Flow-aligning and tumbling in small-molecule liquid crystals: pure components and mixtures

The results of an experimental study to measure the tumbling parameter, λ, for various small-molecule liquid crystals and their mixtures are presented. The methods used include textural observations (twist walls), a direct method, a rheological method, and the oscillatory method developed by Mather, Pearson, and Burghardt in 1995. The single-component results are compared with a molecular theory derived in 1995 by Archer and Larson as well as Kröger and Sellers, which predicts the temperature dependence of λ, while the results from the binary mixtures are compared to a continuum theory derived by Rey in 1996, giving the concentration dependence of λ. The results from the four experimental methods agree with each other for single-component liquid crystals, but not for mixtures. This suggests a failure of the single director Leslie-Ericksen theory to describe the rheology of liquid crystal mixtures.     

Thermally enhanced optical nonlinearity in nematic liquid crystal close to phase transition temperature

This study investigates the beam profile and the liquid crystal (LC) arrangement affected by an optical field on LC thin films at a temperature close to nematic-isotropic phase transition temperature (T_NI). A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal (NLC). An optical field combined with thermal enhancement enhances molecular reorientation and causes additional molecular excitation along the axis of propagation of the beam. The reorientational nonlinearity yields an undulating structure with multi-foci; the length between each pair of foci increases with time, as described.     

Conoscopic observations of shear-induced rotations in nematic liquid crystals

Orientational changes in monodomains of flow-aligning liquid crystals, 4-n-pentyl-4′-cyanobiphenyl and N-(4-methoxybenzylidene)-4-butylaniline, were studied during shear and recovery in a linear shearing device fitted to an optical microscope. Planar alignment (director in the shear plane) allows the study of twist effects and was generated by strong planar anchoring at the walls with orientations in a range of 0–90° with the shear direction. While being held back by the anchoring walls, shear caused the bulk director to rotate towards a steady-state alignment angle in the shear direction (Leslie angle θ_L). The transient director rotation was observed with conoscopy. It was found that increasing the initial alignment towards the vorticity direction increased the measured θ_L. Upon stopping the flow, the bulk director returned to its initial state. With initial alignment orientation changing from parallel to perpendicular to the flow direction, the rate of the twist-driven recovery process increases. This rate increase is not seen in the splay-driven recovery which is constant and consistently faster than twist-driven recovery at all orientations studied. Received: 10 December 1998/Accepted: 7 June 1999