Macroscopic model of self-propelled bacteria swarming with regular reversals

TL;DR

This paper links microscopic stochastic models of reversing bacteria to a macroscopic nonlinear diffusion equation, revealing how reversal frequency influences bacterial spreading and jam resolution during swarming.

Contribution

It introduces a connection between a microscopic stochastic model and a macroscopic nonlinear diffusion equation for bacteria with regular reversals, providing analytical and numerical insights.

Findings

High reversal frequency enhances spreading at high densities.

Critical density p0 marks the transition to collision-dominated diffusion.

Analytical estimates of collision and jam times support the model.

Abstract

Periodic reversals of the direction of motion in systems of self-propelled rod shaped bacteria enable them to effectively resolve traffic jams formed during swarming and maximize their swarming rate. In this paper, a connection is found between a microscopic one dimensional cell-based stochastic model of reversing non-overlapping bacteria and a macroscopic non-linear diffusion equation describing dynamics of the cellular density. Boltzmann-Matano analysis is used to determine the nonlinear diffusion equation corresponding to the specific reversal frequency. Macroscopically (ensemble-vise) averaged stochastic dynamics is shown to be in a very good agreement with the numerical solutions of the nonlinear diffusion equation. Critical density $p_0$ is obtained such that nonlinear diffusion is dominated by the collisions between cells for the densities $p>p_0$. An analytical approximation of the pairwise collision time and semi-analytical fit for the total jam time per reversal period are also obtained. It is shown that cell populations with high reversal frequencies are able to spread out effectively at high densities. If the cells rarely reverse then they are able to spread out at lower densities but are less efficient at spreading out at at higher densities.