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The magnitude and direction of the ultrasonic radiation forces that act on individual particles in a standing-wave field were detemined using a microscope-based imaging system. The forces are calculated from measured particle velocities assuming that the drag force, given b>, Stokes' law, is exactly counterbalanced by the imposed ultrasonic forces. The axial primary radiation force was found to vary sinusoidally with axial position and to be proportional to the local acoustic eneqy density, as predicted by theory. The magnitude of the transverse primary force was detemined by two independent methods to be about 100-fold weaker than the axial force. Separation concepts exploiting the transverse force for cell retention have been successful despite the great disparity in magnitude between the axial and transverse-force components. This may be explained by the reduced hydrodynamic forces on aggregated particles in transverse flow due to their alignment in the sound field. IntroductionThe search for novel solutions to particle-liquid separation problems has engendered a renewed interest in systems that exploit the acoustic forces on particles suspended in a standing ultrasonic wave field. Such systems are distinguished by the fact that they require no physical barrier for separation and no chemical flocculants to enhance aggregation and sedimentation. Although the aggregation of particles in ultrasonic standing waves was first observed more than a century ago (Kundt and Lehmann, 1874), most practical applications and implementations have only recently been identified. A pseudo-standing-wave resonator using modulated ultrasound (Whitworth et al., 1991) and a drifting-field resonator (Benes et al., 1991) were designed to concentrate solid particles by driving them to one end of a chamber. In the inched resonator of Frank et al. (19931, the

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