Concentration-dependent sedimentation of colloidal rods

In this paper we first develop an approximate theory for the leading order concentration dependence of the sedimentation coefficient for rodlike colloids/polymers/macromolecules. To first order in volume fraction phi of rods, the sedimentation coefficient is written as 1+alpha phi. For large aspect ratios L/D (L is the rod length, D its thickness) alpha is found to vary like proportional to>(*) over bar * (L/D)(2)/ln(L/D). This theoretical prediction is compared to experimental results. Then, experiments on fd virus are described, both in the isotropic and nematic phase. First-order in concentration results for this very long and thin (semiflexible) rod are in agreement with the above-mentioned theoretical prediction. Sedimentation profiles for the nematic phase show two sedimentation fronts. This result indicates that the nematic phase becomes unstable with the respect to isotropic phase during sedimentation. (C) 2000 American Institute of Physics. [S0021-9606(00)51338-5].