Effect of viscosity on the volumetric oscillations of a non-equilibrium bubble in free-field and near a free-surface

The effect of viscosity on the behavior of a non-equilibrium bubble is investigated experimentally, in two scenarios; firstly, when the bubble is generated in the bulk of the fluid (termed as “free-field” bubble) and secondly when the bubble is generated near a free-surface (termed as “free-surface” bubble). The bubble is created using a low-voltage spark circuit and its dynamics is captured using a high-speed camera with backlit illumination. The viscosity of the surrounding fluid is varied by using different grades of silicone oil. For a “free-field” bubble, the bubble oscillates radially and as the viscosity of the liquid increases, the number of oscillations, as well as the time period of each oscillation, are increased. At high viscosities, the bubble also becomes stable and does not disintegrate into smaller bubbles. The volumetric oscillations also show good agreement with the Rayleigh-Plesset model incorporating energy loss and viscous effects. For “free-surface” bubbles, two parameters, namely, the initial distance of the bubble from the free-surface and the viscosity of the surrounding fluid are varied. It is observed that beyond a certain initial distance of the bubble from the free-surface, the bubble behaves as a “free-field” bubble with negligible influence of the free-surface on its dynamics. This limiting initial distance decreases as the liquid viscosity is increased and is not dependent on the bubble radius. For these bubbles, different behaviors of the free-surface in each liquid are also presented for distances up to the limiting initial distance. These free-surface behaviors are classified into four different regions on a plot of the Reynolds number (based on the bubble expansion) versus the non-dimensional distance of the bubble from the free-surface.