Morphology and Kinetics of the Isotropic−Nematic Phase Transition in Dispersions of Hard Rods

Abstract

The time evolution of the isotropic−nematic phase separation in dispersions of sterically stabilized colloidal rods was studied with polarization microscopy and static small angle light scattering (SALS). The rods aspect ratio is 14. In the biphasic region (between the isotropic−nematic transition volume fraction φI =12.1% and the nematic−melting volume fraction φN = 35.1%) a SALS ring develops at a wavevector Kmax which shifts to smaller K values in time. Increasing the concentration from φI to φN, polarization microscopy indicates a crossover from nucleation-and-growth to spinodal decomposition. Nucleation is accompanied by a decrease of the intensity at large K as ∼K-4, which is typically found when sharp interfaces have developed in the system. For spinodal decomposition a much less pronounced decrease is observed within the experimental time window. The rate of phase separation was studied by monitoring both the change of the turbidity and the shift of Kmax in time. A maximum rate is found just within the spinodal region.