Rotational Diffusion and Rheological Properties of Liquid Crystals

Abstract

Abstract The rotational diffusion of a rod-like molecule in a liquid crystal is considered in the molecularfield approximation, and the corresponding Fokker-Planck equation is derived. It is shown that the microscopic friction coefficient, which is the parameter of this equation, possesses the usual exponential temperature dependence with the activation energy determined by the isotropic part of the intermolecular interaction energy. The viscous stress tensor of the nematic is obtained by averaging of the corresponding microscopic expression with the non-equilibrium distribution function. Finally, explicit expressions for the Leslie coefficients are obtained in terms of molecular parameters. Their values, signs and temperature variation correspond qualitatively to experimental data.