Liquid micro-droplet dynamics in an RF-driven glow discharge at atmospheric pressure*

The charging and stability of liquid droplets in an electric field have been extensively studied in the context of Coulombic fissions and Rayleigh limit/instability, while the dynamics of charged droplets have been investigated in electrosprays. Although these studies employed mostly unipolar charging of droplets, droplets in a plasma experience ambipolar charging involving both electrons and positive ions. In this contribution, we report on the dynamics of water micro-droplets carried by the gas flow through an atmospheric pressure helium RF glow discharge. The operating conditions allow these droplets to pass through the plasma with minimal evaporation and without reaching the Rayleigh limit. The droplet trajectory in the presence and absence of the plasma provides information on the droplet velocities. In doing so, different forces experienced by the droplet are determined. Using the equilibrium of these forces and the droplet charge estimated from a model, the ambipolar electric field at the edges of the plasma can be estimated. These results are compared with the calculated charging of droplets in multiphase plasma-liquid interaction models.