Energy focusing and cavitation erosion during a single bubble collapse

A cavitation bubble can focus kinetic energy of the surrounding liquid to the bubble center. A prominent effect is a violent bubble collapse with severe damage and erosion even of hardest materials. In contrary to common assumptions, we show that neither the jet that pierces the bubble during the wall-near bubble collapse, nor the ring collapse at the substrate damages hard surfaces. Instead, we demonstrate that cavitation erosion is caused by an additional energy self-focusing mechanism. This shock based self-focusing was revealed with microscopic high-speed imaging at 5 MHz framing rates combined with sub-picosecond exposure times. The correlation of in-situ and ex-situ surface damage with the location of shock-focusing proofs this novel mechanism.