Dynamics of fibrous-type particles: brownian coagulation and the charge effect

The Brownian coagulation of ultrafine chainlike or fibrous type particulate aerosols is theoretically investigated. The coagulation kernel is determined utilizing the concept of the effective collision diameter d_eff. Assuming equal a priori orientational probability, analytical expressions for d_eff are obtained. For monodispersive aerosols with particle length l » d, the diameter, d_eff reduces to l/2 for a collision between a particle and a plane wall, and d_eff ≃ 0.75l for a collision between two particles. In the latter case the coagulation kernel K ≃ 1.2k_BT[ln(l/d) + 0.4]/η where k_B is the Boltzmann constant and η is the viscosity, and the bipolar Boltzmann type particle charge distribution contributes to an increase in K of only about 7%. After corrections to K for slip and polydispersity, the theory agrees fairly well with the available data. The remaining discrepancy is probably caused by the effect of polarization.