MEASUREMENT OF ULTRASONIC FORCES FOR PARTICLE-LIQUID SEPARATIONS

The magnitude and direction of the ultrasonic radiation forces that ac t on individual particles in a standing-wave field were determined usi ng a microscope-based imaging system. The forces are calculated from m easured particle velocities assuming that the drag force, given by Sto kes' 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 energy de nsity, as predicted by theory. The magnitude of the transverse primary force was determined by two independent methods to be about 100-fold weaker than the axial force. Separation concepts exploiting the transv erse force for cell retention have been successful despite the great d isparity in magnitude between the axial and transverse-force component s. This may be explained by the reduced hydrodynamic forces on aggrega ted particles in transverse flow due to their alignment in the sound f ield.