Characterization of the bubble cluster and velocity field in the focal region of a lithotripter

The cavitation bubble cluster and the velocity field in the focal region of a lithotripter are investigated. Images of the bubble cluster were captured which show the bubbles increasing in size over approximately 500 mu s before collapsing rapidly. Velocity measurements were obtained using particle image velocimetry. A few milliseconds after the inception of cavitation the velocity field consisted of localized flows with maximum velocities and shear rates of the order of 100mm s(-1) and 50 s(-1) respectively. At times 40 ms after cavitation an average uni-directional flow of the order of 10mm s(-1) was observed. The mechanism by which the flow is generated is also considered.