Negative Poisson's ratio and semi-soft elasticity of smectic-C liquid crystal elastomers

TL;DR

This paper explores the negative Poisson's ratio and semi-soft elasticity in smectic-C liquid crystal elastomers, analyzing how imperfections affect their soft modes and mechanical response under various stretching geometries.

Contribution

It introduces a simple model to study the impact of imperfections on the soft modes and negative Poisson's ratio in smectic-C elastomers, extending current models.

Findings

Stretching parallel to the layer normal can cause negative stiffness.

Director rotation leads to lateral expansion, indicating negative Poisson's ratio.

Imperfections influence the soft modes and mechanical behavior.

Abstract

Models of smectic-C liquid crystal elastomers predict that it can display soft elasticity, in which the shape of the elastomer changes at no energy cost. The amplitude of the soft mode and the accompanying shears are dependent on the orientation of the layer normal and the director with respect to the stretch axis. We demonstrate that in some geometries the director is forced to rotate perpendicular to the stretch axis, causing lateral expansion of the sample; a negative Poisson's ratio. Current models do not include the effect of imperfections that must be present in the physical sample. We investigate the effect of a simple model of these imperfections on the soft modes in monodomain smectic-C elastomers in a variety of geometries. When stretching parallel to the layer normal (with imposed strain) the elastomer has a negative stiffness once the director starts to rotate. We show that this is a result of the negative Poisson's ratio in this geometry through a simple scalar model.