Dispersion of activity at an active-passive nematic interface

TL;DR

This study investigates how active nematic interfaces influence nutrient mixing, revealing that particle shape affects dispersion, with an optimal shape enhancing activity spread, which may relate to bacterial evolution.

Contribution

The paper introduces a hydrodynamic model coupling active nematics with nutrient transport, highlighting the role of particle shape in optimizing activity dispersion at interfaces.

Findings

Interfacial activity dispersion is subdiffusive due to negative defect barriers.

Dispersion depends non-monotonically on the particle aligning parameter.

An optimal particle shape maximizes activity dispersion at the interface.

Abstract

Efficient nutrient mixing is crucial for the survival of bacterial colonies and other living systems. This raises the question of whether the optimization of mixing through the emergence of active turbulent motion in bacterial swarms played a role in the evolution of bacterial shapes. Here, to address this question, we solve the hydrodynamic equation for active nematics coupled with an advection-diffusion equation for the nutrients. The latter models a conserved activity field and mimics the conservation of nutrients in bacterial swarms. At the interface between active and passive nematic phases, in addition to diffusion, the activity is transported by interfacial flows and in turn modifies them through active stresses. We find that the interfacial dispersion of the conserved activity is subdiffusive due to the emergence of a barrier of negative defects at the active-passive interface, which hinders the propagation of the motile positive defects. Furthermore, we observe a non-monotonic dependence of the generalized diffusion coefficient on the aligning parameter, which is related to the shape of the particles. Our simulations suggest that there is an optimal shape that maximizes the dispersion of conserved activity at the active-passive nematic interface.