Cavitation cloud translation in focused ultrasound

Cavitation mediated effects in liquids exposed to ultrasound, play pivotal roles in a number of industrial arenas, including precision acoustic cleaning (megasonics) and sonochemistry. The spontaneous occurrence of cavitation, and the subsequent interaction with the liquid and the acoustic field, is however poorly understood, which prevents optimization for any given application. In this paper we report on observations made of single isolated cavitation-bubble clouds, exposed to a well characterized burst of propagating focused ultrasound, and the resulting translational motion of the clouds under the action of the primary acoustic radiation force. As may be expected, larger clouds develop under higher intensity insonations, which translate away from the ultrasound source more rapidly, although a larger associated drag force somewhat tempers the effect. Critically, however, a resonant condition is identified whereby small clouds at lower intensities translate much more rapidly than might otherwise be expected. A model is derived from first principles, adapted to the experimental conditions and demonstrates good agreement with the observations, including the frequency resonance. We anticipate the results will have significance for any application in which understanding and predicting a dynamic cavitating liquid is important, particularly under non-standing wave conditions.