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 asp ect ratio of 19 is studied with fluorescence recovery after photobleaching (FRAP) in isotropic and nematic phases. The long-time self-diffusion coeffi cient D-s(L) is found to decrease linearly with concentration up to (L/D)ph i=0.12 (with L the length and D the diameter of the rods, and phi the volum e fraction). In the isotropic phase in coexistence with the nematic phase, D-s(L) remains virtually constant at about 1% of its value at infinite dilu tion. In the nematic phase long-lime self-diffusion is found to be ten time s slower than in the coexisting isotropic phase. In addition, by modifying the FRAP geometry we were able to distinguish between sidewise and lengthwi se diffusion in aligned nematic phases. [S1063-651X(98)11712-9].