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 μs 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 100 mm s⁻¹ and 50 s⁻¹ respectively. At times 40 ms after cavitation an average uni-directional flow of the order of 10 mm s⁻¹ was observed. The mechanism by which the flow is generated is also considered.