Novel Method of Measuring Surface Torsional Anchoring Strength of Nematic Liquid Crystals

Abstract

A simple and accurate method of measuring the surface torsional anchoring strength of nematic liquid crystals is proposed. This method is based on the simultaneous determination of the actual twist angle and the cell gap (optical retardation) of a twisted nematic cell in which the easy axes of the upper and lower substrates are at a particular angle ( e.g., 90°). The TN cell is placed between two polarizers. The TN cell and the analyzer are mounted on rotatable stages. The measurement procedure consists of the following steps. (1) Using monochromatic light, we minimize the amount of light transmitted by rotating both the TN cell and the analyzer (we denote these minimizing angles of the analyzer and the TN cell from the polarizer as Ψ _p ⁽¹⁾ and Ψ₀ ⁽¹⁾, respectively. (2) We set the TN cell at a particular angle from Ψ₀ ⁽¹⁾, and minimize the amount of light transmitted by rotating only the analyzer (we denote this minimizing angle of the analyzer as Ψ _p ⁽²⁾). From the values of Ψ _p ⁽¹⁾ and Ψ _p ⁽²⁾, we can determine both the actual twist angle and the optical retardation using the analytical expression for optical transmissivity based on the Jones matrix method. We discuss the accuracy of this method. Experimental examples are also shown.