When a drop impacts onto a liquid pool under certain conditions, bubble can be pinched off from the dimple forming at the bottom of the rebounding crater1. This process is related to the underwater sound produced by rain2. Herein we study sound emission from such bubbles, when the drop and pool are of same3 and different immiscible liquids4. The frame rate as high as 5 million fps is required to accurately capture the rapid shape evolution. The intricate crater shapes, which include a cascade of dimples, induce interesting scenarios like double-bubble 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).
2Prosperetti, A. & Oguz, H. N., Annu. Rev. Fluid Mech. 25, 577 (1993).
3Thoroddsen et al., J. Fluid Mech. 848, R3 (2018).
4Yang, Tian & Thoroddsen, J. Fluid Mech. 904, (2020).
Ziqiang Yang earned his Bachelor degree in Process Equipment & Control Engineering from China University of Petroleum (Beijing)in 2014. Then he obtained his Master's degree in Mechanical Engineering (Thermofluids direction) in Khalifa University of Science and Technology, Sas Al Nakhl Campus in Abu Dhabi, UAE in 2016. Now Ziqiang Yang is a PhD student at KAUST working under the supervision of Professor Sigurdur Thoroddsen. His research interests focus on using ultra-high-speed video imaging to study the dynamics of free-surface flows. This includes the breakup of drops and bubbles, singularities in hydrodynamics, viscoelastic fluids, fluid acoustics and tomo-PIV, coupled with an interest in computational fluid dynamics. And he is also interested in applying fluid mechanics in biological systems, like DNA separation & biomedical applications.