Defect-mediated dynamics of coherent structures in active nematics

TL;DR

This study reveals how positional coherence and topological defects in active nematics organize chaotic motion through moving attractors and repellers, providing new insights into self-organization in active fluids.

Contribution

It introduces a novel framework linking positional coherence with defect dynamics using dynamical systems theory, experiments, and simulations.

Findings

+1/2 defects act as control centers for collective motion

Regions around +1/2 defects experience high bending and low shear deformations

Stress distribution is minimized at defects, with high differential stress along defect orientation

Abstract

Active fluids, such as cytoskeletal filaments, bacterial colonies and epithelial cell layers, exhibit distinctive orientational coherence, often characterized by nematic order and topological defects. By contrast, little is known about positional coherence -- i.e., how a hidden dynamic skeleton organizes the underlying chaotic motion -- despite this being one of their most prominent and experimentally accessible features. Using a combination of dynamical systems theory, experiments on two-dimensional mixtures of microtubules and kinesin and hydrodynamic simulations, we characterize positional coherence in active nematics. These coherent structures can be identified in the framework of Lagrangian dynamics as moving attractors and repellers, which orchestrate complex motion. To understand the interaction of positional and orientational coherence on the dynamics of defects, we then analysed observations and simulations and see that +1/2 defects move and deform the attractors, thus functioning as control centers for collective motion. Additionally, we find that regions around isolated +1/2 defects undergo high bending and low stretching/shearing deformations, consistent with the local stress distribution. The stress is minimum at the defect, while high differential stress along the defect orientation induces folding. Our work offers a new perspective to describe self-organization in active fluids, with potential applications to multicellular systems.