Rotational dynamics of colloidal tracer spheres in suspensions of charged rigid rods

The short-time rotational dynamics of colloidal silica tracer spheres in suspensions of rigid silica rods is investigated, using time-resolved phosphorescence anisotropy, as a function of tracer radius aT , rod volume fraction f, and the range k 21 of the double-layer repulsions between the like-charged rods and tracer spheres. A large tracer size aT and a small screening length k 21 appear to maximize hydrodynamic hindrance of tracer diffusion for given f. The marked f-dependence of the rotational dynamics is primarily determined by the large excluded volumes of the high-aspect ratio rods. Stokes–Einstein–Debye ~SED! scaling of the rotational diffusion coefficients with the inverse viscosity of the rod suspensions holds fairly well, expect for small aT and large k . The ionic strength dependence of deviations from SED scaling is rationalized in terms of an effective hard-rod model with the bare length L replaced by an effective length L14k. © 2003 American Institute of Physics. @DOI: 10.1063/1.1594182#