Directed collective motion of bacteria under channel confinement

TL;DR

This study demonstrates that bacteria confined in a narrow channel can exhibit stable, unidirectional collective motion driven by fluid flows, with the transition depending on channel width and flow interactions.

Contribution

It reveals how channel confinement stabilizes bacterial collective motion and highlights the role of bacteria-driven flows in this process, supported by experimental and simulation evidence.

Findings

Confinement induces steady unidirectional bacterial flow.

Flow interactions near walls influence bulk cell movement.

Transition point correlates with unbounded swirl size.

Abstract

Dense suspensions of swimming bacteria are known to exhibit collective behaviour arising from the interplay of steric and hydrodynamic interactions. Unconfined suspensions exhibit transient, recurring vortices and jets, whereas those confinedin circular domains may exhibit order in the form of a spiral vortex. Here we show that confinement into a long and narrow macroscopic `racetrack' geometry stabilises bacterial motion to form a steady unidirectional circulation. This motion is reproduced in simulations of discrete swimmers that reveal the crucial role that bacteria-driven fluid flows play in the dynamics. In particular, cells close to the channel wall produce strong flows which advect cells in the bulk against their swimming direction. We examine in detail the transition from a disordered state to persistent directed motion as a function of the channel width,and show that the width at the crossover point is comparable to the typical correlation length of swirls seen in the unbounded system. Our results shed light on the mechanisms driving the collective behaviour of bacteria and other active matter systems, and stress the importance of the ubiquitous boundaries found in natural habitats.