Long-time translational self-diffusion in isotropic and nematic dispersions of colloidal rods

Long-time self-diffusion in dispersions of rigid colloidal rods with an aspect ratio of 19 is studied with fluorescence recovery after photobleaching ~FRAP! in isotropic and nematic phases. The long-time selfdiffusion coefficient Ds L is found to decrease linearly with concentration up to (L/D)f50.12 ~with L the length and D the diameter of the rods, and f the volume fraction!. In the isotropic phase in coexistence with the nematic phase, Ds L remains virtually constant at about 1% of its value at infinite dilution. In the nematic phase long-time self-diffusion is found to be ten times slower than in the coexisting isotropic phase. In addition, by modifying the FRAP geometry we were able to distinguish between sidewise and lengthwise diffusion in aligned nematic phases. @S1063-651X~98!11712-9#