Active turbulence and spontaneous phase separation in inhomogeneous extensile active gels

TL;DR

This paper uses numerical simulations to explore how composition inhomogeneities influence the complex behaviors of inhomogeneous active nematic gels, revealing new dynamical regimes including microphase separation.

Contribution

It demonstrates that composition variations are crucial in active nematic gels, leading to novel regimes like microphase separation without explicit demixing terms in the free energy.

Findings

Identification of three dynamical regimes: defect/vortex patterns, active turbulence, and microphase separation.

Microphase separation occurs even without explicit demixing terms, explained by a theoretical tangent construction.

Large composition variations are characteristic of active turbulence in inhomogeneous active gels.

Abstract

We report numerical results for the hydrodynamics of inhomogeneous lyotropic and extensile active nematic gels. By simulating the coupled Cahn-Hilliard, Navier-Stokes, and Beris-Edwards equation for the evolution of the composition, flow and orientational order of an active nematic, we ask whether composition variations are important to determine its emergent physics. As in active gels of uniform composition, we find that increasing either activity or nematic tendency (e.g., overall active matter concentration) triggers a transition between an isotropic passive phase and an active nematic one. We show that composition inhomogeneities are important in the latter phase, where we find three types of possible dynamical regimes. First, we observe regular patterns with defects and vortices: these exist close to the passive-active transition. Second, for larger activity, or deeper in the nematic phase, we find active turbulence, as in active gels of uniform composition, but with exceedingly large composition variation. In the third regime, which is uniquely associated with inhomogeneity and occurs for large nematic tendency and low activity, we observe spontaneous microphase separation into active and passive domains. The microphase separated regime is notable in view of the absence of an explicit demixing term in the underlying free energy which we use, and we provide a theoretical analysis based on the common tangent construction which explains its existence. We hope this regime can be probed experimentally in the future.