Temporal evolution of plasma sustained in single bubbles in liquid water

Summary form only given. Plasmas directly in or in contact with water has attracted much attention in recently years, due to their potential for environmental and chemical processing applications. Production of plasma without bubble formation has been reported using fast-rising high voltages. However, for most cases, plasma dominantly occurs in a pre-existing vapor phase or vapor phase produced as part of the discharge process itself. Plasma formation mechanism in underwater bubbles is the key to the understanding of plasma formation in liquid water and will help the design of new plasma water treatment devices. In this paper, we report on an experimental investigation of the dynamics of plasmas in isolated bubbles in water. The temporal evolution of single discharges was recorded via an ultra high-speed camera (1,000,000 frames per second). The differences in the discharge appearance and the after-effect on the bubble were systematically studied with different water conductivity, cathode-anode distance and gas species. It was observed that the breakdown mostly happens along the inner surface of the bubble and diffuse into the bubble volume. When the electrode distance is large enough, a secondary bubble would be produced and plasma would be initiated in both bubbles and later be combined to form a larger bubble. Perturbations of the bubble´s fluid boundary due to the streamer are also discussed.