Underwater gas bubbles produced by droplet impact: Mechanism to trigger volumetric oscillations

Abstract

Oscillating gas bubbles formed when droplets collide with a water surface are studied experimentally. Over a short time interval, before and after the bubble detachment, the bubble surface curvature changes drastically, causing a pulse of Laplace pressure. The leading edge of the pulse occurs before the bubble detachment, and the rear (negative) edge falls on the stage of an already closed bubble, which, like a resonator, is excited into volume oscillations on the natural frequency while simultaneously emitting an acoustic wavepacket. The amplitude and steepness of the pulse are inversely related to the size of the bubble, thereby ensuring that the dynamic parameters of the triggering pulse correspond to the natural frequency of the bubble. The release of pressure during the negative trailing edge of the pulse initiates the beginning of volumetric oscillations from the expansion phase and the acoustic packet from the positive half-wave.