Radiation force, equilibrium position, and shape for acoustically levitated bubbles driven above resonance

Abstract

The equilibrium positions and shapes of stably levitated bubbles on the Earth in an ultrasonic standing wave have been measured and calculated for bubbles larger than resonance size (radius≊140 μm). The method of levitating bubbles typically having radii ≳400 μm was previously described [T. J. Asaki, P. L. Marston, and E. H. Trinh, J. Acoust. Soc. Am. 93, 706–713 (1993)]. The equilibrium position gives a measure of the radiation force on the bubble. Trapped bubbles exhibit an oblate shape of primarily quadrupole distortion. They are positioned near to and above the axial pressure minimum in the levitation chamber. These results are in good quantitative agreement with predictions based upon the multipole projections of the acoustic radiation pressure on the bubble, taking into account the monopole acoustic response of the bubble. The monopole response is limited by inertia and radiation damping for the size range of bubbles considered. A simplified theory for the shape (based on the deformation resulting from the Bernoulli pressure of incompressible potential flow around a sphere) is generally consistent with the measurements for the smaller bubbles observed. [Research supported by ONR.]