Visualizing the flow field of E. coli near a boundary

Combining particle tracking velocimetry with digital in-line holographic microscopy, we report measurements of the three-dimensional flow field generated by a freely swimming Escherichia coli near a solid boundary with submicron spatial resolutions. The flow field parallel to the boundary exhibits a pronounced asymmetry with respect to the body-flagellum axis of the bacterium, in contrast to the symmetric four-vortex pattern previously reported in experiments and predicted by the model of two Stokeslets. The difference arises from the counter-rotation of the bacterial body and flagellar bundle near the boundary. By incorporating a pair of oppositely oriented rotlets into the two-Stokeslet model, we successfully reproduce the experimentally observed asymmetry. This extended Stokeslet–rotlet model, informed by our measured flow field, enables a quantitative assessment of the strength of rotlets. Our study highlights the effect of chirality of bacterial swimming gaits on the hydrodynamic flow and provides a foundation for understanding the flow-induced cell-boundary interactions.