Surface-field induced organisation at solid/fluid interfaces

Abstract

Self-assembled monolayers of alkanethiol derivatives have been used to control the anchoring of nematic films of alkylcyanobiphenyl (nCB) liquid crystals. Variation of the surface field, via the ω-functional group presented at the liquid crystal interface, has allowed us to explore both orientational wetting and anchoring transitions. It is found that low-energy CF₃ surfaces give rise to homeotropic anchoring of 8CB whilst CO₂H and OH terminated surfaces yield planar anchoring. The anchoring transition lies close to 5CB on CF₃- and 8CB on CH₃-terminated surfaces. The molecular alignment in the interfacial region is studied using a modified evanescent wave ellipsometric approach in which the position of the Brewster angle represents our integrated order parameter. We find complete orientational wetting by homeotropic nematic films at the substrate/isotropic fluid interface, for sufficiently low energy surfaces. In contrast, in our systems, planar anchoring is not associated with orientational wetting.