Anisotropic self-diffusion in colloidal nematic phases

By computer simulation the anisotropic long-time translational self-diffusi on coefficients are calculated in the nematic phase of colloidal hard spher ocylinders exhibiting Brownian dynamics in a solvent. In particular, the tw o diffusion coefficients D-L(parallel to) and D-L(perpendicular to), parall el and perpendicular to the nematic director, are obtained for aspect ratio s between 4.8 and 16 and for arbitrary densities. Long-time diffusion along the nematic director is nonmonotonic in density: Upon increasing the densi ty, it first increases due to stronger alignment and then decreases due to packing constraints. It is shown that the ratio D-L(parallel to)/D-L(perpen dicular to) and the orientation flip rate essentially scale with the nemati c order parameter. While the ratio D-L(parallel to)/D-L(perpendicular to) i ncreases with density in, the nematic phase, it decreases dramatically with increasing density in the smectic-A phase. The results are compared to rec ent experiments.