Laser-Induced Fluorescence Measurement of Water Vapor and OH Density Distributions Near Droplets in a Plasma

Atmospheric pressure plasma treatment of water has great potential for the decomposition of hazardous materials and wastewater treatment. The use of droplets enables a significant increase in decomposition efficiency due to the large surface to volume ratio compared to bulk liquid 1 . The OH radicals are key enablers of water treatment and their transport from the gas phase to the liquid can be the limiting factor in conversion for low OH radical densities. The OH production in plasmas containing droplets is, however, complicated by droplet evaporation leading to possibly strong gradients in both water vapor and OH radical densities around the droplet in the plasma. In this contribution, we report on measurements of the water vapor and OH distributions near the droplet in an atmospheric pressure diffuse RF glow plasma in helium by laser induced fluorescence. Water vapor concentrations as high as 10 4 ppm were found near the droplet and a strong correlation with the droplet residence time in the plasma was found. The corresponding OH densities near the droplet were found to be in the range of 5×10 13 cm -3 and 2×10 15 cm -3 . This density spans concentrations that suggest transitioning from a flux limited decomposition in the droplet to a solution phase diffusion limited regime which will be discussed in detail.