Self-organized dynamics and the transition to turbulence of confined active nematics

TL;DR

This paper investigates how confinement influences the chaotic behavior of active nematics, revealing transitions to regular patterns and defect dynamics, with experimental and theoretical insights into flow organization.

Contribution

It introduces a confinement method that transforms chaotic active nematic flows into predictable patterns and compares experimental results with a theoretical model, highlighting its limitations.

Findings

Weak confinement leads to circular flows with defect nucleation

Strong confinement causes defect pairs to migrate inward and form doubly-periodic dynamics

Theory captures some defect motions but not all observed behaviors

Abstract

We study how confinement transforms the chaotic dynamics of bulk microtubule-based active nematics into regular spatiotemporal patterns. For weak confinements, multiple continuously nucleating and annihilating topological defects self-organize into persistent circular flows of either handedness. Increasing confinement strength leads to the emergence of distinct dynamics, in which the slow periodic nucleation of topological defects at the boundary is superimposed onto a fast procession of a pair of defects. A defect pair migrates towards the confinement core over multiple rotation cycles, while the associated nematic director field evolves from a distinct double spiral towards a nearly circularly symmetric configuration. The collapse of the defect orbits is punctuated by another boundary-localized nucleation event, that sets up long-term doubly-periodic dynamics. Comparing experimental data to a theoretical model of an active nematic, reveals that theory captures the fast procession of a pair of $+\frac{1}{2}$ defects, but not the slow spiral transformation nor the periodic nucleation of defect pairs. Theory also fails to predict the emergence of circular flows in the weak confinement regime. The developed confinement methods are generalized to more complex geometries, providing a robust microfluidic platform for rationally engineering two-dimensional autonomous flows.