Plasma self-organization in DC discharges with liquid anode: effect of electrode separation, liquid type and working gas

We study the formation of self-organized patterns (SOPs) in the vicinity of a liquid anode in atmospheric pressure DC glow discharges. The discharge is generated in a pin-to-liquid anode configuration with helium (He) flowing into open air. We investigate how the electrode separation and gas mixing influence the formation of SOPs on the surface of a distilled water (DIW) and 1% sodium chloride (NaCl). For a pin-to-liquid gap (g) of ~5 mm and 500 sccm He flow rate, the anode glow has the form of a circle on both DIW and NaCl surfaces. The diameter of this circle increases with increasing current. With increasing gap, multiple circles are first formed which finally transform into several more complex patterns. A ring formed at g = 8 mm changes to a wedge-shaped structure at g = 12 mm. Further increase of the pin-to-liquid gap makes the discharge unstable. The diameters of the patterns at all gap distances are larger for DIW than the NaCl solution. To understand effects of the gas type on the formation of SOPs on liquid anode surfaces, different gases (O₂, N₂ and air) were added as the surrounding gases in addition to the main He gas flow. For all gap distances, mixing He with other gases affected the formation of SOPs. For example, at g=10 mm, the wedge-shaped pattern that forms on the surface of DIW with pure He flow is transformed to a semi-circular wedge with addition of N₂ or air. A similar semi-circular wedge-shaped pattern is formed at g=15 mm without any surrounding gas. Modelling some aspects of the discharge self-organization has been performed using computational tools available to the team. Comparison of simulation results with electrical and optical measurements is being conducted to clarify the nature of SOPs. The presence of electronegative gases and negative ions near the liquid anode surface and the charge transport inside the liquid anode on the pattern formation will be discussed.