A method for separating particles in liquid by exploiting the competition between acoustic radiation force and electrostatic force is investigated. The displacement of particles from the pressure node of an ultrasound standing wave varies according to the effective charges, radii, and stiffness of the particles, and the particles of different radii can thus be separated even when they consist of the same material. When the particles are of different materials, they can also be separated in relations to their effective electric charges or their stiffness. In this study, the efficacy of this method was theoretically estimated and demonstrated by an experiment, with a mixture of polystyrene spheres of different radii in water, using 500-kHz ultrasound and a 3.3-V/mm electric field. The measured ratio of the displacements of polystyrene spheres 10 and 20 μm in diameter from the pressure node was 2:3, which is consistent with the value predicted theoretically. Radiation forces on the polystyrene spheres were also measured by using this technique.
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