Hydrodynamic interactions in rod suspensions with orientational ordering

The effect of hydrodynamic interactions on the diffusion of rods in the nematic phase is studied, both experimentally by time-resolved fluorescence video microscopy and theoretically by mesoscale-hydrodynamics simulations. The aspect ratio of the rods and the relative importance of hydrodynamic interactions-compared to direct interactions-are varied independently. This is achieved in experiments by using charged rod-like viruses (fd-virus) with varying ionic strength, both for the wildtype virus and viruses coated with a brush of polymers. In computer simulations, hydrodynamic interactions are incorporated by a particle-based mesoscopic simulation technique. It is found that translational long-time diffusion coefficients for parallel motion along the nematic director, scaled with the diffusion coefficient at infinite dilution, are significantly affected by hydrodynamic interactions, but are insensitive to the aspect ratio. In contrast, the diffusion anisotropy-defined as the ratio of the diffusion coefficients parallel and perpendicular to the nematic director-shows only a weak dependence on hydrodynamic interactions, but strongly varies with the aspect ratio.