Order and micellar density fluctuations in the biaxial, uniaxial (Nc and ND), and isotropic phases of a lyotropic nematic liquid crystal studied by light beating spectroscopy

摘要

We have performed Rayleigh scattering measurements to study thermally excited fluctuations in a lyotropic nematic liquid crystal system exhibiting uniaxial and biaxial nematic phases. The biaxial nematic order parameter has been defined by de Gennes as a symmetric, traceless, 2nd rank tensor. The dynamics of the order parameter fluctuations predicts 5 normal modes : 2 are associated to the fluctuations in the (uniaxial and biaxial) magnitudes of the order (scalar modes); 3 are associated to the small rigid rotations of the order parameter ellipsoid, around its principal axes (orientational modes). Using a Landau-Khalatnikov formalism we have estimated the temperature dependence of the relaxation rates for these 5 modes, valid for the uniaxial and biaxial phases. We present relaxation rate data obtained from polarized and depolarized Rayleigh scattering in the biaxial, uniaxial Nc, uniaxial N D (Nc-negative birefringence; ND-positive birefringence), and isotropic phases of the K-laurate, decanol, D2O lyotropic nematic system. We have identified the 5 expected modes arising from the order parameter fluctuations. In particular, we present the experimental demonstration of the new orientational mode associated to the rotations of the biaxial « director », and the observation, in ordered phases, of the magnitude fluctuations of the uniaxial and biaxial order parameter. In addition to the 5 modes, we have found a new scalar mode in polarized scattering. This slow mode is tentatively attributed to fluctuations in micellar concentration.