Spatial and temporal dynamics of single nanosecond discharges in air with water droplets*

Discharges in the presence of liquids are applied in many fields, such as water processing (depollution, agriculture, etc.), material treatment (synthesis, functionalization, machining, etc.), and health care (virus deactivation, wound healing, etc.). Regardless of the targeted application, a profound understanding of plasma–liquid interactions is needed to optimize the process. Therefore, extensive research efforts have been put towards analyzing the physical and chemical phenomena occurring at the plasma– liquid interface, particularly during the last decade. Recently, important progress has been made in studying the dynamics of a discharge generated in gaseous medium with millimeter-scale droplets. Specifically, the influence of discharge processing on the properties of the liquid droplets has been assessed by analyzing the hydrodynamic, physical, and chemical phenomena (e.g. internal flows, plasma capillary phenomena, droplets drying under the effect of discharge, and residues left on substrates) occurring in the droplets. This study investigates the electrical characteristics and the spatial-temporal dynamics of nanosecond discharges in air containing one or two millimetric droplets of deionized water. Analysis of the effects of voltage amplitude (V_a) and pulse width on the discharge mode shows that at low V_a, the discharges are run in streamer mode; however, at high V_a, a streamer-to-spark transition is observed. Although we found that the droplet size (diameter between 2 and 4 mm) does not significantly influence the discharge dynamics, its position with respect to the gap (on- or off-axis) has a strong effect. Time-resolved imaging of three droplet configurations (one on-axis droplet, one off-axis droplet, and two on-axis droplets) was used to unveil the ignition and propagation dynamics of streamers and sparks at nanosecond time scale. We also investigated the role of droplet electrical conductivity on the discharge behavior. On the other hand, a simplified fluid model is developped, and the initial results show a great agreement with the experimental results. Therefore, the model can be utilised to determine the role of different physical parameters, such as dielectric permitivity, electrical conductivity, and droplet shape.