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Abstract Particles suspended in either travelling or standing wave fields experience non‐zero time averaged radiation forces. In a standing wave field such forces can result in migration of the particles to preferred positions separated by distances of half an acoustic wavelength. Radiation forces also give rise to either interparticle attraction or repulsion and can exert a torque on suspended particles. The forces depend on some or all of the following properties (i.e. particle size and shape, sound frequency, the square of the sound pressure amplitude) and on the differences between the density and compressibility of the particles and those of the suspending phase. The forces acting on suspended particles are analysed and threshold pressures for the migration of particles to half‐wavelength separations are derived. It is shown that appropriate choice of sound pressure amplitude should lead either to separation of particles of different densities and compressibilities in the sound field or to concentration of mixed samples essentially at the same location. Experimental observations of ultrasonic radiation force effects on cells are reviewed as are applications of radiation forces to effect cell concentration in electrofusion procedures and the use of ultrasound to support cells away from solid surfaces. Examples of cell concentration, cell alignment and cell‐cell interaction are presented.
Coakley et al. (Sun,) studied this question.