The Molecular Theory for the Viscosity of Nematic and Smectic C Liquid Crystals

Abstract

The rotational diffusion of a rod-like molecule in nematic and smectic C liquid crystals, and also in two-component nematic mixtures, is considered in the molecular-field approximation. The microscopic friction constant. which determines the molecular rotation drag, possesses an exponential temperature dependence with the activation energy determined by the isotropic part of the intermolecular interaction energy. The nematic Leslie coefficients are obtained in terms of molecular parameters. Their values, signs and the temperature variation qualitatively correspond to experimental data. An additional activation energy, proportional to the nematic order parameter, appears in the expressions for the rotational viscosity coefficients y₁ and y₂ both in nematics and C smectics. All Leslie coefficients possess complicated behavior with varying mixture concentration. In particular, the rotational viscosity cocfficient y₁ is proportional to a third-order polynomial in concentrations p_A and p_B with coefficients α .

Key Words:

• viscosity
• molecular theory
• nematic-smectic C