Active bacterial pattern formation in evaporating droplets

TL;DR

This study investigates how oli bacteria interact with capillary flows in evaporating droplets, revealing different pattern formations driven by bacterial activity, density, and evaporation conditions, including uniform deposits, localized collective motion, and turbulent plumes.

Contribution

It uncovers the mechanisms of bacterial pattern formation in evaporating droplets, highlighting the role of capillary confinement and bacterial density in self-organization behaviors.

Findings

Uniform contact-line deposits form under fast evaporation or dilute conditions.

Localized collective bacterial motion occurs at critical densities, inducing periodic patterns.

High densities lead to bacterial turbulence and formation of mobile plumes.

Abstract

Bacteria living on surfaces are often confined to droplets. When these droplets evaporate, the motion of the liquid-air interface and the associated internal capillary flow confine the bacteria. Here we study how \emph{E. coli} bacteria interact with this capillary confinement and agglomerate at the droplet's contact line. We identify three different types of bacterial pattern formation that depend on the bacterial activity and the environmental conditions imposed by the evaporating droplet. When the evaporation is fast, the bacteria are slow or the suspension is dilute, a uniform contact-line deposit forms. However, when the capillary confinement concentrates the bacteria at the contact line beyond a critical number density, localized collective motion spontaneously emerges. In that case, the bacteria induce a local stirring of the liquid that allows them to self-organize into periodic patterns and enables them to collectively escape from the contact line. At very high number densities, these periodic patterns get destabilized by bacterial turbulence in the bulk of the droplet resulting in the formation of mobile bacterial plumes at the contact line. Our results show how the subtle interplay between the bacteria and the capillary flow inside the droplet that surrounds them governs their dispersal.