  This work investigates the fundamental role of cavitation bubble clouds in
stone comminution by focused ultrasound. The fragmentation of stones by
ultrasound has applications in medical lithotripsy for the comminution of
kidney stones or gall stones, where their fragmentation is widely assumed to
result from the high acoustic wave energy. However, high-intensity ultrasound
can generate cavitation which is known to contribute to erosion as well and to
cause damage away from the target, although the exact contribution of
cavitation remains currently unclear. Based on in-situ experimental
observations, post-mortem microtomography and acoustic simulations, the present
work sheds light on the fundamental role of cavitation bubbles in the stone
surface fragmentation by correlating the detected damages to the observed
bubble activity. Our results show that not all clouds erode the stone, but only
those located in preferential nucleation sites whose locations are herein
examined. Furthermore, quantitative characterizations of the bubble clouds and
their trajectories within the ultrasonic field are discussed. These include
experiments with and without the presence of a model stone in the acoustic path
length. Finally, the optimal stone-to-source distance maximizing the
cavitation-induced surface damage area has been determined. Assuming the
pressure magnitude within the focal region to exceed the cavitation pressure
threshold, this location does not correspond to the acoustic focus, where the
pressure is maximal, but rather to the region where the acoustic beam and
thereby the acoustic cavitation activity near the stone surface is the widest.
