GB2173420A

GB2173420A - Separation and mixing by sonic waves

Info

Publication number: GB2173420A
Application number: GB08509232A
Authority: GB
Inventors: John Brown Pond
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-04-11
Filing date: 1985-04-11
Publication date: 1986-10-15
Also published as: GB8509232D0; GB2173420B

Abstract

Because the generators of mechanical waves normally operate near resonance, waves of substantial asymmetry for movements of the transmitting medium in opposite directions are not normally encountered. However such waves can be very effective in promoting the drift of inhomogeneities or the interpenetration of substances. The different aspects of the invention consist of different methods of producing waves with substantial asymmetry designed for promoting drift or interpenetration or the results of those actions.

Description

SPECIFICATION Promotion of drift or interpenetration by the use of mechanical waves of special time variation Inhomogeneities in a medium supporting mechanical waves experience the oscillatory movements of the medium surrounding them and may slip. The surrounding medium will act to urge an inhomogeneity in one direction in one part of a wave cycle and in the reverse direction in the next part and if these actions are equal and opposite there will be no net displacement of the inhomogeneity gradually growing over many cycles. There may, however, be inequality for a number of reasons.

For example, due to the strength of the wave, conditions such as the size of the inhomogeneity or the viscosity of the surrounding medium may vary significantly during the wave cycle.

It may also occur if the wave is asymmetric for movements of the medium in opposite directions. Sine waves such as 'A' or 'B' in Fig.

I are symmetrical but consider for example the sawtooth shaped wave of velocity in Fig. II.

Here relatively high accelerations for a short time in one direction (region C-D) are followed by relatively low accelerations for a longer time in the opposite direction (region D-E).

Such asymmetric waves can promote cumulative movement (drift) of inhomogeneities, for example by breaking dynamic bonds when the force exerted by the medium is in one direction rather than the other. As a analogy for that consider a coin on a plank which is repeatedly jerked towards one and then slowly (so as not to disturb the coin) re-extended.

Over a number of cycles of this the coin will migrate (drift) towards the end of the plank.

Asymmetric waves can also affect unbound inhomogeneities in this sort of way since the effect of medium friction is not linear in relative velocity. In that connection it is germane to consider sawtooth-type waves of displacement having, therefore, unequal velocities for opposite directions. Asymmetrical waves having features of both types can be tailored and the best wave to promote drift in a given situation is a matter for design.

Now most generators of mechanical waves, whether natural or contrived, operate at or near resonance (electrical or mechanical or both). This is because near resonance reactive impedances tend to cancel out and the production of oscillations or waves is much easier, particularly at high amplitudes. This implies waves of sine or near-sine shape and it follows that markedly asymmetric mechanical waves are not normally encountered. The novelty of the present invention is the deliberate production of such waves, in special generators or applicators intended to promote interpenetration or drift, by one of the three methods explained below.

Using approximate terms the methods may be characterised as 1. Non-resonant wave generator with the asymmetry placed in its drive (claim 2 refers to this) 2. Wave generator in which the asymmetry is produced by wave passage thought an intervening medium on the way to the target.

(claim 3 refers to this) 3. The combination of waves by various methods so that their combined disturbance in the target produces the required asymmetry (claim 4 refers to this).

In order to illustrate and contrast the methods they are described as applied to phonephoresis in which ultrasound of low megahertz frequency is applied to promote the penetration of topically applied medicines through body tissues. A number of attempts to achieve useful effects in this context have been published but in the prior art there has been no deliberate attempt to produce asymmetric waves. Previously published results have often been disappointing (such success as has been obtained being probably due to the chance production of significant asymmetry by the action described in claim 3). To apply the first method to this problem one design may proceed as follows.

Take a PVDF film transducer, electroded and poled for thickness electrostriction as commonly used in ultrasonics and cement it to a brass plate with the rear electrode in electrical contact (respectively 'A' and 'B' in Fig. III).

More powerful action can be obtained with thicker films but it is counter-productive if more than a half wavelength thick and the film may well be a quarter wavelength thick or thinner. 'E' is a plastics cover, in mechanical contact with the front face of the transducer (assured, for example by moistening with glycerine) allowing the passage of waves but protecting the front electrode from abrasion.

The electrical connector 'D' allows connection to the two electrodes via the plate and the wire 'C'. The transducer is driven via a power amplifier which is fed with a signal containing the desired asymmetry. The signal may be produced by methods which are commonplace in electronics practice. For example a sawtooth shaped voltage drive may be obtained from the normal timebase circuits used in cathode ray oscilloscopes.

To apply the second method to the phonophoresis problem, one design may proceed as follows (as is the example of the first method above and the third method which follows this, the designs are to show the principles and are simple rather than optimal). Refer to Fig. IV. 'A' is a transducer disc of PZT4 ceramic held, together with two lengths of glass tubing 'C' and 'D' by epoxy resin 'B' Wires 'E' and 'F' are soldered to the front and rear electrode faces of the disc and go to a con nector held in the rear end of 'D' by more resin. The inside of 'D' forms an unsealed air chamber which reflects the waves from the back of 'A'.The inside of 'C' forms a water column waveguide, the front end being sealed by a membrane 'G' which allows the passage of the waves. 'A' is normally driven near resonance (series resonance) for best efficiency and 'C' is long enough for the desired asymmetry to develop. Note that the rate at which asymmetry develops, per unit length of wave track, increases with wave amplitude and frequency. In the design currently used by the inventor the wave amplitude is around 1 bar (105 Pa.), the frequency 1 MHz. and the desired asymmetry is obtained with 'C' of length 0.15 m.

To consider the third method, first refer to Fig. I. 'A' and 'B' are two sine or near-sine waves but B is at double frequency and in reverse phase. If these waves are added in a medium then the combined disturbance 'A+B' is asymmetric for medium action in one direction (region F-G) and the reverse (region G-H). These regions are analogous to 'C-D' and 'D-E' in Fig. II. Fig. V shows the principles of an arrangement applied to the phone- phoresis problem. Here the front wall 'A' of a brass housing is a half wavelength thick for waves at 2 Mhz. and thus acts as a window for these waves when resonating. 'B' is a PZT4 transducer disc also half-wavelength thickness resonant at 2 MHz. while 'C' is a similar disc half-wavelength thickness resonant at 1 MHz.Using the major face electrodes, 'B' is soldered to the inside (rear) face of 'A' and 'C' is soldered to the rear face of 'B'. Wires 'D' and 'E' are soldered to the electrical junction 'BC' and the rear face of 'C' and together with the housing contact mean that 'B' and 'C' may be independently electrically driven through the three way connector 'F' mounted on the housing rear cover 'G'. Not only is 'A' a half wavelength window for the 2 MHz waves from 'B' but A+B forms such a window for the 1 MHz. waves from 'C'. Both waves are reflected by the air interface at the rear of 'C'. The couple D-E cannot be earthed to the housing and is driven by a floating transformer secondary.

To keep the required phase relationship between the waves and to be able to vary it as required a signal from the 2 MHz drive for 'B' is taken first through a divide-by-two circuit and then through a variable analogue delay circuit before being used to feed the power amplifier used to drive 'C'. Note that the use of the variable phase delay allows one to cancel or reverse the asymmetry and hence the direction of drift promotion.

Note that the example of ultrasonic phonophoresis is not the only field of use for the principles disclosed above which can be applied at different frequencies for the separation or promotion of many gas or liquid born mixtures. The third method may use two of more separate generators in arrangements such as sketched in Figs. Vl and VII. They may be quite conventional, for example moving coil loudspeakers, and only conventional electronics is needed for driving and phase maintenance.

Claims

1. A method of promoting the drift of elements of one substance or phase in another by the use of asymmetrical mechanical waves

2. A method according to claim 1 where asymmetry is applied in the drive for the wave generator

3. A method according to claim 1 where asymmetry is produced by the passage of the waves through an intervening medium on the way to the target

4. A method according to claim 1 where the asymmetrical waves in the target are obtained by the addition of waves of chosen frequency and phasing. By changing the phasing the promotion can be cancelled or reversed at will. By the same means the normal 'radiation pressure' may be negated.