Morphology and kinetics of the isotropic-nematic phase transition in dispersions of hard rods

The time evolution of the isotropic-nematic phase separation in dispersions of sterically stabilized colloidal rods was studied with polarization micr oscopy and static small angle light scattering (SALS). The rods aspect rati o is 14. In the biphasic region (between the isotropic-nematic transition v olume fraction phi(I) =12.1% and the nematic-melting volume fraction phi(N) = 35.1%) a SALS ring develops at a wavevector K-max which shifts to smalle r K values in time. Increasing the concentration from phi(I) to phi(N), pol arization microscopy indicates a crossover from nucleation-and-growth to sp inodal decomposition. Nucleation is accompanied by a decrease of the intens ity at large K as similar to K-4, which is typically found when sharp inter faces have developed in the system. For spinodal decomposition a much less pronounced decrease is observed within the experimental time window. The ra te of phase separation was studied by monitoring both the change of the tur bidity and the shift of K-max in time. A maximum rate is found just within the spinodal region.