Pattern formation in a liquid-electrode discharge simultaneously excited by alternating- and direct-current power supplies

A fascinating phenomenon in atmospheric pressure liquid-electrode discharge is the formation of self-organized patterns. Up to now, the self-organized patterns are excited predominantly by a direct-current (DC) power source, while only a few studies are implemented with an alternate-current (AC) power supply since the patterns will be destroyed when the dissipated power is high. To enrich the self-organized patterns, two power supplies (DC and AC) are simultaneously employed to excite liquid-electrode discharge in atmospheric pressure air in this work. With varying peak voltage (Vp) of AC power supply, the discharge pattern evolves from a pentagon, a flower-like shape, a gear structure, to a single ring, in which the pentagon and the flower-like patterns have not been reported in the literature. Waveforms of AC voltage, discharge current, and integrated emission signal indicate that the negative discharge is stronger than the positive discharge, whose amplitude increases with increasing Vp. Fast photography reveals that during the positive discharge, there are five spots in the pentagon, while the positive discharge is diffuse in the other patterns. Except the five spots of the pentagon, the patterns are formed during the negative discharge. The emission intensities of N2+, N2, and OH radicals increase continuously with Vp. Based on optical emission spectrum, plasma parameters of the patterns including vibrational temperature, gas temperature, electron density, and electron energy are estimated as well as reduced field, which increase with increasing Vp. Furthermore, as Vp increases, solution conductivity rises while pH decreases.