The transport dynamics of tens of micrometer-sized water droplets in RF atmospheric pressure glow discharges

Abstract

Charging of particles having diameters of tens of microns has been extensively studied at atmospheric pressure in the context of, for example, electrostatic precipitators where the focus was on unipolar charging. The ambipolar charging of particles in atmospheric pressure plasmas, and of droplets in particular, has received less attention. The plasma activation of droplets is of interest for water purification, fertilizer production and materials synthesis, all of which depend on the transport of the droplets through the plasma, which in turn depends on their charging. In this paper, we report on the transport dynamics of water droplets, tens of microns in diameter, carried by the gas flow through an atmospheric pressure RF glow discharge sustained in helium. The droplets 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 insights on the forces acting on the droplet. The measurements were analyzed using results from a three-dimensional fluid model and a two-dimensional plasma hydrodynamics model. We found that the transport dynamics as the droplet enters and leaves the plasma are due to differential charging of the droplet in the plasma gradients of the bounding sheaths to the plasma.

Non‐OH‐driven liquid‐phase chemistry in water microdroplets

“…Unlike formate, PFOA will diffuse from the bulk of the droplet to the interface to accumulate at the interface before reaching the plasma. The droplet takes between 10 and 21 ms to bridge the distance between the dispenser and plasma, depending on its velocity [ 67 ] . This timescale is larger than the diffusion time scale (Table 3) even for the smallest gas flow rate, allowing a significant surface density of PFOA to be formed at the plasma–droplet interface before entering the plasma.…”