Charge and mass transfer by inelastic collisions

Charge-separation in cirrus clouds, a precursor for lightning in the atmosphere, occurs through the inelastic collisions and consequent transfer of mass between ice particles of different sizes. The amount of mass, and thus charge, transferred directly depends on the kinetic energy lost during the collision. We study, using particle-resolving direct numerical simulation (DNS) in two dimensions, the collisions of circular bodies settling under gravity in externally imposed flows. We vary the nondimensional Reynolds number Re = U a₁ / ν, where U is the settling velocity scale, a₁ is the radius of the larger particle and ν is the fluid viscosity; the ratio of particle sizes a₂ / a₁ ; and the scale of the background shear velocity U_f. In the absence of externally imposed shear, we find that wake capture is effective for small Re and leads to collision, but that the relative velocities at collision are small, while larger Re leads to glancing approach between particles of different sizes. Externally imposed shear enables particle collisions at significantly higher relative velocities, and thus to larger mass and charge transfer. We will quantify the dependence of the magnitude of charge transfer on the strength of externally imposed flow.