Experimental investigation and 2D fluid simulation of a positive nanosecond discharge in air in contact with liquid at various dielectric permittivity and electrical conductivity values

Streamer discharges exhibit high reactivity and are pivotal in several plasma-based applications, especially those involving plasma-liquid interactions. This study investigates the effects of liquid dielectric permittivity (& varepsilon;r = 32, 56, 80) and electrical conductivity (sigma = 2, 500, 1000 mu S cm-1 ) on positive nanosecond discharges in ambient air in a pin-to-liquid setup. Increased & varepsilon;r and sigma values lead to higher discharge currents. ICCD imaging reveals that elevated & varepsilon;r decreases the extension of the discharge radially over the liquid surface and lowers the number of filaments at the liquid surface. Similarly, higher sigma values result in a shorter propagation of the discharge. A previously developed fluid model was adapted to include solution conductivity and is utilized to elucidate the discharge dynamics. The results demonstrate that increased & varepsilon;r or sigma decrease the radial component of the electric field produced by the surface ionization wave while increasing the density of electrons in the gap. The simulations and ICCD images are used to determine the charge number (Ns) at the filament front. Ns is in the order of magnitude of Meek's criterion (similar to 108) during propagation and reaches similar to 107 when propagation stops for all & varepsilon;r- and sigma-conditions. We find that Ns is higher for low & varepsilon;r and decreases more rapidly at higher sigma. The findings reported in this paper enhance our understanding of streamer-surface interactions, which are crucial for advancing plasma applications.