Effective elastic theory of smectic-A and smectic-C liquid crystals

TL;DR

This paper derives the effective layer elastic energy for smectic-A and smectic-C liquid crystals, revealing how layer bending rigidity varies with wavelength, tilt angle, and elastic constants, and discusses implications for hydrodynamics and wave propagation.

Contribution

It provides an analytical derivation of the effective layer elastic energy for smectic phases, including new insights into anisotropic bending elasticity and characteristic length scales.

Findings

Layer bending rigidity depends on wavelength and director coupling.

Tilt angle varies with wavelength and elastic parameters, showing discontinuous changes.

Discussions include hydrodynamics and wave propagation in smectic liquid crystals.

Abstract

We analytically derive the effective layer elastic energy of smectic-A and smectic-C liquid crystals by adiabatic elimination of the orientational degree of freedom from the generalized Chen-Lubensky model. In the smectic-A phase, the effective layer bending elastic modulus is calculated as a function of the wavelength of the layer undulation mode. It turns out that an unlocking of the layer normal and the director reduces the layer bending rigidity for wavelengths smaller than the director penetration length. In the achiral smectic-C phase, an anisotropic bending elasticity appears due to the coupling between the layer displacement and director. The effective layer bending rigidity is calculated as a function of the angle $\vartheta$ between the layer undulation wave-vector and the director field. We compute the free energy minimizer $\vartheta=\theta$. It turns out that $\theta$ varies from $0\deg$ to $90\deg$ depending on the tilt angle, undulation wave-length and other elastic constants. We also discover a new important characteristic length and the discontinuous change of $\theta$. Using the elastic constants of Chen-Lubensky model, we determine the parameters of the more macroscopic model [Y. Hatwalne and T. C. Lubensky, Phys. Rev. E {\bf 52}, 6240 (1995)]. We then discuss the hydrodynamics, and demonstrate the alignment of director and the propagation of the anisotropic layer displacement wave in the presence of an oscillatory wall and a vibrating cylindrical source respectively.