Determination of orientational order and effective geometry parameter from refractive indices of some nematics

Abstract

Orientational order parameter and effective geometry parameter have been calculated from the refractive indices of four nematic liquid crystals (LCs). A modified four-parameter model is used to fit the experimental data of temperature-dependent ordinary and extraordinary refractive indices. This four-parameter model can be approximated as a three-parameter model for low-birefringence and high-clearing temperature LCs. Experimental data of a specific LC sample ( ∼ 0.08 and T _c = 155°C) have been used to validate the three-parameter model. The behaviour of effective geometry parameter as a function of temperature and order parameter has also been reported. Different methods of order parameter determination from optical data are compared. The order parameter is calculated by using Haller, Vuks and Neugebauer equations. In addition, an equation describing order parameter in terms of effective geometry parameter is also used to calculate the order parameter. Modified four-parameter model is applied to fit the experimental data of effective geometry parameter and also to fit the order parameter determined by using four different methods. For all the parameters we studied, a very good agreement between theory and experiment is obtained.

Keywords:

• liquid crystals
• refractive indices
• effective geometry parameter
• order parameter