Mechanism of Surface Alignment in Nematic Liquid Crystals

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Abstract
Chatelain hypothesized that orientation of liquid crystals by interaction with the substrate is due to an adsorbed layer of fatty contaminants, but substrate topography has also been postulated as a causal factor. Experiments with chemically cleaned surfaces rubbed without contamination now confirm the importance of the impurity layer, which has been investigated using Auger spectrometry. Through the use of known surfactans, intentionally deformed substrates, and electron microscopy, the roles of surface topography and surface chemistry have been distinguished: substrates with surface energy lower than the liquid crystal surface tension cause homeotropic alignment; otherwise alignment is parallel to the substrate plane, following any long-range order, e.g., grooves, present on the surface.