Observations on shear-induced textures and rheology of a smectic-A phase

Observations are reported of the behaviour of a smectic-A liquid crystal, 4-n-octyl-4’-cyanobiphenyl (8CB), as it is sheared in two geometries. In the first an initially-homeo-tropic sample is sheared in a planar shear flow cell, and it is observed that the shear creates textures which resemble those caused by dilating a smectic. The same textures are seen in the second geometry, where the sample is placed in a transparent cone-and-plate rheometer. The apparent viscosity of the smectic material measured in this geometry is found to increase with the fraction of the sample which contains a certain disordered focal conic texture induced by the shear. It appears that the presence of this texture gives the smectic a finite yield stress.