COAGULATION OF AEROSOLS BY BROWNIAN-MOTION

Experimental data from eight different papers show that the coagulation coefficient of aerosol particles is a unique function of the Knudsen number, up to the value Kn = 15, and is equal to Ko, the product of the Smoluchowski coagulation coefficient, KSm, and the Cunningham-Knudsen-Weber-Millikan factor C. The βF factor of Fuchs is erroneous, making the coagulation coefficient much too small. Above Kn = 18 the coefficient decreases to the gas-kinetic value, KGK, which is independent of Kn but depends on the radius and density of the particles and also the temperature. The value of Kn at which the gas-kinetic value is attained is designated Knχ; because, owing to the persistence of velocity of the particles, their bombardment by a limited number of gas molecules has negligible effect so that the gas-kinetic coagulation coefficient is reached at a value of Kn; which is less than infinite, namely Knχ. The larger the particles the lower is the value of Knχ. An empirical interpolation function, βD, is proposed which is a function of the Peclet number 2Va/D. It operates in the range 15 < Kn < Kn∞, tending to unity for small Kn. The coagulation coefficient for all values of Kn is equal to KSmCβD - this group tending to KGK as Kn approaches Kn∞ and to KSm as Kn approaches zero. © 1979.