High-speed imaging reveals the dynamics of the initiation and subsequent evolution of a nanosecond discharge propagating along the air-water interface

A nanosecond DBD-like discharge in a coplanar electrode configuration propagating along the water-air interface is an efficient source of reactive species to produce plasma-activated water. In this letter, we report on the mechanism of the discharge onset as well as on further developments over hundreds of nanoseconds. We combined ultrafast optical imaging with electrical characteristics to capture basic morphological imprints of individual discharge phases occurring during a single discharge event with high temporal resolution. We show that during the first nanoseconds, a diffuse bi-directional ionizing avalanche expands over the liquid surface at high speed (>5 x 10(5) m s(-1)). Later, discrete plasma filaments form from the diffuse plasma due to an ionization instability. The filaments, during their subsequent elongation (similar to 2 x 10(5) m s(-1)), retain the diffuse plasma at both ends. The filaments re-ignite in the same positions on the liquid surface over several successive (mainly capacitive) current pulses (similar to 1 mu s apart), which result from discontinuities in the driving voltage.