Pulsed electrical discharge in bubbled water

Electrical discharges in water are used for generating hydrogen peroxide, ozone, hydroxyl and oxygen radicals, and other chemically active species for disinfection and purification of water. The interest in electrical discharges in bubbled water is motivated by the need to reduce the power consumption for igniting the discharge. Understanding the electrical and optical characteristics of the discharge will help guide the design of future purification systems. Here we report results of investigations that focus on the study of pulsed electrical discharges in a single bubble of oxygen or argon in water. The single-bubble system consists of a negative needle electrode inside a gas bubble in water and a disk electrode separated from the needle electrode by 5 - 10 mm of water. The bubble is stationary and is subjected to repeated 3-15 kV, 1 μs long voltage pulses. At these voltage levels, the discharge process is confined to the bubble with the water serving as a dielectric material covering the disk electrode. During a single voltage pulse multiple ∼100 ns long discharges occur in the bubble. These discharges deposit sufficient amount of charge on the surface of the bubble to generate a reverse-polarity discharge during the voltage shut-off. Experiments with Ar and O₂ bubbles show the same optical emission intensity at 309 ± 5 nm (OH emissions). These results indicate that OH radicals are generated by electron-driven processes in water vapor present in all bubbles.