Hierarchical defect-induced condensation in active nematics

TL;DR

This study reveals that phase-separated active nematics can form -1/2 topological defects characterized by dense, condensed cores, driven by active fluxes, which differ from defects in homogeneous systems and are controllable via density and persistence length.

Contribution

The paper introduces the discovery of defect-induced condensation in phase-separated active nematics, supported by agent-based simulations and a new hydrodynamic theory, highlighting a novel second-order collective state.

Findings

Phase-separated active nematics form -1/2 defects with dense cores.

Defects and arcs emerge from anisotropic active fluxes.

Control parameters include particle density and persistence length.

Abstract

Topological defects play a central role in the formation and organization of various biological systems. Historically, such nonequilibrium defects have been mainly studied in the context of homogeneous active nematics. Phase-separated systems, in turn, are known to form dense and dynamic nematic bands, but typically lack topological defects. In this paper, we use agent-based simulations of weakly aligning, self-propelled polymers and demonstrate that contrary to the existing paradigm phase-separated active nematics form ${-1/2}$ defects. Moreover, these defects, emerging due to interactions among dense nematic bands, constitute a novel second-order collective state. We investigate the morphology of defects in detail and find that their cores correspond to a strong increase in density, associated with a condensation of nematic fluxes. Unlike their analogs in homogeneous systems, such condensed defects form and decay in a different way and do not involve positively charged partners. We additionally observe and characterize lateral arc-like structures that separate from a band's bulk and move in transverse direction. We show that the key control parameters defining the route from stable bands to the coexistence of dynamic lanes and defects are the total density of particles and their path persistence length. We introduce a hydrodynamic theory that qualitatively recapitulates all the main features of the agent-based model, and use it to show that the emergence of both defects and arcs can be attributed to the same anisotropic active fluxes. Finally, we present a way to artificially engineer and position defects, and speculate about experimental verification of the provided model.