Drift capability of swimming bacteria in shear flows

The ability of microorganisms to control their swimming direction is crucial for navigating complex flow environments at low Reynolds numbers. We study how swimming Escherichia coli generate transverse locomotion across streamlines, defined as drift swimming, i.e., perpendicular to the flow direction, resulting from the chiral coupling between their flagellar rotation and an imposed shear flow. The drift velocity increases linearly with shear rate at low shear and saturates at a maximum value comparable to the intrinsic swimming speed. This maximum drift grows monotonically as cells approach the channel wall and is strongly enhanced under geometric confinement. Reorientation dynamics reveal a marginally stable fixed point along the vorticity axis, with oscillations around it arising from thermal noise and body–flagellum misalignment. Wall-induced hydrodynamic constraints suppress these fluctuations and amplify the drift. Our results demonstrate an enhanced navigation capacity under extreme shear relevant to bacterial colonization in confined environments.