Ultrasonic separation of suspended particles

The forces on suspended particles in acoustic fields are reviewed briefly and the theoretical modelling of ultrasonic separators based on piezoelectrically excited layered resonators is described. Two flow-through resonator chamber concepts for ultrasonic particle (bio-cell) separation are investigated: (a) the coagulation or sedimentation approach, (b) the so-called h-shaped ultrasonic separator. The h-shaped ultrasonic separator is analysed by combining for the first time the mathematical modelling of the laminar flow with the acoustic force based velocity field of the particles relative to the suspension medium. This allows a complete modelling of the resonator's particle separation performance. Examples for separation chamber designs optimized by use of the mathematical model are presented and the calculated particle traces in the h-resonator are shown and compared with experimental results. For direct comparison of different ultrasonic flow through separator concepts a separation performance figure is introduced and its value is given for the two investigated separator concepts for the sample suspensions of polystyrene spheres, yeast and spirulina cells in (salt) water. The presented results are of importance for the state of the art design of acoustic cell filters for perfusion type bioreactors, as recently launched at the biotechnology market, as well as for the ultrasonic separation of plant (algae) cells under low gravity conditions, where the sedimentation concept fails.

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