Surface ordering and anchoring behaviour at liquid crystal surfaces laden with semifluorinated alkane molecules

Abstract

We study the surface ordering and anchoring behaviour at liquid crystal/air interfaces laden with the semifluorinated alkane C₁₈H₃₇–C₁₂F₂₅. On alkanes, the semifluorinated alkane is known to form Gibbs films exhibiting a sharp transition from a dilute state at higher temperatures to a dense state at lower temperatures. We observe a similar transition in Gibbs films of this compound on the surface of cyanobiphenyl liquid crystals. The transition temperature can be tuned by controlling the C₁₈H₃₇–C₁₂F₂₅ concentration in the liquid crystal bulk phase. Our results show that the transition in the Gibbs film changes the anchoring of the liquid crystal from homeotropic at higher temperatures to planar at lower temperatures. When the transition temperature of the Gibbs film is tuned so that it takes place above the nematic–isotropic or smectic–isotropic transition of the liquid crystals, a pronounced effect on the nematic or smectic surface order is observed. Our study demonstrates for the first time that the ordering and anchoring behaviour of thermotropic liquid crystals can be tuned at a liquid crystal/air interface.