Generation Mechanism of Hydroxyl Radical in Electrolyte Streamer Discharge

Abstract

During the last two decades atmospheric pressure non-thermal plasmas in electrolyte have been spotlighted in view of environmental and medical applications. The simultaneous generation of ultra violet (UV) radiation and active hydroxyl radical (OH) makes the plasmas discharge in particularly suitable for decontamination, sterilization, purification, and lesion ablation purposes. Especially in lesion ablation, the electrolyte plasma has been spotlighted because of the advantage of minimized damage to surrounding tissue. The existing studies, the OH generation was increased through high applied power rather than investigation on the effect of the electrolyte on the electrolyte plasma discharge. The high applied power risen thermal damage is the basis of resulted in a limit in lesion removal rate using plasma. In this paper the mechanism of hydroxyl radical production in electrolyte streamer discharge is discussed based on the electric field. The emphasis is on their generation mechanisms and their physical characteristics. The electric field is considered as the key parameter since the plasma discharge, hydroxyl radical generation and loss reaction rate are determined by electric field. The streamer discharge shows high electron energy over 10 eV of electron energy with ionization degree of 10−5 – 10−4. These properties are quite different from the typical plasma properties known from low pressure gas discharges. In the plasma physics literature hydroxyl radical production is primarily ascribed to be due to electron, metastable induced or thermal dissociation of water, processes which are dominant in (low pressure) gas discharges and in combustion and hot flames. Several previous work, reviewed in this work, are focused on the chemical kinetics with fixed boundary system for hydroxyl radical generation. In the electrolyte, the electrolyte surface is unfixed boundary, rising challenging physical issues. Since the effect of fluid dynamics and electrolyte ion on electric field profile within the plasma effect, the discharge in electrolyte plasma is considered as chaotic phenomena. Thus the mechanisms of OH generation and electrolyte plasma discharge are investigated by analyzing the electrical signal and optical signal from the plasma with the developed fluid-plasma hybrid model. In order to apply the breakdown electric field in the electrically conductive electrolyte, the electrolyte-immersed metal electrode surface should be completely covered by vapor as an insulating layer. Vapor coverage significantly influences on the discharge gas component and heat transfer. Compared to vapor coverage from electrolysis or external gas injection, vapor coverage formed through film boiling is advantageous because it prevents the chemical erosion of electrode, suppresses the increase in electrolyte temperature, and facilitates OH generation. Thus it is important to maintain the electrode at a temperature of at least 300 oC for forming the vapor coverage through film boiling. Negative streamer discharge is dominant in the vapor coverage because the sodium cation, the mobility of which is higher than that of the chlorine anion, is accumulated on the vapor surface. The electric field inside the vapor coverage can be described by the ratio between the voltage difference across the coverage and effective thickness of the vapor. Because the thickness of the vapor coverage oscillates with respect to time owing to heat exchange with the viscous electrolyte, the electrolyte exhibits periodic streamer discharge. The mechanism of streamer propagation inside the electrolyte vapor is electron impact ionization, which is non-photo-ionization caused by the vibrational and rotational energy of the discharge gas H2O and different from ionization in air. Water molecules are dissociated to OH in ionization front of streamer by impact of electron, accelerated to 18.2 eV by electric field of 52 kV/cm. This ionization front is not extinguished when it reaches the surface of the vapor coverage, and it propagated along the vapor surface because the Taylor cone instability phenomenon sustains the electron energy above 15 eV, which corresponds to and electric field of 34 kV/cm. The OH is mainly formed through the discharge that propagates along the surface of the vapor coverage because the OH in the vapor volume is lost by recombination to hydrogen peroxide (H2O2). Thus the widening the surface area of the vapor coverage is important to enhance the formation of OH electrolyte plasma.