Microbubble shape oscillations excited through an ultrasound‐driven parametric instability

An ultrasonically driven air bubble can become shape‐unstable through a parametric instability. Here, we report time‐resolved optical observations of shape oscillations (mode n=2 to 6) of micron‐sized single air bubbles for a range of acoustic pressures. The observed mode number n was found to be linearly related to the resting radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which as expected is minimum at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small, nonspherical linear perturbation into a Rayleigh‐Plesset‐type equation model which includes a physical thermal damping mechanism describing heat and mass transport in the thin boundary layer at the bubble‐to‐water interface. Indeed, a parametric instability is responsible for the shape oscillations, and the Rayleigh‐Plesset‐type equation captures the e...