HIFU-induced cavitation bubble cloud dynamics and its role in model kidney stone erosion*

Cavitation bubble clouds generated by high-intensity focused ultrasound (HIFU) contribute to non-invasive clinical procedures to erode bodily tissues up to ablation. Independently of the ultrasound energy, the role of the bubble clouds themselves in the erosion process is not clear, and the physics governing the cloud dynamics remains elusive. We herein present experiments of the bubble cloud dynamics under HIFU in (i) a boundary-free environment, and (ii) with a stone in the focal region of the ultrasonic transducer. In both cases, intricate time-dependent cloud morphologies are reported. This partially results from the convergence and merging of sub-clouds under the acoustic forces. In the absence of a stone, the monotonic trajectories of the clouds are enveloped within the ultrasound beam. The presence of a stone in the focal region however generates a standing wave which modifies the canonical cloud trajectories observed. When cavitating close enough to the focal region, clouds violently hit the stone and hammer the surface. This results in a suspended layer of fragments that experiences a gravity-driven instability once the ultrasound is turned off. These results are supported with high-speed conventional and X-ray imaging, as well as microscopy of the damaged stones.