INTERACTION OF LITHOTRIPTER-GENERATED SHOCK-WAVES WITH AIR BUBBLES

The shock wave-induced collapse and jet formation of pre-existing air bubbles at the focus of an extracorporeal shock wave lithotripter is i nvestigated using high-speed photography. The experimentally obtained collapse time, ranging from 1 to 9 mus for bubbles with an initial rad ius R0 of 0. 15 to 1. 2 mm, agrees well with numerical results obtaine d using the Gilmore model. The collapse time is not linearly dependent on the initial bubble diameter since the temporal profile of the lith otripter wave contains a stress wave. The bubbles, positioned below a thin plastic foil, show strong jet formation in the direction of wave propagation with peak velocities of up to 770 m/s at the moment of col lapse. Bubbles of initial radii between 0.3 and 0.7 mm always induce p erforation of the foil by the jet (hole diameter 80-300 mum). Averagin g the jet flow speed over 5 mus immediately after the collapse results in velocities from nearly zero up to 210 m/s, depending on the initia l bubble size, with a maximum at R0=550 mum. This maximum is related t o the temporal profile of the shock wave and to the effective cross se ction of the bubble for shock wave energy transfer. As cavitation bubb les are generated in the focal region of the lithotripter, the results are discussed with respect to the processes in a cavitation bubble fi eld, which are of importance in cavitation erosion as well as in extra corporeal shock wave lithotripsy.