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A fractionation method for fine-particle suspensions using resonant ultrasonic fields coordinated with bidirectional fluid flow fields is described. The basis for the separation is differences in the speed of response of particles to the imposition of a resonant acoustic field. As such, the method is sensitive to the particle size and the acoustic contrast between the solid particles and their suspending fluid. Both batch and continuous fractionation processes can be developed from a two-step acoustic-flow cycle. An analytical model was constructed from equations that describe the trajectories of particles as they respond to the acoustic and flow fields. Model predictions indicate how the fractionation can be controlled through choice of cycle parameters. The method was implemented experimentally. Results for the fractionation of 325-mesh polystyrene spheres indicate that sharp fractionations can be achieved. The experimental results are generally in agreement with the model predictions.

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Acoustics fields are being used to manipulate fine particles (0.1–20 μm) in liquid suspensions held within a thin rectangular channel. Ultrasonic fields are applied across the channel, resulting in particle migration in this direction. Particles with different physical characteristics (size, density, compressibility, shape) respond with different speeds to the sound field [Z. I. Mandralis and D. L. Feke, Fluid/Part. Sep. J. 3, 115–121 (1990)], resulting in partial separation across the channel. These partial separations are amplified into useful separations by applying cyclic, coordinated, bidirectional laminar flows. The response of the particles to the standing wave field will be described. A mathematical model that simulates the fractionation scheme has been developed. Results from the simulation demonstrate the effectiveness of the technique. Experimental results from a batch fractionation of 325 mesh polystyrene particles are presented. Scaleup and continuous operation schemes will be discussed.