Sounds of a drop impact on a liquid pool

We investigate the impact of a drop into a deep liquid pool, with focus on bubble entrapment and sound emission during the crater collapse¹. This process is related to the underwater sound generated by rain. Herein we study sound emission from such bubbles, when the drop and pool are of same and different immiscible liquids. The frame rates as high as 5 million fps are required to accurately capture the rapid shape evolution. The intricate crater shapes, which include a cascade of dimples, induce interesting scenarios like double bubbles pinch-offs. We synchronize the phase of the bubble oscillation, recorded by high-speed camera, to the acoustic signal from hydrophone and confirm that the onset of sound generation is at the instant of neck pinch-off. The tiny bubble from the initial drop contact with the liquid pool will also be induced to oscillate by the oscillation of the large bubble from the air cylinder pinch-off. The dynamic process of bubble oscillation is analyzed from the audio signal and we find good agreement with the Minnaert frequency when using the fluid properties of the drop. This applies reasonably well even for very small bubbles where the frequency ∼ 100 kHz.

¹Pumphrey H. C. & Elmore P. A., J. Fluid Mech. 220, 539 (1990).