A non-traditional view on the modeling of nematic disclination dynamics

TL;DR

This paper introduces a novel energy model for nematic disclination dynamics that overcomes singularities, enabling the study of defect equilibria and interactions, with implications for both liquid crystal and solid mechanics.

Contribution

It proposes an augmented Oseen-Frank energy framework that models non-singular disclination dynamics and draws parallels with dislocation behavior in solids.

Findings

Non-singular equilibrium solutions for disclinations including half-integer strengths.

Gradient flow dynamics fail to capture defect evolution.

A new 2D model for disclination dynamics validated through various defect interactions.

Abstract

Nonsingular disclination dynamics in a uniaxial nematic liquid crystal is modeled within a mathematical framework where the kinematics is a direct extension of the classical way of identifying these line defects with singularities of a unit vector field representing the nematic director. It is well known that the universally accepted Oseen-Frank energy is infinite for configurations that contain disclination line defects. We devise a natural augmentation of the Oseen-Frank energy to account for physical situations where, under certain conditions, infinite director gradients have zero associated energy cost, as would be necessary for modeling half-integer strength disclinations within the framework of the director theory. Equilibria and dynamics (in the absence of flow) of line defects are studied within the proposed model. Using appropriate initial/boundary data, the gradient-flow dynamics of this energy leads to non-singular, line defect equilibrium solutions, including those of half-integer strength. However, we demonstrate that the gradient flow dynamics for this energy is not able to adequately describe defect evolution. Motivated by similarity with dislocation dynamics in solids, a novel 2D-model of disclination dynamics in nematics is proposed. The model is based on the extended Oseen-Frank energy and takes into account thermodynamics and the kinematics of conservation of defect topological charge. We validate this model through computations of disclination equilibria, annihilation, repulsion, and splitting. We show that the energy function we devise, suitably interpreted, can serve as well for the modeling of equilibria and dynamics of dislocation line defects in solids making the conclusions of this paper relevant to mechanics of both solids and liquid crystals.