We present experimental observations of microjets formed by cavitation microbubbles. An underwater electric discharge, applied beneath a flat free surface, produces a primary compression wave, 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 secondary cavitation bubbles in the liquid, some of which form microjets upon collapse. The tuning of these reflections enables an effective control of the microjet direction in the bubble cloud. All of the jets of the microbubbles between the spark bubble and free surface are directed radially away from the spark bubble. The mechanical response of an alumina plate placed between the electrodes and free surface generates a quasi-planar compression wave, which, following its multiple reflections from the free surface and plate, orients the microjets in the same direction toward the plate. These observations 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 cavitation bubble.

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