Line defects in nematic liquid crystals as charged superelastic rods with negative twist--stretch coupling

TL;DR

This paper develops an elasticity theory for line defects in nematic liquid crystals, revealing their negative twist-stretch coupling and providing quantitative measurements of their elastic properties through combined theory, simulation, and experiments.

Contribution

It introduces a unified framework modeling nematic line defects as superelastic rods and demonstrates their negative twist-stretch coupling through comprehensive analysis.

Findings

Line defects behave as superelastic rods with measurable elastic moduli.

Twisted line defects tend to unwind under stretching, showing negative twist-stretch coupling.

Experimental confirmation of theoretical predictions using patterned nematic cells.

Abstract

Topological defects are a ubiquitous phenomenon in diverse physical systems. In nematic liquid crystals (LCs), they are dynamic, physicochemically distinct, sensitive to stimuli, and are thereby promising for a range of applications. However, our current understanding of the mechanics and dynamics of defects in nematic LCs remain limited and are often overwhelmed by the intricate details of the specific systems. Here, we unify singular and nonsingular line defects as superelastic rods and combine theory, simulation, and experiment to quantitatively measure their effective elastic moduli, including line tension, torsional rigidity, and twist--stretch coefficient. Interestingly, we found that line defects exhibit a negative twist--stretch coupling, meaning that twisted line defects tend to unwind under stretching, which is reminiscent of DNA molecules. A patterned nematic cell experiment further confirmed the above findings. Taken together, we have established an effective elasticity theory for nematic defects, paving the way towards understanding and engineering their deformation and transformation in driven and active nematic materials.