Abstract
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 rod volume fraction for various sphere and rod dimensions. The sphere friction factor, which was always the same for diffusion and sedimentation, is determined only by the macroscopic rod viscosity when the rods are relatively mobile (dilute regime). However, when the host-rod dynamics is slow (semi-dilute regime) the tracer friction is much smaller than expected from the host viscosity, and markedly dependent on the sphere/rod size ratio. These experiments on well defined rod-sphere mixtures, supported by low-shear rheology and birefringence measurements, confirm that current models for hindered tracer dynamics do not (adequately) incorporate host mobility.
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