Thomas J. Asaki Philip L. Marston
Phys. Dept., Washington State Univ., Pullman, WA 99164-2814
Acoustic levitation has been used as a method for measuring the frequency and dissipative properties of shape oscillation modes of drops and bubbles [P. L. Marston and R. E. Apfel, J. Acoust. Soc. Am. 67, 27--37 (1980); T. J. Asaki et al., J. Acoust. Soc. Am. 93, 706--713 (1993)]. One of the problems in the interpretation of such measurements is the effect of the levitating field. Some of the relevant issues for slightly deformed drops or bubbles, where the equilibrium shape is spheroidal and R is the radius of the sphere of the same volume, are examined. The interaction of shape oscillations with the external acoustic flow is approximated for a spheroid at the velocity antinode of a standing wave in the long-wavelength limit (kR->0) with the result that the effective restoring force for the quadrupole mode is increased. The curvature of the effective potential well is increased. The analysis makes use of the known solution for the potential flow past an incompressible spheroid in the kR->0 limit. [Work supported by ONR.]