Acoustic transient generation by laser-produced cavitation bubbles near solid boundaries

Abstract

The acoustic transients emitted after breakdown and cavitation bubble collapse upon focusing a Q-switch laser pulse into a liquid are investigated with special emphasis on their modifications induced by a solid boundary. For measuring the form p(t)/pmax of the pressure pulses an optical technique with a resolution of 10 ns has been developed. When p(t)/pmax is known, the pressure amplitude can be determined even when a transducer with a rise time much longer than the pulse duration is used. The duration of the transients (20–30 ns) and their pressure are nearly the same after breakdown and spherical bubble collapse. During spherical collapse, a maximum pressure of about 60 kbar is developed inside a bubble with Rmax=3.5 mm, and on average 73% of the bubble energy loss is transformed into acoustic energy. The sound emission near a solid boundary strongly depends on the normalized distance γ between the bubble and the boundary. The highest pressures at the boundary are achieved for γ→0; for γ=0.2 and Rmax =3.5 mm it has been found that p=2.5 kbar. These results are discussed with respect to the mechanisms of cavitation erosion important for hydraulic cavitation, laser lithotripsy, and ocular surgery.