Penetration of Gas Discharge Through the Gas–Liquid Interface Into the Bulk Volume of Conductive Aqueous Solution

Gas discharge plasma generated above the surface of conductive aqueous solutions in a glass capillary was used to study penetration of the discharge from the bubble (imitated by the space above meniscus of liquid surface in the capillary) into the bulk liquid. The experiments were conducted at both polarities with a high-voltage needle electrode placed above the liquid surface. Different aqueous solutions were examined (distilled water, conductive saline solutions). High-speed shadowgraphy was used as the main diagnostic tool for the study of the disturbances at the plasma-liquid interface. It has been found that electric field just beneath the liquid surface and the liquid/plasma conductivity ratio have a decisive effect on the development of plasma-liquid interface instabilities. Experiments with negative electrode above the liquid surface showed that this surface in the place of the largest current density recedes. This receding is caused by the reaction pressure resulting from liquid evaporation. Thus, long cavities with plasma inside can be formed. The cavity elongation speed is of the order of 1 m · s^-1, and it depends on current density. The liquid surface remains smooth, when the liquid conductivity is larger than the conductivity of adjacent plasma. In the opposite case, if the liquid conductivity is smaller than the conductivity of adjacent plasma, the distribution of current density on the plasma-liquid boundary is unstable: any initial surface disturbance boosts the current density in a local surface valley simultaneously causing a detriment of the surrounding current density. Consequent stronger liquid evaporation in the valley causes its deepening, and hence, next enhancement of the inhomogeneity of current density distribution. The dips created in this way subsequently transform into negative streamers, when electric field larger than 1 MV · m^-1 appears near the liquid surface. Experiments with the positive electrode above the liquid surface significantly showed more intense liquid evaporation than the experiments with the negative one - under otherwise the same conditions. Therefore, elongation speed of the gas cavities is also significantly higher. The development of spikes on liquid surface is also dependent on the liquid conductivity. However, electric field larger than 10 MV · m^-1 near the liquid surface is necessary for the development of positive streamers.