Spontaneous oscillations and geometric cutoff in confined bacterial swarms

TL;DR

This paper presents a physical model explaining spontaneous oscillations in confined bacterial swarms, linking bacterial swimming dynamics with fluid flow and predicting oscillation onset conditions.

Contribution

It introduces a minimal linear response framework that captures the emergence of oscillations and provides analytical predictions matching experimental data.

Findings

Model predicts critical bacterial density for oscillations.

Maximum film thickness for sustained oscillations is analytically derived.

Quantitative agreement with experimental observations.

Abstract

Self-organized dynamic patterns in dense active matter are striking manifestations of non-equilibrium physics. A prominent example is the macroscopic elliptical motion observed in quasi-2D bacterial suspensions, which has lacked a physical explanation. Here, we examine a minimal linear response framework coupling bacterial swimming dynamics with fluid flow, treating long-range hydrodynamic interactions as a macroscopic communication channel. We demonstrate that microscopic swim motion, via Jeffery coupling, manifests as a ``phase-leading'' response to local shear flows. System-wide sustained oscillations, on the other hand, require both a critical bacterial density and strict geometric confinement. By analytically predicting the onset cell density and maximum film thickness, our model achieves excellent quantitative agreement with experiments, establishing a unified physical framework for self-organized periodic motion of elongated body in active fluids.