Statistical analysis of the dynamic behavior of individual discharges during the ignition and continuous phases of contact glow discharge electrolysis

Contact Glow Discharge Electrolysis (CGDE) denotes a plasma inside a vapor layer surrounding a gas-evolving electrode immersed in an aqueous electrolyte and operated at high voltages. We used a high-speed camera to image the formation of the vapor layer as well as its dynamic behavior during continuous CGDE on a Au wire cathode. The plasma ignites with a spark within a large bubble at the tip, which expands along the wire to the top, leaving a stable glow within the vapor layer behind. Using an in-house developed open-source Python-based software we deduced, from a thorough statistical analysis of images taken during continuous CGDE, a vapor layer thickness between 0.1 and 0.4 mm. Furthermore, we provide information on the dynamic behavior of individual discharges through the vapor layer from a series of images. The discharges are confined within the vapor layer and, thus, the extent of the discharges is similar to the vapor layer thickness. We find that the discharges have approximately the shape of oblate spheroids, which appear either as circles or ellipses in the camera images, depending on the orientation of the discharge with respect to the camera. We discuss the relevance of our results for the fundamental understanding of atomic scale surface structural changes and products formed in the solution in the presence of the plasma.