Shape oscillations of bubbles driven by modulated ultrasonic radiation pressure: The complex frequency response.

Abstract

The shape of a bubble in water should change in response to the radiation pressure of an ultrasonic wave. Modulation of the radiation pressure at the resonance frequencies for shape oscillations should facilitate the stable excitation of such modes [P. L. Marston, J. Acoust. Soc. Am. 67, 15–26 (1980)]. Levitation and detection of shape oscillations were previously discussed for bubbles in the size range 1–5 mm diameter [P. L. Marston etal., J. Acoust. Soc. Am. 89, 1885(A) (1991)]. Levitator design improvements have allowed stable trapping and shape oscillation detection for bubbles in the size range 1–12 mm diameter. The quadrupole mode was observed with resonance frequencies in the range 400–15 Hz. Oscillations over a more limited range were studied with a laser light scattering method that allowed the response phase and amplitude to be measured. The phase is referenced to the amplitude modulation of the acoustic field. Further experiments have involved the effects of surfactants, levitation of thin liquid shells, splitting of bubbles, and surface instabilities. [Research supported by ONR and NASA.]