Activity driven orientational order in active nematic liquid crystals on an anisotropic substrate

TL;DR

This study explores how anisotropic substrates influence the turbulent behavior and defect alignment in active nematic liquid crystals, revealing active stress-driven global order and orientation-dependent flows.

Contribution

It introduces a model with anisotropic friction and viscosity to show how substrate anisotropy induces nematic order in active defects, a phenomenon absent in passive systems.

Findings

Global nematic order of defects depends on substrate anisotropy and active stress

Active flow around defects is orientation-dependent, causing defect alignment

Passive liquid crystals do not exhibit defect ordering under anisotropic viscosity

Abstract

We investigate the effect of an anisotropic substrate on the turbulent dynamics of a simulated two dimensional active nematic. This is introduced as an anisotropic friction and an effective anisotropic viscosity, with the orientation of the anisotropy being defined by the substrate. In this system we observe the emergence of global nematic order of topological defects that is controlled by the degree of anisotropy in the viscosity and the magnitude of the active stress. No global defect alignment is seen in passive liquid crystals with anisotropic viscosity or friction confirming that ordering is driven by the active stress. We then closely examine the active flow generated by a single defect to show that the kinetic energy of the flow is orientation dependent, resulting in a torque on the defect to align them with the anisotropy in the substrate.