Crack patterns of drying dense bacterial suspensions

TL;DR

This study investigates how the motility of bacteria influences crack patterns in drying bacterial suspensions, revealing that active swimming leads to circular cracks while immotility results in spiral cracks, linking microscopic behavior to macroscopic patterns.

Contribution

It introduces a novel connection between bacterial motility and crack pattern formation in drying suspensions, using elastic and poroelastic theories to explain the phenomena.

Findings

Active bacteria produce circular cracks due to tensile drying stress.

Immotile bacteria form spiral crack patterns.

Microscopic swimming behavior influences macroscopic crack morphology.

Abstract

Drying of bacterial suspensions is frequently encountered in a plethora of natural and engineering processes. However, the evaporation-driven mechanical instabilities of dense consolidating bacterial suspensions have not been explored heretofore. Here, we report the formation of two different crack patterns of drying suspensions of \textit{Escherichia coli} (\textit{E. coli}) with distinct motile behaviors. Circular cracks are observed for wild-type \textit{E. coli} with active swimming, whereas spiral-like cracks form for immotile bacteria. Using the elastic fracture mechanics and the poroelastic theory, we show that the formation of the circular cracks is determined by the tensile nature of the radial drying stress once the cracks are initiated by the local order structure of bacteria due to their collective swimming. Our study demonstrates the link between the microscopic swimming behaviors of individual bacteria and the mechanical instabilities and macroscopic pattern formation of drying bacterial films. The results shed light on the dynamics of active matter in a drying process and provide useful information for understanding various biological processes associated with drying bacterial suspensions.