Abstract:

The acoustic radiation force on polystyrene spheres was measured through observation of the sphere movement in a 500-kHz ultrasonic standing wave. As predicted by Yosioka, the linear dependency of the force on the cube of the sphere radius began to fail when the sphere radius was below 5 (mu)m. This failure can be accounted for by the presence of a shell layer that increased the effective radius of the sphere and it suggests that acoustic radiation might be used in handling microspheres smaller than previously thought [K. Yasuda and T. Kamakura, Appl. Phys. Lett. (in press)]. A method for concentrating particles using an acoustic radiation force by the superposition of the higher harmonics on fundamental ultrasound has also been investigated. The efficiency obtained when blood cells are concentrated by using the superposition method should theoretically be 20% better than that obtained when using the sine wave. Although the improvement obtained experimentally was only 5% because the waveform was incomplete, the potential usefulness of the superposition method has been demonstrated. [K. Yasuda, Jpn. J. Appl. Phys. 36, 3130--3135 (1997)].