Coagulation of highly concentrated aerosols

A number of material synthesis processes such as flame, plasma and laser ablation have been developed for production of films and powders at low pressure and high temperature. At these conditions particle growth typically takes place by coagulation in the free molecule and transition regimes. As economic manufacturing of these materials favors operation at high particle concentrations, classic coagulation theory may not be sufficient to describe the ensuing aerosol dynamics, especially if fractal-like particles are formed. The coagulation rate of highly concentrated, polydisperse aerosols is investigated here from the free molecule to the continuum regime by solving the corresponding Langevin dynamics (LD) equations. The LD simulations are validated by monitoring the attainment of the self-preserving size distribution (SPSD) for dilute particle volume fractions, phi(s), below 0.1%. High particle concentrations in the free molecule regime lead to deviations of the aerosol dynamics from the kinetic theory of gases especially during instantaneous coalescence (completely inelastic particle-particle collisions) resulting in slower coagulation rates and slightly narrower SPSDs than in conventional dilute aerosols. In the transition regime, the coagulation rate of highly concentrated aerosols is progressively higher than that for dilute aerosols as growing particles enter the continuum regime where coagulation rates are 2-30 times higher than that of classic Smoluchowski theory. At high particle concentrations (phi(s), > 1%), a SPSD is approached (sigma(g,n) = 1,42) that does not exhibit the characteristic minimum at the transition regime of dilute aerosols. A relationship is developed for the aerosol coagulation rate of highly concentrated aerosols from the free molecule to continuum regime. (C) 2008 Elsevier Ltd. All rights reserved.