Nonlinear aspects of the acoustic levitation of compressible spheres

Abstract

The use of acoustic levitation cells has become increasingly popular in both basic research and industrial materials processing. Most theoretical studies of the forces involved have assumed a linear response of the levitated object to the acoustic field. In this study some of the effects of the nonlinear responses of highly compressible spheres (i.e., gas bubbles in a liquid) on the time-average radiation force are described. The results show that, for example, as the pressure amplitude increases (1) the radius at which the force changes sign decreases; (2) the positions in the field at which the force is maximal shift relative to the positions of the maxima determined using linear theory for radii less than the fundamental resonance; (3) at a fixed position in the field, harmonic resonance responses produce local maxima in the acoustic force. The presence of the higher-order harmonic resonances also may be seen in calculations of the levitation number for a suspended bubble. Implications for levitated liquid drops will be discussed. [Work supported by NIH.]