Anisotropic self-diffusion in colloidal nematic phases

By computer simulation the anisotropic long-time translational self-diffusion coefficients are calculated in the nematic phase of colloidal hard spherocylinders exhibiting Brownian dynamics in a solvent. In particular, the two diffusion coefficients D-L(parallel to) and D-L(perpendicular to), parallel and perpendicular to the nematic director, are obtained for aspect ratios between 4.8 and 16 and for arbitrary densities. Long-time diffusion along the nematic director is nonmonotonic in density: Upon increasing the density, 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(perpendicular to) and the orientation flip rate essentially scale with the nematic order parameter. While the ratio D-L(parallel to)/D-L(perpendicular to) increases with density in, the nematic phase, it decreases dramatically with increasing density in the smectic-A phase. The results are compared to recent experiments.