DAMAGE OF SOLID-SURFACES BY SINGLE LASER-PRODUCED CAVITATION BUBBLES

The erosion of solid surfaces by single cavitation bubbles is investig ated. To this end, a bubble of given size is generated by focusing an intense laser pulse into water at a given distance from the surface of a pure aluminium specimen. The bubble dynamics, in particular the for mation of a fast liquid jet, is observed with a high speed camera. Wit h an initial bubble distance from the surface of 2.4 mm, a shallow pit is observed on the surface. Yet, at a reduced bubble distance, a circ ular damage structure without a central pit is produced. As shown by h igh speed photography at a million frames per second, the vortex ring induced by the jet flow mediates this damage, as its toroidal bubble c ore decays into several microbubbles collapsing in contact with the su rface. It is conjectured that in the range of bubble sizes and distanc es investigated here it is the second collapse that is responsible for the damage.