Arrested development and traveling waves of active suspensions in nematic liquid crystals

TL;DR

This paper investigates how active particles in nematic liquid crystals exhibit arrested bend instabilities and traveling waves, revealing how fluid elasticity and anchoring influence the transition from alignment to complex flow states.

Contribution

It introduces a model combining active suspension theory with nematic elasticity, analyzing the transition to arrested states and traveling waves in active nematic suspensions.

Findings

Active suspensions undergo a bend instability beyond a critical activity level.

Fluid elasticity arrests the development of fully turbulent states, resulting in steady flows.

Phase transitions occur at finite anchoring strengths, leading to diverse dynamic states.

Abstract

Active particles in anisotropic, viscoelastic fluids experience competing stresses which guide their trajectories. An aligned suspension of particles can trigger a hydrodynamic bend instability, but the elasticity of the fluid can drive particle orientations back towards alignment. To study these competing effects, we examine a dilute suspension of active particles in an Ericksen-Leslie model nematic liquid crystal. An anchoring strength linking the active and passive media tunes the system between active suspension theory in Newtonian fluids in one limit, and active nematic theory in another. For extensile active stresses, beyond a critical active Ericksen number or particle concentration, the suspension first comes into alignment, then buckles via a classical bend instability. Rather than entering the fully developed roiling state observed in isotropic fluids, the development is arrested into a steady, flowing state by fluid elasticity. Arrested states of higher wavenumber appear at yet larger extensile activity, and a phase transition is identified at finite anchoring strength. If the active particles are motile, the particles can surf along the bent environment of their own creation. More exotic states are also observed, including a oscillatory 'thrashing' mode. Moment equations are derived, compared to kinetic theory simulations, and analyzed in asymptotic limits which admit exact expressions for the traveling wave speed and both particle orientation and director fields in the first arrested state.