Electrostatic enhancement of particle collision rates in atmospheric flows

Abstract

Collisional growth of tiny particles is a fundamental process governing the growth of cloud droplets and the aggregation of ash particles in volcanic plumes, with direct implications for precipitation formation, cloud lifetime, and ash plume dynamics. The particles in these scenarios often carry electric charges. In this study, we investigate the collision dynamics of a pair of like charged dielectric spheres subjected to a uniaxial compressional flow, an important linear flow that captures key features of atmospheric straining motions. Finite particle size leads to electrostatic interactions that deviate from the point charge approximation, resulting in far field repulsion and near-field attraction, which in turn generate nontrivial particle trajectories and critical collision thresholds. For certain combinations of charge and size, the interplay between hydrodynamic and electrostatic forces creates strong radially inward particle relative velocities that substantially alter particle pair dynamics and modify the conditions required for contact. For uncharged particles, collision efficiency increases monotonically with particle size ratio. However, in the presence of electrostatic forces with high charge ratio values, the collision efficiency exhibits a nonmonotonic dependence, attaining a maximum at small size ratios and decreasing as the ratio increases, with a crossover beyond which larger particles become less favorable for collision. These results demonstrate that the same polarity charges on finite sized atmospheric particles do not necessarily inhibit collisions. Instead, they can enhance collisional growth for specific charge and size ratio combinations, revealing counterintuitive pathways relevant to cloud microphysical processes and volcanic ash aggregation in electrified atmospheric environments.