Vorticity phase separation and defect lattices in the isotropic phase of active liquid crystals

TL;DR

This paper investigates how substrate friction influences the formation of vortex lattices and defect patterns in active liquid crystals, revealing a phase separation of vorticity driven by friction that enables control over active flow structures.

Contribution

It introduces the role of substrate friction as a tuning parameter for emergent structures in active liquid crystals, linking pattern formation to vorticity phase separation.

Findings

Emergence of vortex lattices with topological defects at intermediate activity levels.

Flow vortices trapping and chasing pairs of +1/2 defects in chiral arrangements.

Vortex pattern length scale governed by the flow screening length and friction.

Abstract

We use numerical simulations and linear stability analysis to study the dynamics of an active liquid crystal film on a substrate in the regime where the passive system would be isotropic. Extensile activity builds up local orientational order and destabilizes the quiescent isotropic state above a critical activity value, eventually resulting in spatiotemporal chaotic dynamics akin to the one observed ubiquitously in the nematic state. Here we show that tuning substrate friction yields a variety of emergent structures at intermediate activity, including lattices of flow vortices with associated regular arrangements of topological defects and a new state where flow vortices trap pairs of $+1/2$ defect that chase each other tail. These chiral units spontaneously pick the sense of rotation and organize in a hexagonal lattice, surrounded by a diffuse flow of opposite rotation to maintain zero net vorticity. The length scale of these emergent structures is set by the screening length $l_\eta=\sqrt{\eta/\Gamma}$ of the flow, controlled by the shear viscosity $\eta$ and the substrate friction $\Gamma$, and can be captured by simple mode selection of the vortical flows. We demonstrate that the emergence of coherent structures can be interpreted as a phase separation of vorticity, where friction plays a role akin to that of birth/death processes in breaking conservation of the phase separating species and selecting a characteristic scale for the patterns. Our work shows that friction provides an experimentally accessible tuning parameter for designing controlled active flows.