The stability of microbubbles and sonoluminescence

Sonoluminescence (SL), the light emission of a micron-sized bubble of gas trapped in water by an acoustic field, is associated with the collapse of a bubble nonlinearly oscillating under the sound field. Recent advances have made it possible to trap a single bubble at the pressure antinode of a standing wave, where it collapses and reexpands with the periodicity of the applied sound and emits light with every cycle. A critical feature of SL is the spherical symmetry of the bubble. If the collapse is violent, irregularities in the spherical shape of the bubble may develop towards the end of the collapse. As a result, the bubble may get destroyed shortly after the collapse. Due to the microscopic size of the bubble, molecular fluctuations constitute an additional force to which the gas–liquid interface is exposed. This additional force causes irregularities in the spherical shape of the bubble. The stability of a bubble is discussed by analyzing results obtained from hydrodynamic calculations of the bubble dynamics and the fluid dynamical processes outside the bubble taking molecular fluctuations into consideration. Results of these calculations reveal a surprising stability behavior and are in good agreement with observed behavior in single-bubble SL experiments. a)ua@dmu.ac.uk