Interaction between plasma and water surface: Formation and dynamic behavior of water surface depression and its effect on aqueous chemistry

Abstract

In this work, we report that apart from the phenomenon of the Taylor angle, depression of the water surface was formed in a negative pin-to-water discharge. The depression extent of the water surface decreases gradually along with the operation of discharge. Real-time detection of the aqueous NO3− concentration in the treated distilled water implies that this depression behavior depends strongly on the solution conductivity. The solution conductivity was expected to increase steeply in the beginning, which corresponds to an unstable stage of the discharge. In this unstable stage, a deep dimple was formed on the water surface, and it swung and induced significant turbulent gas dynamics above the water surface, confirmed by fast schlieren photography. This unstable depression behavior and its resulted turbulent gas dynamics enhanced the process of air diffusion to the discharge column and, correspondingly, the production of reactive nitrogen and oxygen species, which again resulted in abundant production of aqueous NO3−. This process was slowed down with the rise of solution conductivity, which corresponds to a decrease in the depression extent of the water surface and finally the discharge tended to be stable. Further analysis indicates that the dimple was induced by the accumulated negative charges on the water surface which created a repellent Coulomb force relative to the external electric field. The accumulated charges reduced due to the formation of a conductive path in the solution with the increase in the conductivity and, correspondingly, resulted in the decrease in depression on the water surface.