  The effect of viscosity on the behaviour 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 back-lit
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. 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 behaviours of the free-surface in each liquid are also presented as a
function of the two parameters.
