OH Radicals Distribution and Discharge Dynamics of an Atmospheric Pressure Plasma Jet Above Water Surface

In recent years, plasma in contact with water has received much attention because of their biological and medical applications. Hydroxyl radicals (OH) are believed to play an important role in these applications. In this paper, the dynamic of a nanosecond atmospheric-pressure plasma jet (APPJ) above water surface and the OH radicals produced by the APPJ were investigated experimentally. The distributions of OH both in free space and above the water surface were recorded by a laser-induced fluorescence (LIF) technique. It was observed that for cases above water surface, OH LIF intensity was 2-3 times stronger using the same operating parameters. Specially, a second OH intensity peak would appear above the liquid surface, potentially leading to a significant increase of OH radical flux diffused from the gas phase to dissolve in the liquid. Discharge dynamic recorded by high speed intensified charge-couple device camera showed that after the primary discharge, a secondary discharge would form and propagate backward from the gas-liquid interface toward the nozzle. This phenomenon was previously believed to exist only above conductive surfaces, such as metal. This paper showed the importance of using in situ diagnostics for plasma jet biomedical applications, as the spatial distribution of the reactive species produced during the discharge process can be significantly influenced by the introduction of wet surfaces, such as cultivated bacteria or human tissue.