Experimental and 2D Fluid Simulation of a Negative Nanosecond Discharge in Air Above a Liquid Surface with Different Dielectric Permittivity and Electrical Conductivity

Plasma–liquid interaction remains a crucial phenomenon influencing numerous applications. Plasmas produced by electrical discharges exhibit properties that depend on the voltage polarity as well as on the liquid properties. In this study, we investigate the impact of liquid permittivity (\({\upvarepsilon }_{{\text{r}}} = { }32,{ }56,{\text{ and }}\,80\)) and water electrical conductivity (σ = 2, 500, and 1000 μS/cm) on negative discharges initiated in air at atmospheric pressure. Using a negative pulsed nanosecond high-voltage setup with a pin-to-liquid configuration, experimental results demonstrate that increasing \({\varepsilon }_{r}\) leads to faster discharge ignition and higher discharge current. ICCD imaging reveals a decrease in the maximal radial extension of the discharge over the liquid surface with increasing \({\varepsilon }_{r}\). Also, rising σ lead to an increase of the discharge current, and the ICCD images show a decrease in the radial propagation of the discharge over the solution. To gain deeper insights into the discharge dynamics and properties, a 2D fluid model is employed to simulate the various conditions. The results indicate that increasing \({\varepsilon }_{r}\) decreases the radial E-field produced by the surface ionization wave and increases the electron density in the air gap. Regarding σ, high-conductivity conditions result in lower radial E-field in the front of the surface ionization wave, explaining the shorter radial propagation of the discharge. Comparing negative with positive discharge, we observe that the former travels a shorter distance over the liquid surface due to its more diffuse front. Moreover, we note the absence of filamentation in the negative surface discharge, unlike the positive counterpart. This disparity is attributed to a relatively lower space charge contained in the front, thereby prohibiting the formation of individual filaments.