Investigation of OH Species in a Helium Atmospheric Pressure Plasma Jet: from Gas Phase to Liquid Phase through the Plasma–Liquid Interface

This work develops a semiempirical 1D numerical model with average measured [^•OH_(g)] (denoted as [^•OH_(g)]_M) as the boundary condition and measured [^•OH_(aq)] (denoted as [^•OH_(aq)]_M) to calibrate the accumulated [^•OH_(aq)] modeled (denoted as [^•OH_(aq)]_S) in the solution treated by a plasma jet. The [^•OH_(g)]_M obtained in the plasma plume is integrated from the [^•OH_(g)] distribution detected in the radial direction at position 1 mm above the interface. The [^•OH_(aq)]_M in the solutions is determined from the fluorescence measurements by exciting 2-hydroxyterephthalic acid at 310 nm and detecting the fluorescence emitted at 425 nm for cases with different plasma treatment times. The developed numerical model considers both the diffusion and convection for the domain covering 1 mm above the interface with the dominant generation and consumption mechanisms considered in the discharge plume to evaluate the incoming flux of ^•OH_(g) through the interface, which is calibrated with [^•OH_(aq)]_M in the solution treated. The simulated results show that the transport behavior (i.e., diffusion and convection) plays only a minor role in the contribution of [^•OH_(aq)]_S, while the electron-impact dissociation reactions play significant roles in the generation of ^•OH_(g) in the discharge plume, leading to the high local [^•OH_(g)] and incoming flux of ^•OH_(g) to the interface. The self-association reactions of ^•OH_(g) contribute to the remarkable consumption of ^•OH_(g). The simulated [^•OH_(g)] distribution increases from the [^•OH_(g)]_M determined at the upstream boundary to its maximum near the central region as the density reaches 9.5 × 10¹⁹ m^–3 and decreases rapidly above the interface.