Electromechanical coupling mechanisms at a plasma–liquid interface

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.