Bend and splay elastic constants of diheptylazoxybenzene.

Abstract

In this study we report the values of the bend (${\mathit{K}}_{3}$) and splay (${\mathit{K}}_{1}$) elastic constants of the nematic liquid crystal diheptylazoxybenzene, using a Fr\'eedericksz-transition technique. Both the bend and the splay constants were measured over a range of temperatures approaching the nematic--smectic-A (NA) transition. The ratios of ${\mathit{K}}_{3}$/${\mathit{K}}_{1}$ follow the trend of increasing towards the NA transition, which makes the conditions favorable, in the homeotropic geometry, for striped instabilities to set in. The value of ${\mathit{K}}_{3}$/${\mathit{K}}_{1}$ for our material is fairly small at higher temperatures when compared to similar materials, which is consistent with the decreasing of ${\mathit{K}}_{3}$/${\mathit{K}}_{1}$ with an increasing alkyl-chain length. The bend critical exponent ${\mathrm{\ensuremath{\rho}}}_{3}$ was determined to be 0.68\ifmmode\pm\else\textpm\fi{}0.04, which is in close agreement with a prediction of de Gennes [Solid State Commun. 10, 753 (1972)]. The effect of the nematic range on this critical exponent is examined in terms of a critical-multicritical crossover.