Role of surface adsorption in the surface-induced alignment of nematic liquid crystals on evaporated SiO films

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Abstract
It is shown that a layer of liquid crystal molecules adsorbed on an evaporated SiO film plays an important role in aligning the bulk of the liquid crystal, apart from the topographical structure of the SiO film. On an SiO film evaporated from the substrate normal, various textural patterns were observed depending on how the liquid crystal was introduced onto the film. These patterns were invariably stable against the nematic-isotropic phase transition, indicating the presence of a firmly adsorbed layer of liquid crystal molecules. In particular, when the liquid crystal was injected into the cell with a unidirectional flow in the nematic phase, a stable quasi-homogeneous alignment could be obtained. For this sample, the structure of the SiO film-liquid crystal interface was studied in detail to compare its aligning mechanism with that of an obliquely evaporated SiO film, by measuring the wall-induced pretransitional birefringence and the contact angle of the isotropic liquid resting on the substrate in equilibrium with the nematic phase. The results of these measurements are in close agreement with the previous observations for an SiO film evaporated at 60° from the substrate normal, implying that the molecular processes occurring on both types of SiO films are not so much different, and concomitantly the presence of such an adsorbed layer even on the 60° evaporated SiO film. The effect of the adsorbed layer on the liquid crystal alignment on an obliquely evaporated SiO film is discussed with a particular emphasis on the temperature variation of the tilt angle. Finally, it is qualitatively argued on the basis of the present measurements that the molecules in the adsorbed layer are oriented with a considerable randomness on the molecular level rather than well aligned.