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Nematic liquid crystals in electric and magnetic fields Frisken, Barbara Joan


External electric and magnetic fields affect the orientation of liquid crystal molecules. Several aspects of these effects have been studied in the liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) with special attention focused on geometries where one field is applied along the direction of initial orientation and a second field is applied perpendicular to this direction to break the uniaxial symmetry of the sample. Preliminary studies involved measuring the magnetic susceptibility of a nematic liquid crystal; for the first time, these measurements were made with a SQUID magnetometer. Details of the nematic ordering in 5CB over the temperature range of 15-40°C and in a magnetic field of 0.060T have been deduced from these measurements. The influence of fields on molecular ordering has been studied theoretically by extending Maier Saupe mean field theory to include two fields. The influence of fields on fluctuations of the nematic director has been studied using continuum theory. These studies show that the effect of fields on fluctuations of the nematic director should dominate the experimental observations. . Optical measurements are reported which demonstrate for the first time that a biaxial nematic phase can be induced by applying two fields to a sample with positive susceptibility anisotropics. This biaxial phase was studied in 5CB at 33.4°C for magnetic fields between .08-.42T and electric fields between 0-6x10⁴V/m. The results agree with those predicted from fluctuation theory. As the magnitude of the symmetry breaking field is increased, a transition to an elastically deformed state takes place; this Freedericksz transition is usually second order. Theory and measurements are reported here which show for the first time that this transition can be first order in a number of geometries for the liquid crystal 5CB. In conjunction with these measurements, a novel modulated phase has been, observed. This appears to be a stable, equilibrium phase in which the director remains in the plane defined by the initial alignment and the distorting field. Experimental investigations, of this phase are presented and a simple model is proposed which predicts this behavior.

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