Stress anisotropy in confined populations of growing rods

TL;DR

This study investigates how confinement and particle shape influence stress anisotropy and alignment in colonies of growing rod-shaped bacteria, revealing a strong link between shape, stress inversion, and nematic order through in-silico simulations.

Contribution

The paper demonstrates, through simulations, the relationship between particle aspect ratio, stress anisotropy, and alignment, supported by an asymptotic theory and analysis of different confinement geometries.

Findings

Large aspect ratio rods exhibit stress inversion linked to alignment.

Strong alignment decouples active and passive stresses.

Robust effects observed in less restrictive confinement geometries.

Abstract

Order and alignment are ubiquitous in growing colonies of rod-shaped bacteria due to the nematic properties of the constituent particles. These effects are the result of the active stresses generated by growth, passive mechanical interactions between cells, and flow-induced effects due to the shape of the confining container. However, how these contributing factors interact to give rise to the observed global alignment patterns remains elusive. Here, we study, in-silico, colonies of growing rod-shaped particles of different aspect ratios confined in channel-like geometries. A spatially resolved analysis of the stress tensor reveals a strong relationship between near-perfect alignment and an inversion of stress anisotropy for particles with large length-to-width ratios. We show that, in quantitative agreement with an asymptotic theory, strong alignment can lead to a decoupling of active and passive stresses parallel and perpendicular to the direction of growth, respectively. We demonstrate the robustness of these effects in a geometry that provides less restrictive confinement and introduces natural perturbations in alignment. Our results illustrate the complexity arising from the inherent coupling between nematic order and active stresses in growing active matter which is modulated by geometric and configurational constraints due to confinement.