluid boundary and, in the methods outlined above, this is done typically with an 'expander' type piezoelectric

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transducer which vibrates at its resonant frequency when driven by an external A.C. signal, or when incorporated into an oscillator circuit. However, power input and coupling efficiency is not good and these transducers are inappropriate for large scale applications.

In considering alternative transducers for high power, high frequency generation for water and effluent treatment, there remains the problem of process convenience. Any system envisaged should ideally be able to operate, without the need to change the method of sample transportation or containment.

It is one object of this invention to provide improved apparatus and a method for the generation of acoustic fields in liquids, which is capable of generating large volume fields at high power levels, efficiently and conveniently.

Accordingly, the present invention consists in one aspect in apparatus for the generation of acoustic fields in liquids, comprising a displaceable first electrically conducting plate mechanically coupled with the liquid, a second electrically conducting plate positioned in capacitive relationship with the first plate, and power supply means for supplying high frequency electrical power to the capacitive plates to effect periodic displacement.

Preferably, the first displaceable plate comprises a thin metal plate fixed at its margin and the second plate is relatively thick.

Suitably, the power supply means comprises a radio frequency power amplifier and a D.C. source.

Advantageously, the apparatus forms part of suspended particle separation means comprising a liquid chamber in association with said displaceable plate and means for directing away from the liquid, particles moving in said acoustic field.

In another aspect the invention consists in a method for the generation of acoustic fields in liquids comprising the steps of coupling with the liquid one displaceable plate of a plate capacitor arrangement and supplying high frequency electrical power to the capacitor to effect periodic displacement.

One requirement is to provide a method of displacing a surface with sufficient amplitude at megahertz frequencies over a long liquid carrying conduit such as to cause a standing wave to occur transversally to the long axis of the conduit. This means that one expects to sweep particulate matter (> 0.1 micron) up to the flow boundary at the top of the conduit and then bleed off that part of the flow at successive points. The sweep is achieved by amplitude or frequency modulating the radio-frequency signal applied to the transducer.

A fixed potential is applied to.the capacitor(s) and a high frequency signal is superimposed by capacitive coupling to the plates. If, say, a 2000 volt D.C. potential is modulated by a 1000 volt peak to peak A.C. voltage, the forces experienced across the plates will become dynamic. In short, assuming one plate is rigid, the other flexible plate will vibrate at the frequency of the applied A.C.

Consider an air spaced electrode pair (capacitor) where the capacitance is given by:

E„ E _r A

C =

where C is the capacitance in Farads, E ₀ the permittivity of free space, E. the relative permittivity (air = 1) , A the

electrode area, and d the electrode spacing. Transposing gives:

E ₀ E _r A d =

but, in a capacitor the capacitance is given in Farads (Coulombs per Volt) viz; C = Q/V where Q is the charge and V the applied voltage. Thus substituting;

E ₀ E _r A V d = 3

It follows then that if a parallel plate capacitor is designed with one plate fixed (i.e. very thick) , and the other is flexible, then by varying the potential across the plates a force will be felt between the plates so long as the potential varies the charge on the plates. This is due to the direct electrostatic attraction across the plates between +ve and -ve charges. Let a charge of l micro Coulomb exist on each plate spaced within 0.005 meter, over an area of 1 m ² , then the force by Coulomb's law is;

d =

47T E _n d' 1

substituting values,

F = 360 Newton.

Thus it is seen that small charges can produce large forces. Given Q = VC, a 1 nanofarad capacitor charged with 1000 volts, will produce the charge of 1 micro Coulomb as in the example above.

Displacement is determined by modulating the applied D.C. with an A.C. voltage of appropriate magnitude and frequency. The effect is for the standing force of attraction between the two plates set up by the charge to be nulled by the applied A.C. The mechanical displacement is small across a large area.

Let a parallel plate capacitor of unit area be charged by a 2000 volt D.C. potential. The A.C. varies the D.C. by 1000 volts. Calculation using equation 3 gives a displacement of approximately 2 x 10 ^"8 m.

At 1 MHz a capacitor of the dimensions given has a reactance X _c of l/2π fC which is approximately 159 Ohms. This low reactance means that even with modest voltages the current, and hence charge and force, is substantial.

The invention will be further described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a part diagrammatic circuit diagram illustrating the present invention;

Figure 2 is a power supply graph;

Figure 3 is an exploded view of one embodiment of this invention;

Figure 4 is a sketch illustrating the use of apparatus as shown in Figure 3 as part of a particle separator; and

Figure 5 is a sketch view of a further embodiment of this invention.

With reference to Figure 1, the capacitive transducer 10, comprising a thin flexible plate 12 and a thick rigid plate 14 is held at a constant bias voltage by means of a D.C. source 16. A radio frequency circuit comprises an oscillator 18 capable of being tuned in the range 500KHz to 5MHz, which in

turn drives a radio frequency power amplifier 20 either directly or though a step-up transformer. This is connected to plate 14 of the capacitor 10 via one or a series of external coupling capacitors C. These serve to block the high voltage D.C. from appearing at the output of the R.F. amplifier. The other plate 12 connected to ground in a path which may include a current limiting resistor R. An inductor L may also be used in the ground path to form a series tuned circuit to minimise impedance losses.

The nature of the resultant voltage applied to the capacitor is shown in Figure 2.

The air spaced capacitor formed by the displaceable plate and the fixed plate requires that attention be given to the reduction of dielectric breakdown, due to moisture etc, in the air gap between the two. Furthermore, the fixed electrode must remain virtually static whilst the thinner conduit electrode displaces. One possibility is to form a series of independent separation columns all served by individual sealed capacitive transducers.

A.preferred form of capacitive transducer is shown in Figure 3. A relatively thick, circular plate 30 serves as the base of the transducer 32 and forms the fixed electrode. An annular spacer assembly 34 is suitably constructed from rigid polymer and supports the margin of the thin electrode plate 36. The sealed void 38 between the plates 30 and 36 contains dry nitrogen.

A separation plant utilising the transducer at Figure 3 is illustrated schematically in Figure 4.

Six separation columns 50 to 60 are arranged in two groups of three. Each column has at its base a transducer 32 of the form shown in Figure 2. Column 50 has a flow inlet 62

at the foot of the column and a radial filtrate outlet 64 towards the top. A concentrate outlet 66 pipe extends axially downward into the chamber.

Preferably, the form and dimension of the transducer 32 and column 50 are selected in accordance with the disclosure of W091/13674, to which reference is directed. In this way it can be arranged that on powering of the transducer 32, particles within the liquid flow congregate axially as shown scnematically at 68 and can be moved upwardly in a travelling acoustic wave for collection by the concentrate outlet pipe 66.

The structure of the remaining columns 52 to 60 is analogous and will not be described further. The manner of interconnection of the columns is as follows. The concentrate outlets of columns 50, 52.and 54 are connected in parallel to feed the flow inlet of column 56. Similarly, the concentrate outlets of columns 56, 58 and 60 are connected in parallel to feed a particle outlet 70. The filtrate outlet 64 of each of columns 50, 52, 56 and 58 forms the flow inlet of the next column. The filtrate outlet 64 of each of columns 54 and 60 is connected to a further processing equipment, as appropriate.

An alternative approach is appropriate where acoustic wave generation is required along the length of a conduit. Here, the thin electrode may form part of the conduit wall.

Referring to Figure 5 an electrode plate 100 forms a thin walled section extending axially of the conduit 102. The remainder 104 of the conduit may be formed of polymer or even of the same thin metal wall. The electrode plate 100 will usually be grounded but can be covered with an insulating layer to prevent conduction to the liquid.

The thick plate 106 may be arcuate as shown or have a half round upper surface to conform to the wall section 100.

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An insulating gap is provided in the form of a simple air gap or an evacuated or nitrogen filled insulating chamber. The plates 100 and 106 are held in suitable sealing strips 108.

Figures 4 and 5 show different geometries for particle separation. In Figure 3, as has been described each column has an independent standing wave generated by the capacitive transducer situated at the base. The wave is modulated to form a central core of concentrated particulate matter. The flow stream forces purer fluid successively through each column and the particulate matter is fed into coaxial tubes which pass concentrated contaminating particles into a series of further columns, eventually all the particulate matter is concentrated and harvested or discarded as appropriate. In the conduit form of Figure 5, the fluid passes down a series of vertical sound fields and is swept to the top of the flow stream. Here, the concentrated stream is bled off and passed for further processing in the same way. A variety of further configurations are of course possible.

The two plates of the capacitor may optionally be separated by a solid or liquid dielectric sufficiently flexible to accommodate the desired flexure of the thin plate. This will be relatively small; typically less than one micron. Displacement in ways other than flexure of a thin plate held at its margin can easily be envisaged. The other plate is preferably rigid but the alternative exists of two displaceable plates having a common rigid mounting and associated with respective liquid chambers.

Whilst this invention is primarily concerned with the use of acoustic fields for the harvesting of suspending particles, the purification of liquids by removal of suspended particles, or in general the separation of phases by density, it will find other applications in the generation of acoustic fields, standing or travelling, in liquids.