Light Scattering Study of Critical Phenomena at the Nematic-Isotropic Transition in a Lyotropic Liquid Crystalline System.

A static and dynamic quasi-elastic light scattering study of critical behavior at the nematic-isotropic transition in the ternary lyotropic liquid crystalline system, potassium laurate-decanol-water, is reported. For the first time, light scattering from order parameter fluctuations in the nematic phase are observed. These observations allow the determination of the critical exponent (gamma)'; which is found to be 1 (+OR-) .16. This result is in agreement with the prediction for (gamma)' near a tricritical point and in disagreement with the mean field result of 0.5 for a first order transition. An experimental search for the Landau point in this system is inconclusive as no deviation from (gamma) = 1 is found for the concentrations studied. Additionally, angular dissymmetry measurements are conducted at both the nematic-isotropic and nematic-reentrant isotropic transition. These studies show the reentrant isotropic transition to behave similarly to the usual N-I transition. These measurements allow the determination of the correlation length in both isotropic phases. In the upper isotropic phase the bare correlation length is 22 (+OR-) 1.8 (ANGSTROM) and the correlation length at the transition is 400 (ANGSTROM) while the bare correlation length in the reentrant isotropic phase is 18 (+OR-) 1.6 (ANGSTROM), growing to 900 (ANGSTROM) at the transition. The viscosity through both isotropic-uniaxial nematic transitions and the uniaxial-biaxial nematic transition is studied. Flow alignment effects in this lyotropic are enhanced with respect to thermotropics and results in an extremely large reduction in viscosity upon entering the nematic phase from the isotropic. As predicted, the transport properties show no divergences at the uniaxial-biaxial transition.