Brownian motion of particles in concentrated suspensions

An intuitive treatment of Brownian motion is described, mainly based on the Langevin and generalized Smoluchowsky equation. It is assumed that spheric al particles undergo Brownian forces as well as mutual hydrodynamic and int reparticle interactions when in thermal equilibrium. The velocity auto-corr elation function summarizes the temporal behavior and the effects on the fu nction due to Brownian forces and particle interactions are represented by parameters called a Brownian relaxation time, tau(B), a hydrodynamic relaxa tion time, tau(H), and an interaction relaxation time, tau(I). The function is closely related to the mean square displacement of the particles. The m ean square displacement is directly related to the average scattering of li ght from Brownian particles. The resulting effect is, in the frequency doma in, the frequency broadening and, in the time domain, the intensity fluctua tion. The latter is more suited in experimental observations of the tempora l behavior of Brownian motion, although the detector size must be limited w ithin the coherence area. (C) 2000 Elsevier Science Ltd. All rights reserve d.