Droplet Emission in a 1 atm DC Glow Discharge with Liquid Electrode: Observations and Potential Mechanisms*

The thermodynamics and transport processes in a multiphase plasma-liquid interaction can couple with ionized species, chemical reaction, and charge dissipation and thus are complex. The intense mass and heat transfer between the gas and liquid phase provide channels for reactive species in the form of evaporation, gas dissolving, and convection. Interestingly, the injection of liquid droplets in a DC glow discharge using liquid as the electrode is also observed and could be an important coupling mechanism for multiphase plasma. For the liquid cathode, Taylor cone can be formed due to charged surface deformation and give rise to droplets injection into the gas phase. On the other hand, a similar injection phenomenon can also be found with a liquid anode. Such injection starts from the bubble accumulation in liquid phase and subsequent emergence from the anode plate. There appears to be a relationship between the impinging of the bubbles at the interface and the emission of droplets. The electric field in the gas phase can significantly affect these micron-scale droplets' trajectories. Spectroscopic analysis of discharge light emission indicated that droplets that contact the hot cathode electrode emit a glow signature that is similar to that observed in the plasma column. This observation suggests that the strong discharge glow at the interface and in the plasma column might partially originate from the continuous evaporation of these droplets in the hot plasma column, which ultimately introduces  electrolyte ions into the gas phase.