In-Water Plasma Generation on a Liquid Wall Using a Compact Device and Dedicated Power Supply

Plasmas that are generated on and in liquids are generally the subject of pure research at universities; however, they have considerable practical potential for use in material processing, water purification, and sterilization. Their chief drawback is that they require a large device to generate in-water plasmas and a bulky power supply. If the device size and the volume of the power supply could be reduced, it might be possible to realize water sterilization in home electric appliances, such as washing machines. We have developed a compact device with a unique structure and a dedicated power supply that provides high voltage at high frequencies for generating in-water plasmas. Our dedicated power supply occupies one-sixth of the volume of comparable types. The device can generate in-water plasmas in an air stream using ambient air introduced from outside using a pump. Hydroxyl (OH) radicals in in-water plasmas were detected by optical emission spectroscopy, and their spatial distribution was observed in the air steam using an intensified charge-coupled device camera and a bandpass filter of 309 nm. Hydroxyl radicals in water were detected as 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO)-OH signals using electron spin resonance spin trapping, both before adding DMPO to water and after doping the plasma-treated water with DMPO. It was found that OH radicals were generated in in-water plasmas and persisted in plasma-treated water. Using the detection of DMPO-OH signals employing the postdoped method, OH radicals were measured at 0.86 nmol/cc; they remained in the water for a long time after turning OFF the power supply. Finally, we demonstrated the decomposition rate of indigo carmine using our device and power supply to be about 13-fold that of the comparable device, despite its consuming about one-seventh of the input power. Hydroxyl radicals have high oxidation potential, so in-water plasmas as a source of radicals may be applicable to water sterilization in home elect- ic appliances.