Interaction of lithotripter-generated shock waves with air bubbles

Abstract

The shock wave‐induced collapse and jet formation of pre‐existing air bubbles at the focus of an extracorporeal shock wave lithotripter is investigated using high‐speed photography. The experimentally obtained collapse time, ranging from 1 to 9 μs for bubbles with an initial radius R₀ of 0.15 to 1.2 mm, agrees well with numerical results obtained using the Gilmore model. The collapse time is not linearly dependent on the initial bubble diameter since the temporal profile of the lithotripter 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 collapse. Bubbles of initial radii between 0.3 and 0.7 mm always induce perforation of the foil by the jet (hole diameter 80–300 μm). Averaging the jet flow speed over 5 μs immediately after the collapse results in velocities from nearly zero up to 210 m/s, depending on the initial bubble size, with a maximum at R₀=550 μm. This maximum is related to the temporal profile of the shock wave and to the effective cross section of the bubble for shock wave energy transfer. As cavitation bubbles are generated in the focal region of the lithotripter, the results are discussed with respect to the processes in a cavitation bubble field, which are of importance in cavitation erosion as well as in extracorporeal shock wave lithotripsy.