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Measurements of the elastic constants of a liquid crystal Morris, Stephen William


The bend and splay elastic constants of the nematic liquid crystal octylcyanobiphenyl (8CB) are measured as a function of temperature using an electric-field-induced deformation. The capacitance and birefringence of a sample cell, temperature controlled to 0.1mK, were simultaneously measured as a function of applied voltage. The splay constant is determined from the critical voltage at the onset of the deformation (the Freedericksz transition). The bend constant is found by fitting the data above the critical voltage to the theory of Deuling, which is derived in full. The data is also analysed in the high- and low-field limits of the theory. The bend elastic constant displays a pretransitional divergence near the nematic-smectic A phase transition due to smectic fluctuation effects. The divergent part is fit by a power law with a critical exponent of 1.0±0.1. The Deuling theory, which is based on the assumption of linear elasticity, shows systematic deviation from the data at high voltages at all temperatures, with the disagreement increasing rapidly as the smectic phase is approached. This result suggests that the linear elastic theory of Dueling/ fails for large deformations and where smectic fluctuations contribute signifigantly to the bend elasticity. The failure near the smectic transition may be explainable by the quenching of smectic fluctuations by the deformation.

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