Sphere dynamics in isotropic colloidal rod fluids

Long-time self-diffusion and sedimentation of tracer spheres in dispersions of rigid colloidal host rods has been measured in situ as a function of ro d volume fraction for various sphere and rod dimensions. The sphere frictio n factor, which was always the same for diffusion and sedimentation, is det ermined only by the macroscopic rod viscosity when the rods are relatively mobile (dilute regime). However, when the host-rod dynamics is slow (Semi-d ilute regime) the tracer friction is much smaller than expected from the ho st viscosity, and markedly dependent on the sphere/rod size ratio. These ex periments on well defined rod-sphere mixtures, supported by low-shear rheol ogy and birefringence measurements,confirm that current models for hindered tracer dynamics do not (adequately) incorporate host mobility.