shape oscillation dynamics of a single bubble in a cavitation-induced acoustic field

The final stable shape taken by a fluid-fluid interface when it experiences a growing instability can be important in determining features as diverse as atmospheric weather patterns and the growth of cells and viruses. A laboratory-scale example is that an air bubble driven by an acoustic field when it becomes shape-unstable through a parametric instability. The excitation of volume/shape oscillations and oscillation-mode transitions in a single bubble system have been investigated. However, the role of absorbed energy by the acoustically excited gas bubble in the shape taken by a fluid-fluid interface and transition from volume oscillation to shape oscillation is missing. In this paper, we focus on the dynamics of a single rising air bubble exposed to an acoustic pressure field induced by a cavitation bubble. The air bubble is injected into the water pool through a needle, and then the laser beam is focused on the vicinity of the air bubble to generate the cavitation bubble. We elucidate the effects of acoustic source intensity, the distance from the acoustic source, and air bubble size on the air bubble final oscillation regime. If the acoustic wavelength is much larger than the bubble size, the initial bubble response is spherical oscillations of the radius. However, the forcing of large amplitude volume oscillations leads to the onset of shape oscillations. Finally, a dimensional analysis indicates the critical threshold for the excitation of the shape oscillation from an energy absorption perspective.