The final stage of the collapse of a cloud of bubbles close to a rigid boundary.

The final stage of the collapse of a hemispherical cloud of bubbles close to a rigid boundary was investigated by ultra high-speed photography with up to 200 million frames/s. Our investigations reveal two types of secondary shock wave emission during cloud rebound. In the first case, the secondary shock wave emission is a consequence of the free collapse of a bubble within the cloud by the ambient pressure in the fluid. In the second case, it is a consequence of the interaction of the cloud-collapse-induced shock wave with microbubbles situated close to the collapse site of the cloud. The latter can be very powerful, resulting in a secondary shock wave emission with a maximum amplitude of about 0.5 GPa.