Discharge in air in contact with water: influence of electrical conductivity on the characteristics and the propagation dynamics of the discharge

Due to the high reactivity and the non-thermal properties of streamer discharges, they are applied in various fields, such as water treatment and medicine. Streamer discharges are usually produced in the gas phase before interacting with a liquid or solid surface. Although the dynamics of a streamer discharge in gases is well described, its propagation at liquid surfaces remains poorly understood. In this study, we investigate the influence of water electrical conductivity ( σ ), between 2 and 1000 µ S cm −1 , on the characteristics and propagation dynamics of pulsed positive DC nanosecond discharges with the solution serving as a cathode. σ strongly influences τ r (the dielectric relaxation time), and two discharge modes may be obtained, depending on whether τ r is shorter or longer than the delay to achieve breakdown ( τ pulse ). This latter can be indirectly modified by adjusting the voltage amplitude ( V a ). In the case of V a = 14 kV, the breakdown voltage ( V bd ) at low σ is lower than that measured at high σ , probably because τ pulse < τ r and > τ r , respectively. In the case of V a = 20 kV, V bd decreases slightly with σ , probably because of the decrease of the resistivity of the global electrical circuit as τ pulse ∼ τ r for high σ . In addition to the electrical characterization, the dynamics of the discharge at the solution’s surface is investigated using 1 ns-time-resolved imaging. Its morphology was found to evolve from a disc to a ring before it splits into highly organized plasma dots (streamers’ head). The number ( N dots ) and propagation velocity of plasma dots are determined as a function of σ . At V a = 14 kV, N dots does not vary significantly with σ despite the increase of V bd ; this latter likely compensates the neutralization of charge accumulated at the surface by ions in solution. In the case of V a = 20 kV, N dots decreases with σ , and it can be related to a decrease of accumulated charge at the water surface. Finally, based on the electrical measurements, we found that the charge per plasma dot ( Q dot ) increases with σ , which does not correlate with the imaging results that show a short length of propagation at high σ . Then, considering the plasma dot mobility at low σ and the instantaneous propagation velocities at high σ , a more realistic Q dot is measured.