Sedimentation of large particles in a suspension of colloidal rods

The sedimentation at low Reynolds numbers of large, non-interacting spherical inclusions in networks of model monodisperse, slender colloidal rods is investigated. The influence of rod concentration, rod length, and inclusion stress on the inclusion's creeping motion is investigated. The decrease in sedimentation speeds as a function of rod concentration is compared to the Stokes law, using the zero-shear viscosity from the Doi-Edwards theory for semi-dilute colloidal rod solutions. The experimental speeds display the same concentration dependence as the zero-shear viscosity and are, thus, strongly dependent on the rod length. The speed is, however, a fraction of 2 and 4 lower than expected for rods of 0.88 mu m and 2.1 mu m, respectively. The results for both rod lengths superimpose when plotted against the overlap concentration, hinting at an extra dependence on the entanglement.