Nanosecond plasma discharge dynamics on a levitated water droplet

Abstract

The interaction between nanosecond pulsed discharges and acoustically levitated droplets is investigated using time-resolved ICCD imaging, optical emission spectroscopy (OES), electrical diagnostics, and numerical simulations. Experiments are carried out in a pin-to-pin electrode configuration in air at atmospheric pressure with ultra pure water droplets (1 µS.cm−1) and saline droplets (16 mS.cm−1). The results show that the droplet acts as an obstacle contoured by the plasma channel while enhancing the local electric field through polarization, thereby facilitating plasma ignition. For ultra pure water, two streamers develop between the electrodes and the droplet and subsequently merge into a single plasma channel enveloping the droplet surface after approximately 2 ns. In contrast, for saline droplets, electron neutralization at the liquid interface slows streamer propagation along the surface, delaying the merging process. After about 10 ns, however, the discharge evolution becomes similar for both droplet types, regardless of conductivity. The droplet behaves as a capacitor, with its capacitance governing the charging time and influencing early streamer dynamics. Finite element numerical simulations confirm the electric field enhancement near the droplet poles, while evaporation measurements indicate negligible plasma-induced heating, excluding significant Joule effect. Overall, these findings advance the understanding of plasma–droplet interactions and highlight the decisive role of liquid conductivity and dielectric properties in discharge dynamics. The insights gained provide a foundation for optimizing plasma–liquid processes involving droplets in environmental, biomedical, and material science applications.