Understanding Plasma–Liquid Interface Instabilities Using Particle Image Velocimetry and Shadowgraphy Imaging Methods

Plasma-induced fluid convection is a poorly understood phenomenon originating at the plasma-liquid interface. Diffusive transport, both natural and driven, as well as convection effects determines the rate at which reactive species are produced at the interface and those diffusing from gas phase, ultimately mix within the solution. Efficient mass transfer rates are important to a number of applications ranging from water purification to plasma medicine. In order to understand the mechanisms of plasma-generated active species' transport through the plasma-liquid interface, a new 2-D Hele-Shaw-like cell was constructed to allow shadowgraph and particle image velocimetry (PIV) measurements of the interface. The plasma-induced flow field in the liquid water as observed using PIV revealed the formation of vortices as well as significant velocity shear, particularly near the bubble interface in the bulk liquid. These flows appear to be related to streamer strikes at the interface. The stability analysis was carried out to investigate the presence of the Kelvin-Helmholtz instability and to determine if the observed perturbations to the flow field were stable. The circulation flows were found to be consistent with Marangoni convection. Additionally, shadowgraph images showed that the streamers impact at the interface locally compress the fluid, thereby driving diffusive flow, density gradients, and interfacial capillary waves.