Effect of substrate preparation on smectic liquid crystal alignment: A structural study

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Abstract
Octylcyanobiphenyl, a bilayer smectic A liquid crystal at room temperature, was deposited on gratings, grids, and flat surfaces, and was studied using high‐resolution x‐ray scattering at shallow angles of incidence. One surface of the film was in contact with air and the other was in contact with the glass or silicon substrate, which had been treated with one of two different silane solutions. At the air interface, surface tension forces caused the liquid crystal molecules to align perpendicularly with respect to the plane of the substrate. Competing with the liquid crystal–air interface, which is a strong aligner, a grating at the liquid crystal–substrate interface can produce distortions in the smectic layering which results in excess elastic energy and favors alignment parallel to the substrate and the grooves. Our results show that in films less than ∼30 μm thick, the homeotropic orientation was maintained throughout the film due to the constraint of perpendicular alignment at the air interface. However, for thicker films on gratings, Bragg scattering from molecules lying parallel to the grooves was observed. The free‐air surface was thus the strongest aligning force, followed by the surface topology, while surface anchoring was not found to play a role.