Flow Alignment in Cyanobiphenyl Liquid Crystals

Abstract

The cyanobiphenyl compounds 5CB (4-cyano-4'-n-pentyl-biphenyl) and the corresponding hexyl, 6CB, and heptyl, 7CB, form a homologous series of nematic liquid crystals. The flow of these liquid crystals in a torsional shear flow apparatus was studied. The shear induced alignment of the director field gives a corresponding induced birefringence, which is observed between crossed polarizers. At a given shear rate a nearly uniform flow alignment throughout the liquid crystal layer is obtained, except at two boundary layers where the director is changing rapidly in space, adjusting to the normal boundary conditions. The size of the boundary layers can be estimated by studying the way in which they decrease in thickness as the shear rate is increased. From the observed optical anisotropy we compute the flow alignment angle and the ratio of rotational viscosity coefficients for the three cyanobiphenyl liquid crystals as a function of temperature.