An entropic effect essential for surface entrapment of bacteria

TL;DR

This paper reveals that an entropic effect from hydrodynamic interactions is essential for understanding how bacteria become trapped near surfaces, providing a model that explains experimental observations and predicts entrapment zones.

Contribution

It introduces a novel entropic effect into the physics of bacterial surface entrapment, with an analytic model matching experimental data and predicting entrapment conditions.

Findings

The model reproduces bacterial nose-down configuration.

It explains the anticorrelation between pitch and wobbling angles.

Predicts an entrapment zone in parameter space.

Abstract

The entrapment of bacteria near boundary surfaces is of biological and practical importance, yet the underlying physics is still not well understood. We demonstrate that it is crucial to include a commonly neglected entropic effect arising from the spatial variation of hydrodynamic interactions, through a model that provides analytic explanation of bacterial entrapment in two dimensionless parameters: $\alpha_1$ the ratio of thermal energy to self-propulsion, and $\alpha_2$ an intrinsic shape factor. For $\alpha_1$ and $\alpha_2$ that match an {\it Escherichia coli} at room temperature, our model quantitatively reproduces existing experimental observations, including two key features that have not been previously resolved: The bacterial "nose-down" configuration, and the anticorrelation between the pitch angle and the wobbling angle. Furthermore, our model analytically predicts the existence of an entrapment zone in the parameter space defined by $\{\alpha_1,\alpha_2\}$.