Observation of Spherical Growth and Collapse of a Spark Bubble in Water

An experiment is described where the evolution of a single vapor bubble in water was observed. The bubble was generated by an electric spark between two electrodes designed to provide minimal interference to both liquid motion and the propagation of acoustic waves. The electrostatic energy of the discharge was varied to obtain vapor bubbles with radii ranging from about 2 to 5 centimeters. A high-speed camera was used to visualize bubble evolution, and the films showed that even the largest bubbles maintain spherical shape almost entirely during their lifetime. This confirms that gravity has little influence on the oscillation of bubbles of this size. Acoustic pressure records were found to be in good agreement with a simple model that has been proposed for free oscillation of vapor bubbles. The dependance of peak pressure on bubble radius showed that for this arrangement an optimal value exists for the electrostatic energy that maximizes the acoustic output. This may give indications to increase the efficiency of sound sources based on the implosion of a spark bubble.