Effects of confinement and shear on the properties of thin films of thermotropic liquid crystal

The surface forces apparatus technique was used to measure the normal forces, thin film viscosity, and lateral (frictional) forces between two surfaces interacting across 4'-n-octyl-4-cyanobiphenyl (8CB), to determine the effects of confinement (film thickness) and shear (sliding velocity) on the ordering of the smectic-A and the nematic phases. The surface roughness and hydrophobicity were altered by different adsorbed surfactant monolayers to study the effects on the orientation and anchoring of 8CB. The positional order increases as the surface separation decreases, and the orientational ordering increases with increasing shear rate. The friction force in the planarly oriented nematic phase resembles results for alkanes, while the better ordered smectic-A phase exhibits lower friction forces. The liquid crystal orients perpendicularly to surfactant-coated surfaces. On a loose-packed surfactant layer, it becomes strongly anchored, which increases the resistance to sliding, while on a close-packed monolayer the friction force is low, but the liquid crystal is easily removed by the applied load or pressure.