Plasma dynamics, instabilities and OH generation in a pulsed atmospheric pressure plasma with liquid cathode: a diagnostic study

Abstract

While plasma-liquid interactions have been an important focus in the plasma research community, the impact of the strong coupling between plasma and liquid on plasma properties and processes remains not fully understood. In this work, we report on the impact of the applied voltage, pulse width and liquid conductivity on the plasma morphology and the OH generation for a positive pulsed DC atmospheric pressure plasma jet with He-0.1% H2O mixture interacting with a liquid cathode. We adopted diagnostic techniques of fast imaging, 2D laser induced fluorescence (LIF) of OH and Thomson scattering spectroscopy. We show that plasma instabilities and enhanced evaporation occur and have a significant impact on the OH generation. At elevated plasma energies, it is found that the plasma contracts due to a thermal instability through Ohmic heating and the contraction coincides with a depletion in the OH density in the core due to electron impact dissociation. For lower plasma energies, the instability is suppressed/delayed by the equivalent series resistor of the liquid electrode. An estimation of the energy flux from the plasma to the liquid shows that the energy flux of the ions released into the liquid by positive ion hydration is dominant, and significantly larger than the energy needed to evaporate sufficient amount of water to account for the measured H2O concentration increase near the plasma-liquid interface.

Plasma‐enhanced evaporation and its impact on plasma properties and gaseous chemistry in a pin‐to‐water pulsed discharge

“…Atmospheric pressure discharges formed with a liquid electrode are gaining intense attention in the last decades both for plasma fundamentals and promising applications such as synthetic ammonia and hydrogen (both are sustainable and clean energy) [1,2] , plasma biomedicine [3,4] , surface modification [5,6] , nanomaterial synthesis [7,8] , and so on. When the discharge is on, the liquid surface evaporates under the effects of interfacial exothermic reactions such as ion hydration, H and OH recombination, and plasma-induced heat transfer [9] . When water vapor transfers to the gaseous phase region, solvent and dissolved components may also diffuse to the discharge column along with the vapor [10] .…”

“…A large impact of evaporation on the plasma kinetics is demonstrated, such as the production of the gaseous OH, H 2 O + , and NO x (if air is mixed or used as background gas) [15,17,18] . Important effects of coupled plasma-liquid surface deformation have been shown on the discharge breakdown and plasma instability, as well as enhanced in-volume mixing, convection, and energy dissipation into the aqueous phase [19][20][21] . Extremely limited penetration of electrons [22,23] and reactive neutral radicals in the plasma-liquid interface was proved by sensitive diagnostic and simulation works [24,25] .…”

“…Based on the CET-LIF methods, Riès et al [33] discussed the different effects of the nature of targets as well as gas flow and voltage amplitude in the production of OH and H 2 O in a plasma jet. Yue et al [21] studied the plasma dynamics and instabilities in a pulsed pin-to-liquid discharge and measured the water vapor density in the adjacent region to the liquid surface. Meanwhile, several researchers applied a plasma jet to the solution and in situ diagnosed and analyzed various important active particles including OH, H, O, H 2 O 2 , and so on above the treated solution surface [34][35][36] .…”

Plasma-Induced Selective Propylene Epoxidation Using Water as the Oxygen Source

“…The plasma jet used in this study is similar to the one reported in the previous literature . The plasma was generated at the tip of a tungsten needle electrode.…”

“…The plasma jet used in this study is similar to the one reported in the previous literature. 39 The plasma was generated at the tip of a tungsten needle electrode. A negative pulsed direct current (DC) voltage was applied to the tungsten needle by a DEI PVX-4110 high-voltage pulser.…”