Swimming in circles: Motion of bacteria near solid boundaries

TL;DR

This paper presents a hydrodynamic model explaining why E. coli bacteria swim in circles near solid surfaces, attributing the behavior to hydrodynamic interactions that cause circular trajectories and trapping close to the boundary.

Contribution

The study introduces a hydrodynamic model that accounts for bacteria's circular motion near surfaces, linking trajectory curvature to bacterial body length and validating with experimental data.

Findings

Circular trajectories result from hydrodynamic interactions with boundaries.

Trajectory radius increases with bacterial body length.

Model aligns reasonably with experimental observations.

Abstract

Near a solid boundary, E. coli swims in clockwise circular motion. We provide a hydrodynamic model for this behavior. We show that circular trajectories are natural consequences of force-free and torque-free swimming, and the hydrodynamic interactions with the boundary, which also leads to a hydrodynamic trapping of the cells close to the surface. We compare the results of the model with experimental data and obtain reasonable agreement. In particular, we show that the radius of curvature of the trajectory increases with the length of the bacterium body.