Hydrodynamic hovering of swimming bacteria above surfaces

TL;DR

This paper investigates the physical mechanisms behind bacteria hovering above surfaces, revealing that cell shape asymmetries and tilt are crucial for stable swimming at a finite height.

Contribution

It introduces a combined numerical and theoretical framework that explains the hydrodynamic hovering of bacteria, emphasizing the roles of cell shape and asymmetry.

Findings

Hovering requires elongated cell bodies.

Hovering results from a tilt away from the wall.

A minimal model captures key hovering features.

Abstract

Flagellated bacteria are hydrodynamically attracted to rigid walls, yet past work shows a 'hovering' state where they swim stably at a finite height above surfaces. We use numerics and theory to reveal the physical origin of hovering. Simulations first show that hovering requires an elongated cell body and results from a tilt away from the wall. Theoretical models then identify two essential asymmetries: the response of width-asymmetric cells to active flows created by length-asymmetric cells. A minimal model reconciles near and far-field hydrodynamics, capturing all key features of hovering.