Jet orientation of a collapsing bubble near a solid wall with an attached air bubble

The interaction between a cavitation bubble and a non-oscillating air bubble attached to a horizontal polyvinyl chloride plate submerged in de-ionized water is investigated using a low-voltage spark-discharge setup. The attached air bubble is approximately hemi-spherical in shape, and its proximity to a spark-induced oscillating bubble (represented by the dimensionless stand-off distance H′) determines whether or not a jet is formed in the oscillating bubble during its collapse. When the oscillating bubble is created close to the plate, it jets towards or away from the plate. The ratio of oscillating bubble oscillation time and the wall-attached bubble oscillation time (T ′) is found to be an important parameter for determining the jet direction. This is validated with numerical simulations using an axial-symmetrical boundary element model. Our study highlights prospects in reducing cavitation damage with a stationary bubble, and in utilizing a cavitation collapse jet by controlling the jet's direction.