Fully Coupled Simulation of the Plasma Liquid Interface and Interfacial Coefficient Effects

Abstract

There is a growing interest in the study of coupled plasma-liquid systems because of their applications to biomedicine, biological and chemical disinfection, agriculture, and other areas. Without an understanding of the near-surface gas dynamics, modellers are left to make assumptions about the interfacial conditions. For instance it is commonly assumed that the surface loss or sticking coefficient of gas-phase electrons at the interface is equal to 1. In this work we explore the consequences of this assumption and introduce a couple of ways to think about the electron interfacial condition. In one set of simulations we impose a kinetic condition with varying surface loss coefficient on the gas phase interfacial electrons. In a second set of simulations we introduce a Henry's law like condition at the interface in which the gas-phase electron concentration is assumed to be in thermodynamic equilibrium with the liquid-phase electron concentration. It is shown that for a range of electron Henry coefficients spanning a range of known hydrophilic specie Henry coefficients, the gas phase electron density in the anode can vary by orders of magnitude. Varying reflection of electrons by the interface also has consequences for the electron energy profile. This variation in anode electron density and energy as a function of the interface characteristics could also lead to significant variation in near-surface gas chemistries when such reactions are included in the model; this could very well in turn affect the reactive species impinging on the liquid surface. We draw the conclusion that in order to make more confident model predictions about plasma-liquid systems, finer scale simulations and/or new experimental techniques must be used to elucidate the near-surface gas phase electron dynamics.