Brownian coagulation of aerosols in rarefied gases

Abstract

The kinetics of coagulation of aerosols is well understood when the particle radius is much larger or much smaller than the mean free path of the surrounding gas (very small or very large Knudsen numbers). However, no adequate theory is available in the so-called transition regime between these limiting cases. This is an experimental investigation of Brownian coagulation in the transition regime. Monodisperse aerosol [di(2-ethylhexyl)sebacate, particle radius 0.2 μm] was produced in a falling-film condensation aerosol generator. The evolution of the size distribution, and hence the rate of coagulation, was monitored as the aerosol flowed through a series of light scattering cells. Measurements were performed at five different pressures, ranging from atmospheric pressure to 67 Torr, thereby scaling the Knudsen number from 1 to 12. The influence of diffusional deposition, sedimentation, and gradient coagulation in vaporization was kept to a minimum, and the uniqueness of the inversion technique was demonstrated. The experimental results are in good agreement with Fuchs’ semiempirical interpolation equation, assuming a coalescence efficiency of unity.