Acoustics of oscillating bubbles when a drop hits the water surface

Abstract

In experiments on a collision of a drop with the water surface, video recording of gas bubbles detached from underwater cavities was accompanied by simultaneous measurement of acoustic radiation. In each experiment, a number of video frames, in which the bubbles exhibit a rotational symmetry, were extracted and processed. The curvatures and volumes of the bubbles were derived, from which time series of volumetric oscillations and an external pressure component introduced by surface tension were calculated. The frequencies found from the spectra of volumetric and acoustic oscillations match well with both each other and the natural frequency defined by the bubble size at rest. An eventual mechanism of the bubble oscillations is an abrupt fall of an external pressure caused by a loss of the curvature during detachment of the bubble. The pressure fall leads to an expansion of gas inside the bubble, triggering the volumetric oscillations and accompanying acoustic radiation, which starts with a positive half-wave.