Sound emission from oscillating bubbles trapped by the collapse of drop-impact craters

TL;DR

This study investigates the sound produced by oscillating bubbles trapped during drop impacts on a pool, revealing how bubble dynamics and impact conditions influence acoustic signals through high-speed imaging and acoustic measurements.

Contribution

It provides detailed insights into the mechanisms of sound generation from entrapped bubbles during drop impacts, combining ultra-high-speed imaging with synchronized acoustic data.

Findings

Sound amplitude increases with smaller bubbles pinched off.

Acoustic frequency matches Minnaert theory for spherical bubbles.

Interplay between large dimple bubbles and tiny entrapped bubbles affects sound signals.

Abstract

When a drop impacts a deep pool, it forms a crater which subsequently rebounds. Under certain conditions, a dimple forms at the crater bottom, which pinches off to entrap a small bubble. The oscillation of this entrapped bubble is the primary source of the underwater sound produced by rain. We use simultaneous ultra-high-speed video imaging and synchronized acoustic recording, with an immersed hydrophone, to investigate the details of the sound formation, over a range of impact Weber numbers and different dimple shapes. With frame-rates as high as 5 million fps, we can track the shape evolution of the pinched off dimple-bubble, which experiences large volumetric compression, by as much as 50%. The subsequent volume oscillations are consistent with the observed $\simeq 125$ Pa acoustic pressure amplitude, for the strongest compression. For our configuration the sound amplitude increases for smaller bubbles pinched off from the dimple. The acoustic forcing mechanism is therefore the inertial focusing of the momentum of the liquid outside the dimple, as it pinches off. The acoustic frequency agrees well with the Minnaert theory for freely oscillating spherical bubbles, of the same size. The finer details of the acoustic signal reveal an interplay between the larger dimple bubble and the tiny bubble entrapped during the initial contact between the drop and pool. For the singular dimple where no bubble is pinched off, the sound generation has a broader range of frequencies, with the tiny bubble oscillating at $\sim 100$ kHz, after being deformed by the rapid vertical retraction of the dimple below the singular jet.