Measurement of Director Orientation at the Nematic—Isotropic Interface Using a Substrate-Nucleated Nematic Film

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Abstract
A flat and stable interface between the nematic and isotropic phases was prepared in a sandwich-type cell under thermal equilibrium conditions to determine the director orientation at the nematic-isotropic interface. The preparation method is based on the fact that a rubbed polyvinylalcohol film nucleates a surface nematic film when the isotropic liquid of a nematogen containing a small amount of impurity is cooled down passing through the clearing point. The impurity causes the nematic and isotropic phases to coexist over a finite temperature range in which the nucleated nematic film keeps its interface between the isotropic phase parallel to the substrate. Using the nematic-isotropic interface obtained this way, we determined the preferred orientation at the interface for a nematogen, 4-cyano-4′-n-pentylbiphenyl, mixed with a small amount of hexamethylbenzene, by observing the electric field-induced deformations in the nematic layer. For this purpose, a simple procedure which requires little knowledge about the material constants was developed theoretically. The director was found to be tilted by 28.1 ± 0.6° from the interface, and its anchoring energy, or equivalently the anisotropy of the interfacial tension, was estimated to be on the order of 10-3 erg/cm2.