Critical properties of nematic-isotropic transition near the Landau point and line defects in biaxial nematics

This research work consists of two parts: microscopic study of defects and flow alignment in uniaxial and biaxial nematics, and magnetic field induced birefringence at isotropic-nematic transition near the Landau point. The lyotropic liquid crystal used in the study is a mixture of potassium laurate, 1-decanol and D{dollar}sb2{dollar}O. It is one of the very few systems in which the biaxial nematic and the Landau point are realized.; The microscopic studies were conducted in cylindrical capillaries. The defect textures of biaxial nematics under this geometry were well established. They are characteristically different than that of uniaxial nematics. This result can be used to identify future biaxial nematics. Of the two topologically different defects in the biaxial nematics, two were observed.; The flow alignment studies show, both theoretically and experimentally, that the director forms a conic structure under the flow in capillaries. The cone angle is equal to the flow alignment angle. By measuring this angle under the microscope, the ratio of viscosity coefficients were determined for the two uniaxial nematics.; By analyzing the phase shift data of field induced birefringence measurement, the Landau expansion coefficients were determined. The absolute value of the cubic term B decreases and goes to zero as the Landau point is approached, indicating that transition is second order at the Landau point. The results also show that there is a B = 0 line above the N{dollar}sb{lcub}rm c{rcub}{dollar} - I transition, running through the Landau point into the biaxial nematic phase. This is in agreement with the theoretical predications.