Liquid purification or separation – Processes – Utilizing electrical or wave energy directly applied to...
Patent
1992-11-16
1996-01-16
Cintins, Ivars
Liquid purification or separation
Processes
Utilizing electrical or wave energy directly applied to...
210542, B01D 4300
Patent
active
054845378
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the manipulation of particles in an ultrasonic field.
It is well known that particles whose acoustic properties differ from those of the medium in which they are suspended, may move when exposed to an acoustic standing wave to form regions of high concentration at half wavelength intervals. By the use of a plane acoustic wave, it has been shown that particles can be concentrated in bands corresponding to pressure nodes or anti-nodes. Techniques have been described for the manipulation of biological cells in this way providing--for example--the separation of blood cells from plasma. Reference is directed in this connection to: Biology Society pp. 154-156 Peterson et al "Development of an Ultrasonic Blood Cell Separator". Cells in a Standing Wave Field".
In a typical known arrangement, bands of particles are formed along the axis of a cylindrical chamber and then by movement of the standing wave field by frequency or phase shifting, the banded particles are moved to a collection point. In a number of important applications, the speed at which the bands can be moved is considerably slower than would be wished.
It is an object of this invention to provide improved apparatus and method for the manipulation of particles in an ultrasonic field and, in an important example, to provide such method and apparatus which permits a more rapid collection of particles.
Accordingly, the present invention consists in one aspect in apparatus for the manipulation of particles in an ultrasonic field, comprising a chamber for containing the particles in fluid suspension, the chamber being at least in part defined by a cylindrical pressure-release wall such that acoustic fluid pressure is small at the fluid/wall boundary; acoustic transducer means adapted to generate an acoustic wave along the cylindrical axis through excitation of a single acoustic mode and transducer drive means, the arrangement being such that on excitation, the particles are radially displaced symmetrically of the cylindrical axis.
Preferably, the arrangement is such that on excitation, the particles are radially displaced inwardly to form a column along the cylindrical axis.
Advantageously, the acoustic transducer means is adapted to excite the lowest axially symmetric acoustic mode.
Suitably, the acoustic pressure amplitude at the fluid/wall boundary is no greater than 20% (and preferably no greater than 10%) of the amplitude on the cylindrical axis.
In one form of the invention, the cylindrical pressure-release wall is of circular cross section and the acoustic transducer means is adapted to produce an acoustic wave having a radial distribution of amplitude approximating to a Bessel function.
By the production of a single mode field, in contrast to the plane waves listed hitherto, the present invention enables the formation of an axial column of particles which, depending on the particle and field parameters, may be divided axially into bands. It is found experimentally that once particles have been formed into a column, it is generally possible to move them much more quickly in the axial direction than is the case with planar bands. It is believed that an explanation for this phenomenon is that particles concentrated in a radial column move in unison as a body with reduced viscous drag.
The radial displacement provided in the present invention may also permit radial separation of particles in accordance with particle size, density and adiabatic compressibility. It is also found that the use of a single-mode field substantially eliminates the acoustic streaming which is generated at the chamber wall if a plane-wave field is used.
In another aspect, the present invention consists in a method for the manipulation of particles in an ultrasonic field, comprising the steps of positioning the particles in fluid suspension within a cylindrical pressure-release wall such that acoustic fluid pressure is small at the fluid/wall boundary; and generating an acoustic wave along the cylindrical axis through excitation of a single acoustic mode
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Published WO application for PCT/GB87/00363 (Unilever Plc).
International Search Report list of references for PCT/GB91/00373 and Annex.
Jacobi, William J., "Propagation of Sound Waves along Liquid Cylinders", Journal of the Acoustical Society of America, vol. 21, No. 2 (Mar. 1949), pp. 120-127.
Peterson, Stephen et al., "Development of an Ultrasonic Blood Cell Separator", IEEE 8th Conf., 1986.
R. T. Beyer, Nonlinear Acoustics, (The Naval Ship Systems Command Dept. of the Navy, 1974) Chap. 9, pp. 309-311.
R. K. Gould and W. T. Coakley, Proc. of the 1973 Symposium on Finite Amplitude Wave Effects in Fluids, Ed. by L. Bjorno (Pergamon, Guilford, 1974), pp. 252-257.
W. T. Coakley, D. W. Bardsley, M. A. Grundy, F. Zamani, and D. J. Clarke, J. Chem. Tech. Biotechnol. 44, 1989, pp. 43-62.
Cintins Ivars
Public Health Laboratory Service Board
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