Motion of Bubbles in a Moderate Sound Field

Abstract

A standing acoustic wave in a liquid exerts on a bubble a net translational force that, under proper conditions, propels the bubble through the liquid. Measurements are presented of the velocity at which bubbles, driven below resonance, are drawn toward an acoustic pressure antinode of a standing wave in a cylindrical container. Bubble velocities, ranging to 25 cm/sec, were measured by photographing the bubble under stroboscopic illumination as it moved in the sound field. Bubble radii ranged from 20 to 80 μ, and pressure amplitudes from 0.0 to 0.9 bar. It is found that, if the acoustic pressure and its gradient are known accurately as a function of position, then the observed bubble motion agrees with the motion predicted analytically. Experimental results also indicate a threshold for an instability in the rectilinear motion of the bubble. This threshold appears to be identical with the threshold for the observed onset of dancing motion of stationary bubbles trapped in a sound field. Theory indicates that this instability threshold is caused by the onset of nonspherical oscillations of the bubble. [Work supported by the Office of Naval Research.]