Interfacial Ordering in a Liquid Crystal Near the Nematic - Isotropic Transition

Abstract

The molecular ordering of the liquid crystal 8CB near a surfactant-coated glass surface has been studied using evanescent-wave ellipsometry. Surface molecular ordering was measured in the isotropic phase, and both bulk and surface ordering were measured simultaneously in the nematic phase in the vicinity of the nematic-isotropic transition temperature T _NI.Two kinds of behavior were observed, depending on the strength of the surface-liquid crystal interaction in a given sample. (1) For samples with a strong surface-liquid crystal interaction, at temperatures above T _NI the interface is wet by a homeotropically oriented, ordered layer whose thickness diverges as the temperature T approaches T _NI. For T < T _NI, the interface is wet by an surface region of enhanced nematic order whose thickness increases as T → T _NI but remains finite at T _NI. (2) For samples with a weak surface-liquid crystal interaction, at temperatures above T _NI the interface is wet by an ordered layer whose thickness increases but remains finite as T → T _ni. For T < T _NI, the interface is wet by a disordered surface region whose thickness increases but remains finite as T → T _NI. The results are analyzed in terms of the Landau-de Gennes theory and found to be in qualitative agreement with theoretical predictions.

Key Words:

• Interfacial ordering
• surface ordering
• nematic-isotropic transition
• ellipsometry
• liquid crystal interface