Equilibrium shape of an acoustically levitated bubble driven above resonance

Abstract

The equilibrium shapes of stably levitated bubbles on the Earth in an ultrasonic standing wave have been measured and calculated for bubbles larger than resonance size. The shape and equilibrium position are sensitive to the acoustic field used for levitation. Measured static oblate shapes are in good agreement with predictions based upon the quadrupole projection of the radiation pressure on a compressible sphere taking into account the monopole response of the bubble. For the size range of bubbles considered, the monopole response is limited by inertia and radiation damping. A simplified theory (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 of the bubbles observed.