Rotational and translational self-diffusion in colloidal sphere suspensions and the applicability of generalized Stokes-Einstein relations

We investigate long-time translational and rotational self-diffusion of flu orocarbon tracer spheres (100 nm radius) in aqueous dispersions of silica h ost spheres (10 nm radius). Diffusion is measured as a function of ionic st rength (0-10 mM NaCl) and for host-sphere volume fractions up to 37% using depolarized dynamic light scattering. Both translational and rotational sel f-diffusion are strongly hindered and have a similar concentration dependen ce as the low-shear viscosity, except at the lowest ionic strength. Our res ults show that the Stokes-Einstein (SE) relations, linking viscosity and di ffusion at infinite dilution, can be extended to dense colloidal systems wh enever the dynamics of the host spheres is fast compared to the experimenta l time scale. However, when the host-sphere dynamics is relatively slow, su ch as in the case of very large host particles or charged particles at low ionic strength, tracer-sphere diffusion (in particular rotation) generally is faster than the SE relations.