Electromechanical coupling mechanisms at a plasma–liquid interface

A Dickenson,M I Hasan,J L Walsh

doi:10.1063/5.0045088

Abstract

The direct interaction between a non-equilibrium gas discharge and a liquid volume leads to the generation of a plasma activated liquid. This interaction induces a flow in both the gas above the liquid and within the liquid volume. The physical mechanisms behind the induced flows are complex. In this work, a two-dimensional experimentally validated numerical model was developed to determine the dominant mechanism driving the liquid flow at the plasma–liquid interface. The model followed the evolution of the plasma and the flow fields in both phases, describing a pin-water discharge configuration operating in air, which was used to treat a de-ionized water sample and a tap water sample. Two potential physical mechanism were investigated, the electrohydrodynamic (EHD) flow induced in the gas phase and the electric surface stresses across the interface. It was found that the dominant mechanism driving the liquid flow is correlated with the charge relaxation time of the liquid. For liquids with a charge relaxation time longer than the characteristic time of the plasma, such as de-ionized water, the liquid behaves as a dielectric, and the electric surface stresses dominate the flow in the liquid phase. For liquids with a charge relaxation time shorter or in the same order of the plasma’s characteristic time, such as tap water, the liquid behaves as a conductor, and the EHD flow induced in the gas phase dominates the flow in the liquid phase.

Highlights

Cold atmospheric plasmas (CAPs) have been the focus in recent years due to their unique ability to generate a mixture of highly reactive chemical species under ambient conditions
Two potential physical mechanism were investigated, the electrohydrodynamic (EHD) flow induced in the gas phase and the electric surface stresses across the interface
For liquids with a charge relaxation time shorter or in the same order of the plasma’s characteristic time, such as tap water, the liquid behaves as a conductor, and the EHD flow induced in the gas phase dominates the flow in the liquid phase

Summary

Introduction

Cold atmospheric plasmas (CAPs) have been the focus in recent years due to their unique ability to generate a mixture of highly reactive chemical species under ambient conditions. The number of applications where the use of CAP has been explored is growing rapidly, and promising results have been obtained spanning the domains of agriculture,[1,2] plasma medicine,[3–5] and materials processing.[6,7]. In many of these applications, the discharge inevitably interacts with some form of liquid, be it moisture on freshly harvested vegetables, right through to the deliberate treatment of a liquid volume to create plasma activated liquid (PAL). Using PAL offers the intriguing possibility of retaining some chemical activity initiated by the plasma for several days,[8–10] which is a major advantage for most applications This advantage comes at a price, as the introduction of a plasma–water interface vastly increases the complexity of the physiochemical processes at play. In non-contact sources, the plasma is generated some distance away from the interface, and the reactive species generated by the plasma are carried to the interface by diffusion, convection, or both.[11,12] Such sources include surface barrier discharges (SBDs),[11,13] gliding arc discharges,[14,15] and plasma jets, where the plasma terminates before arriving at the interface.[12,16] In direct-contact sources, on the other hand, the plasma physically contacts the liquid surface, creating a complex gas–water–plasma interface

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