Effect of viscosity on the dynamics of a spark-generated non-equilibrium bubble in free-field and near a free-surface

The effect of viscosity on the behaviour of a spark-generated non-equilibrium bubble is investigated experimentally. In specific, the dynamics of the bubble in two scenarios, namely, when the bubble is generated in the bulk of the fluid (``free-field'' bubble) and when the bubble is generated near a free-surface (``free-surface'' bubble) are investigated. The bubble is created using a low-voltage spark circuit and its dynamics is captured using a high speed camera with back-lit illumination. The viscosity of the surrounding medium is varied by using different grades of silicone oil. It is observed that for a ``free-field'' bubble, the bubble oscillates radially and with an increase in the viscosity of the liquid, both the number of oscillations as well as time period of each oscillation are increased. For ``free-surface'' bubbles, our experiments reveal a variety of distinctive bubble and re-entrant jet behaviours as 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.This limiting initial distance is observed to decrease as the viscosity increases.