Needle-to-Liquid DC Discharge in Atmospheric Air: Electrical Characteristics and Impact on Potassium Halide Solutions

Abstract

This study investigates the electrical and chemical characteristics of a non-thermal atmospheric pressure DC plasma discharge in a needle-to-liquid configuration. A high-voltage (HV) needle is placed at 2 mm above the liquid surface, while the ground electrode is submerged in a potassium halide solution (potassium iodide (KI) or potassium chloride (KCl)). The reactive species in the liquid are estimated based on their reaction with KI, producing iodine (I₂), either through direct titration of the plasma-treated KI or back titration of the plasma-treated KCl. Different discharge regimes are identified for each polarity: Trichel corona, unstable glow, and stable glow for negative polarity, and onset streamer, Hermstein glow corona, unstable glow, and stable glow for positive polarity. The presence of a liquid surface allows for higher voltage and current ranges without sparking and facilitates the establishment of a stable glow discharge, which is challenging in the case of a needle-to-plate configuration. The concentration of reactive species in the liquid is significantly lower in corona regimes compared to glow regimes, due to their lower power consumption, and the absence of direct contact between the plasma and the liquid. Moreover, the positive unstable glow is three times more efficient than the negative stable glow. One explanation is that the flow induced within the liquid during the positive unstable glow discharge enhances mixing of reactive species, preventing their saturation at the gas-liquid interface and improving their penetration into the liquid phase.