Multiple observations of cavitation cluster dynamics close to an ultrasonic horn tip.

Bubble dynamics in water close to the tip of an ultrasonic horn (∼23 kHz, 3 mm diameter) have been studied using electrochemistry, luminescence, acoustics, light scattering, and high-speed imaging. It is found that, under the conditions employed, a large bubble cluster (∼1.5 mm radius) exists at the tip of the horn. This cluster collapses periodically every three to four cycles of the fundamental frequency of the horn. Following the collapse of the cluster, a short-lived cloud of small bubbles (each tens of microns in diameter) was observed in the solution. Large amplitude pressure emissions are also recorded, which correlate temporally with the cluster collapse. Bursts of surface erosion (measured in real time using an electrochemical technique) and multibubble sonoluminescence emission both also occur at a subharmonic of the fundamental frequency of the horn and are temporally correlated with the bubble cluster collapse and the associated pressure wave emission.