Shear dynamics of an inverted nematic emulsion

TL;DR

This paper theoretically investigates the complex shear-induced behaviors of nematic liquid crystal droplets in 2D and 3D, revealing various steady states, defect dynamics, and the influence of anchoring conditions, with implications for experimental studies.

Contribution

It provides a comprehensive theoretical analysis of shear dynamics in inverted nematic emulsions, highlighting new defect behaviors and steady states based on anchoring strength and geometry.

Findings

Weak anchoring leads to droplet rotation and stretching.

Intermediate anchoring causes tilting and tumbling of 2D droplets.

Strong anchoring results in defect detachment and flow dragging of topological defects.

Abstract

Here we study theoretically the dynamics of a 2D and a 3D isotropic droplet in a nematic liquid crystal under a shear flow. We find a large repertoire of possible nonequilibrium steady states as a function of the shear rate and of the anchoring of the nematic director field at the droplet surface. We first discuss homeotropic anchoring. For weak anchoring, we recover the typical behaviour of a sheared isotropic droplet in a binary fluid, which rotates, stretches and can be broken by the applied flow. For intermediate anchoring, new possibilities arise due to elastic effects in the nematic fluid. We find that in this regime the 2D droplet can tilt and move in the flow, or tumble incessantly at the centre of the channel. For sufficiently strong anchoring, finally, one or both of the topological defects which form close to the surface of the isotropic droplet in equilibrium detach from it and get dragged deep into the nematic state by the flow. In 3D, instead, the Saturn ring associated with normal anchoring disclination line can be deformed and shifted downstream by the flow, but remains always localized in proximity of the droplet, at least for the parameter range we explored. Tangential anchoring in 2D leads to a different dynamic response, as the boojum defects characteristic of this situation can unbind from the droplet under a weaker shear with respect to the normal anchoring case. Our results should stimulate further experiments with inverted liquid crystal emulsions under shear, as most of the predictions can be testable in principle by monitoring the evolution of liquid crystalline orientation patterns or by tracking the position and shape of the droplet over time.