Study of electro-migration and capture of particles by a charged pendant droplet

Abstract

To understand the particle scavenging mechanisms by charged droplets in air cleaning technologies, we investigate the phenomena of non-incorporation/non-attachment and return/rebound of airborne particles/clusters upon impacting charged droplets. Using high-speed imaging, the motion and fate of particles and clusters lifted from a surface and colliding with a stationary droplet suspended from an electrified needle are examined in detail. Aggregates of glass beads and carbon particles are employed to explore how material characteristics, such as hydrophobicity/hydrophilicity, conductivity, and dielectric constant, influence their capture by the droplet. Analysis of observed cluster trajectories along with numerical modelling has been carried out to provide insights into their charges and velocities. DC and AC electric field experiments unequivocally demonstrate that the Faradaic induction mechanism at both the ground and droplet surfaces causes the acquisition or reversal of particle charges. The migration of particles and clusters is predominantly governed by Coulombic force, not dielectrophoretic force (DEP). Hydrophobic carbon particle clusters with high conductivity exhibit increased rebound/non-incorporation from the droplet, whereas hydrophilic glass beads with low conductivity do not show significant rebound and readily integrate into the droplet. The study demonstrates that (i) particles deposited on surfaces suffer resuspension and (ii) the capture of particles striking a charged droplet surface may not take place universally but could have a strong dependence on the chemical and material nature of the particles. Although the study is based on aggregated clusters and not on individual particles, it opens up avenues for future research using smaller particles and droplet systems relevant to air-cleaning applications.