Oscillatory dynamics of swimming E coli bacteria at walls in Poiseuille flow*

Swimming microorganisms respond to flows in highly diverse and complex environments, at scales ranging from open oceans to narrow capillaries. The combined effects of fluid flow and boundaries lead to preferred swimmer orientation breaking the up/down-stream and left/right symmetry. To date, this so-called bacterial surface rheotaxis has been quantified by measuring instantaneous orientation distributions or average transport velocities, but a complete picture is still missing.

We investigate the time-resolved orientation dynamics of E.coli bacteria, theoretically and experimentally, as a function of applied shear close to walls. With increasing flow, we identify four regimes separated by critical shear rates: (I) circular swimming with a bias along the direction of vorticity ("to the right"); (II) upstream swimming without oscillations; (III) oscillatory motion, increasingly more to the right; (IV) coexistence of swimming to the right  and to the left, with dynamical switching between these states. By modeling bacterial rheotaxis comprehensively - accounting for their chiral nature, wall interactions, elongation, fore-aft asymmetry and activity - we explain the full dynamics.