Mechanism of gas bubble shoot-off and motion during spark discharge in liquid

The conditions of the excitation of a pulsed plasma discharge on the surface of a processed metal (copper) sample immersed in a conducting aqueous solution have been studied. Cathode polarization of the metal was provided by a high-voltage capacitor bank (4$\mu$F) charged to $U$ = 200–1100 V after each discharge. It is established that electric breakdown with a duration not exceeding 0.1 $\mu$s is always preceded by the formation of small hydrogen bubbles (with radii $r\approx$ 37–40 $\mu$m) on the polarized metal surface, which takes about 139–159 $\mu$s. A mechanism of passivation of the processed metal surface by these hydrogen bubbles, which are synchronously shot off from the electrode surface under the action of the spark discharge, is proposed. Consistent matching of the experimental data and model calculations is used to estimate the main parameters determining the kinetics of bubble detachment from the electrode surface at various voltages U on the capacitor bank.