Dynamics of macro- and micro-bubbles induced by nanosecond discharge in liquid water

Gaseous micro-bubbles dispersed in liquid water represent perturbations of the homogeneity of the liquid and influence the onset of electrical discharge in the bulk liquid. In this study, we systematically examined shadowgraph images to analyse the gaseous structures occurring in response to nanosecond micro-discharges produced in deionised water. The images revealed the dynamics of resolved bubbles and unresolved sub-micrometric structures starting from nanoseconds after the onset of discharge. We provide absolute counts and the radii distributions of micro-bubbles that occur near the anode needle and show how this depends on the amplitude and repetition frequency of the applied high-voltage pulses, when the latter varies between 0.1 and 100 Hz. A systematic statistical analysis showed that the probability of producing bubble-assisted nanosecond discharge in the liquid phase rapidly increases with the discharge repetition rate (>0.5 Hz). Although the cavitation bubble formed around the anode disintegrates and disappears from the anode region within the first millisecond, the sub-micrometric structures remain for tens of milliseconds, and fragmented micro-bubbles survive even for hundreds of milliseconds. Our findings impose strict limitations on the experimental setups used to investigate the mechanisms of direct discharge in liquid water.