Bubble bursting and sound generation in suspensions

Suspension rheology becomes solid-like as its particle volumetric fraction Φ increases, but how this affects bubble bursting and sound generation, is poorly known. Here we conduct lab experiments to investigate this effect by injecting a bubble (volume V) into a refractive index-matched suspension. We find that depending on Φ and V, sound with diverse waveforms are excited, covering a frequency band of f = O(10-10⁴) Hz. In a suspension of Φ=< 0.3 or in a suspension of Φ = 0.4 with a V smaller than critical, the bubble bursts after it forms a hemispherical cap at the surface and excites a high-frequency (HF) wave (f = O(10⁴ Hz)) with an irregular waveform, which likely originates from film vibration. However, in a suspension of Φ = 0.4 and with a V larger than critical, the bubble bursts as soon as it protrudes above the surface, and its aperture opens slowly, exciting Helmholtz resonance with f = O(10³) Hz. Superimposed on the waveform are an HF wave component excited upon bursting and a low-frequency (f = O(10)Hz) air flow vented from the deflating bubble, which becomes dominant at a large V. We interpret this transition as a result of the bubble film of a solid-like Φ = 0.4 suspension, being stretched faster than the critical strain rate such that it bursts by brittle failure.