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

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.

References in corpus (13)

• Motility-Induced Phase Separation
• Statistical Mechanics of Interacting Run-and-Tumble Bacteria
• Active Turbulence
• Excitable Patterns in Active Nematics
• Dynamics of active liquid interfaces
• Active nematic materials with substrate friction
• Submersed Micropatterned Structures Control Active Nematic Flow, Topology and Concentration
• Probing the shear viscosity of an active nematic
• Flow states and transitions of an active nematic in a three dimensional channel
• Activity-suppressed phase separation
• Fluctuations can induce local nematic order and extensile stress in monolayers of motile cells
• Active liquid crystals powered by force-sensing DNA-motor clusters
• Helical Flow States in Active Nematics

Cited by in corpus (2)

• Vortex Lattices in Active Nematics with Periodic Obstacle Arrays
• Pattern selection and the route to turbulence in incompressible polar active fluids