The collision efficiency of small drops

Abstract

The forces and couples on a pair of spheres rotating and translating in a viscous fluid have been obtained by Davis (1969) and O'Neill and Majumdar (1970) from accurate solutions of the Stokes equations. These forces and couples have been used to calculate the collision efficiencies of pairs of small cloud droplets. The results show that the collision efficiencies are finite for all the pairs of droplets considered, in contrast with the results of Hocking (1959) who predicted that for droplets of radius less than 19 μm the collision efficiencies were zero. The collision efficiencies of droplets of radius less than 20 μm is shown to vary considerably with the critical minimum gap between the droplets below which collision is assumed always to occur. The approximations involved are discussed and the effects of electric fields or charges on the drops are shown to be negligible in the early stages of cloud droplet growth.