Jetting from cavitation bubbles due to multiple shockwaves

Abstract

We present experimental observations of microjets formed by cavitation microbubbles. Anunderwater electric discharge, applied beneath a flat free surface, produces a primary compressionwave, which undergoes several phase inversions upon reflections from the free surface and spark-bubble interface. The first reflection yields a tension wave, which produces a cloud of secondarycavitation bubbles in the liquid, some of which form microjets upon collapse. The tuning of thesereflections enables an effective control of the microjet direction in the bubble cloud. All of the jetsof the microbubbles between the spark bubble and free surface are directed radially away from thespark bubble. The mechanical response of an alumina plate placed between the electrodes and freesurface generates a quasi-planar compression wave, which, following its multiple reflections fromthe free surface and plate, orients the microjets in the same direction toward the plate. These obser-vations imply that the jet direction is determined mainly by the secondary compression wave,which is the first and thus most energetic compression wave acting on a sufficiently grown cavita-tion bubble.