Surface Anchoring of Nematic Liquid Crystals

Abstract

Existing experimental techniques for measuring the orientational anchoring strength at a substrate-nematic interface are critically analyzed with a particular emphasis on their macroscopic basis and on the range of applicability and precision. Conceptual basis of the anchoring strength is first clarified by making a distinction between the macroscopic and the microscopic approaches toward the substrate-nematic interface, and the significance of the Gibbs’ dividing surface is emphasized together with some thermodynamic properties of the anchoring strength and pretilt angle. The principles of some typical experimental techniques are next reviewed from a fundamental viewpoint, and possible complications and sources of error in these methods, when applied to an interface with strong anchoring, are discussed. Drawing upon the result of the “high-electric-field technique” which yields an anchoring energy apparently larger than 10⁻³ J/m², the greatest value ever observed, for a planar-aligned nematic on rubbed polymer surface, it is argued through the consideration of the dividing surface that the observed anchoring strength, though still meaningful as a thermodynamic parameter, does not necessarily imply the existence of strong substrate-nematic orientational coupling on the microscopic scale.