Temporal evolution of self-organized spiral patterns formed in a liquid-electrode discharge sustained in atmospheric pressure air

Being stabilized in a docile glow discharge phase, liquid-electrode discharge (LED) at atmospheric pressure is one of the popular plasma sources. Extremely fascinating in LED is the appearance of various self-organized patterns at the surface of the liquid when it is at anode potential. However, the fascinating spiral patterns have to date not been reported in the literature about LED. In this paper, the spiral patterns with various arm numbers are observed in LED excited by a sinusoidal voltage. The results indicate that the arm number of the spirals decreases with time, and multi-arm spirals evolve to a single-arm spiral at last. During this evolution, the current waveform keeps a constant profile, which starts with a short pulse and is followed by a long hump. In addition, the intensity of the pulse increases, while that of the hump keeps almost constant as time elapses. Fast photography reveals that the single-arm spiral forms in the hump discharge phase and develops on a timescale of several microseconds. To shed light on the formation mechanism of the self-organized spirals, the spiral patterns are simulated based on the reaction diffusion model, which reveals that the evolution of the spiral patterns results from the variation in ion strength of the liquid electrode. Finally, from optical emission spectroscopy, plasma parameters including electron density, electron temperature, vibrational temperature, and gas temperature are investigated for the spiral patterns, all of which show a growing trend during the evolution from the multi-arm spiral to the single-arm one. The obtained results are of great significance for the pattern formation dynamics of self-organized spirals in different systems.