Critical Phenomena at the IsotropicBlue Phase Transitions in Esters of Cholesterol.
Abstract
This study is an experimental investigation of pretransitional effects in cholesteric liquid crystals that have blue phases. The experiments were designed to measure the critical exponents associated with the divergence of the electric susceptibility. Other aims of the study were the testing of the validity of the present version of the Landau theory and the use of the harmonic approximation. Both light scattering and Kerr effect studies were used. If the harmonic approximation proved to be adequate then the light scattering data would be used to measure the pitch parameter ((kappa)) which plays a central role in the theory of blue phases in cholesteric esters. This dimensionless parameter is a quantative measure of the chirality of cholesterics which form blue phases. From the results of the Kerr effect studies one could also determine the Kerr constants in the isotropic and blue phases. Both of these experiments have never been done before. In the present version of the Landau theory, the order parameter is a five component tensor (the anisotropic part of the dielectric tensor). Each component represents a normal mode. Light scattering in general is from a combination of these normal modes. The above methods were chosen because coupling with only specific normal modes of the order parameter could be achieved. The validity of the harmonic approximation has been well established. The inverse intensities are linear and the two polarization configurations have the same slopes. The inverse induced birefringences also vary linearly with temperature. These results imply that the critical exponent (gamma) = 1 as expected. The Landau theory in its present form cannot explain the experimental phase diagram. It predicts a pitch parameter which is larger by a factor of two or three from what is seen.
 Publication:

Ph.D. Thesis
 Pub Date:
 1986
 Bibcode:
 1986PhDT.......133S
 Keywords:

 Physics: General