Stokes friction coefficient of spherical particles in the presence of polymer depletion layers Analytical and numerical calculations, comparison with experimental data

The Stokes friction coefficient has been calculated for the case of polymer depletion near a spherical particle surface, to provide a theoretical background for the interpretation of dynamic light scattering data. In a quasi-flat Newtonian approximation an analytical solution for arbitrary viscosity profiles was obtained. Numerical integration of the NavierStokes equation confirmed the large range of applicability of the approximate analytical solution. Explicit equations for an exponential and a step viscosity profile are given. To compare experiment and theory, dynamic light scattering data of liposomes with different radii in 110 kDa dextran are presented. For the first time evidence of a depletion layer relaxation effect has been obtained. This relaxation caused an effective reduction of the depletion layer thickness for small particles. A linear correction for the relaxation effect is suggested.