Collision efficiency of cloud droplets in quiescent air considering lubrication interactions, mobility of interfaces, and noncontinuum molecular effects

Abstract

The gravitational collision efficiency of a pair of cloud droplets settling in quiescent air is computed using various models for the aerodynamic interaction forces. The employed models consider (i) the effect of spherical fluid drops with mobile interfaces, and (ii) the noncontinuum molecular effect which changes short-range lubrication forces when the gap size between a pair is comparable to the air mean free path. These cases are compared with the widely used case that considers the droplets to be spherical rigid particles. We find that assuming rigid particles is accurate for water droplets interacting in air, but noncontinuum lubrication has to be taken into account.