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

The concentration dependence of the short-time rotational diffusion coefficient Hsr(φ) of charged tracer spheres with radii of 67, 103, and 137 nm in suspensions of charged rigid rods was measured as a function of the ionic strength. The ionic strength dependence was qualitatively similar to that observed previously for tracer-host sphere mixtures. Reducing the Debye screening length κ-1 from 20 to 2.3 nm had no significant effect on the tracer mobility, but a further reduction of κ-1 to 0.96 nm led to a reduction of Hsr(φ), due to stronger hydrodynamic coupling between the tracer and the rods. Results in the high salt limit were compared with earlier TPA results for rotational dynamics of tracers in suspensions of host spheres with a diameter of 2αH≅L