Generation mechanisms and physical properties of electrical discharges in and above water

During the last two decades, electrical discharges in or near water have been drawing a lot of attention in view of environmental and medical applications. Indeed, the simultaneous generation of intense UV radiation, shock waves and active radicals makes these discharges particularly suitable for decontamination, sterilization and purification purposes. The physics of these complex discharges is not fully understood, although in the last few years considerable progress has been made. This chapter reviews the current status (end 2007) of research on the generation mechanisms and physical properties of electrical discharges in water. Four discharge types covering the main basic physics of electrical discharges in water will be discussed thoroughly. (1) The pulsed direct liquid phase discharge consisting of the streamer and spark/arc discharge, generated by high voltage pulses on sharp electrodes in bulk water. (2) Electrical discharges in gas/vapor bubbles surrounded with water or in contact with a metal electrode. (3) Discharges in the gas phase (typically at atmospheric pressure) where either one or both metallic electrodes are replaced by a ‘water electrode’. (4) Diaphragm or capillary discharges, where a conductive current is forced to flow through a submerged diaphragm or capillary, leading to evaporation with subsequent electrical breakdown and plasma formation in the vapor phase. Next, different experimental configurations and generation methods will be discussed, with an assessment of their technical and physical features in relation to the application potential.