Underwater spark-discharge methods have been widely utilized for experimental studies in many fields such as material processing, water treatment, and cavitation bubble dynamics. However, the precise control of bubble size using this method has been difficult. This poses challenges to better understand the complex interactions of non-spherical cavitation bubble growth and collapse, which require fine and careful control of bubble size. A novel low-voltage (60.0 V) underwater spark-discharge method using a metal-oxide-semiconductor field effect transistor is presented here. We are able to repeatedly generate oscillating bubbles of consistent maximum radius, a. The dependency of the total circuit resistance to spark-generated bubble size in this method is discussed.
An oscillating bubble near another (stationary) bubble can give rise to interesting interactions. Such a nonequilibrium (oscillating) bubble can create a jet in a smaller nearby (initially stationary) bubble as demonstrated in this study both experimentally and numerically. In the experimental study, a spark-generated bubble (through a short circuit with two electrodes) was generated near a stationary smaller bubble. In order to keep the millimeter-sized bubble stationary, it was trapped in a droplet of silicone oil attached to one of the electrodes. The jet in the initially stationary bubble can reach velocities up to 250 m/s, but the velocity becomes lower for bubbles that are larger or situated further away. The current article also describes some experiments with the appearance of a crown-like secondary jet on the free surface (regarded as a large stationary bubble) relatively long after the bubble has collapsed. Some other interesting interactions of a spark-generated bubble with more than one stationary bubble are presented.
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.
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