Bend Instabilities and Topological Turbulence in Shear-Aligned Living Liquid Crystal

TL;DR

This study investigates how shear-induced alignment in a living liquid crystal with microswimmers leads to bend instabilities and topological turbulence, revealing the interplay between active forces and passive nematic elasticity.

Contribution

It demonstrates the transition from shear alignment to topological turbulence driven by microswimmers in a nematic liquid crystal, highlighting the role of bend instabilities and disclination dynamics.

Findings

Active forces induce bend undulations that grow and saturate.

Disclination pairs nucleate and multiply, forming turbulence.

Energy spectra show distinct wavevector dependencies.

Abstract

Flagellated microswimmers B. Subtilis dispersed in a nematic phase of a lyotropic chromonic liquid crystal form a living liquid crystal (LLC). The combination of the passive and active components allows us to analyze how the active component transitions from the shear-imposed alignment into topological turbulence. The lateral extension of the experimental cell is 1000 times larger than the 10-micrometer thickness, to avoid the effect of lateral confinement on the dynamic patterns. The surface anchoring is azimuthally degenerate to avoid permanent anisotropy. After the shear cessation, active forces produced by the microswimmers trigger self-amplifying bend undulations. The amplitude of undulations grows with time and then saturates, while the wavelength increases only slightly. Strong bend stresses at the extrema of undulations are released by nucleating disclination pairs, which multiply to produce topological turbulence. The nucleating pairs and the symmetry axes of the +1/2 disclinations are orientationally ordered. In highly active LLCs, this alignment causes a nonmonotonous time dependence of the disclinations' number, as a fast-moving +1/2 disclination of one pair annihilates a -1/2 disclination of the neighboring pair. The spectra of elastic and kinetic energies exhibit distinct wavevector dependencies caused by the energy cost of the deformed passive nematic background. The study demonstrates how applied shears and a passive viscoelastic background affect the dynamics of active matter. Combined with the previously determined viscoelastic properties of the lyotropic chromonic liquid crystal, the results complete a comprehensive description of the experimentally assessable type of active matter.