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Swimming in Circles: Motion of Bacteria near Solid Boundaries

Eric Lauga ^*, Willow R. DiLuzio ^* , George M. Whitesides  and Howard A. Stone ^*

^* Division of Engineering and Applied Sciences, and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts

Correspondence: Address reprint requests to E. Lauga, E-mail: lauga{at}mit.edu. E. Lauga's present address is Dept. of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139.

Near a solid boundary, Escherichia 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, the radius of curvature of the trajectory is observed to increase with the length of the bacterium body.