Effective diffusion coefficient Brownian dynamics

When multiple scattering is discarded from the measured signal, DLS can be used to study the dynamics of concentrated suspensions, in which the Brownian motion of individual particles (self-diffusion) differs from the diffusive mass transport (gradient or collective diffusion), which causes local density fluctuations, and where the diffusion on very short time-scales (r < c lD) deviates from those on large time scales (r c D lones and Pusey 1991 Banchio et al. 2000). These different diffusion coefficients depend on the microstructure of the suspension, i.e. on the particle concentration and on the interparticle forces. For an unknown suspension it is not possible to state a priori which of them is probed by a DLS experiment. For this reason, a further concentration limit must be obeyed when DLS is used for basic characterisation tasks such as particle sizing. As a rule of thumb, such concentration effects vanish below concentrations of 0.01-0.1 vol%, but certainty can only be gained by experiment. 

A different formahsm in which the diffusion of a Brownian gas in a fluid under stationary and non-stationary flow has been analyzed is mesoscopic nonequilibrium thermodynamics (MNET) (Perez-Madrid, 1994 Rubi Mazur, 1994 Rubi P rez-Madrid, 1999). This theory uses the nonequUibrium thermodynamics rules in the phase space of the system, and allows to derive Fokker-Planck equations that are coupled with the thermodynamic forces associated to the interaction between the system and the heat bath. The effects of this coupling on system s dynamics are not obvious. This is the case of Brownian motion in the presence of flow where, as we have discussed previously, both the diffusion coefficient and the chemical potential become modified by the presence of flow (Reguera Rubi, 2003a b Santamaria Holek, 2005 2009 2001). 

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