Some remarks about the coagulation of aerosol particles by Brownian motion

The development of the theory for collision and coagulation of aerosols in Brownian motion is reviewed. Theoretical conclusions resulting from the application of continuum theory are compared with calculations based on the kinetic theory of gases. These two approaches lead to different results which are applicable only to charged dilute aerosol clouds, in a limited range of the ratio of the mean free path of the suspending medium λ to the particle radius R. The present theories suggest that (a) the rate of coagulation of aerosols reaches a maximum between the ratio λ/R = 5 and 10, (b) charging of particles should have an increasing effect on coagulation rates as the ratio of the average charge per particle to the particle radius increases. There is a substantial region of aerosol behavior for which the present theory and experimental work are incomplete, particularly in accounting for the effects of charged particles. It is likely that a completion of the theory for coagulation of aerosols must await new developments in the physics of ionized dense gases.