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 (sigma), between 2 and 1000 mu S cm(-1), on the characteristics and propagation dynamics of pulsed positive DC nanosecond discharges with the solution serving as a cathode. sigma strongly influences tau (r) (the dielectric relaxation time), and two discharge modes may be obtained, depending on whether tau (r) is shorter or longer than the delay to achieve breakdown (tau (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 sigma is lower than that measured at high sigma, probably because tau (pulse) < tau (r) and > tau (r), respectively. In the case of V (a) = 20 kV, V (bd) decreases slightly with sigma, probably because of the decrease of the resistivity of the global electrical circuit as tau (pulse) similar to tau (r) for high sigma. 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 sigma. At V (a) = 14 kV, N (dots) does not vary significantly with sigma 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 sigma, 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 sigma, which does not correlate with the imaging results that show a short length of propagation at high sigma. Then, considering the plasma dot mobility at low sigma and the instantaneous propagation velocities at high sigma, a more realistic Q (dot) is measured.