Fig. 4

Sketch of the oscillatory rheotaxis mechanism. Here the bacterium is initially oriented towards the right and slightly downstream, and red arrows show the projection of the cell onto the surface. Then, the oscillations can be envisaged as a 4-step process: a The vorticity pushes the body down onto the surface and lifts the flagella up. b Then the flow advects the flagella faster than the body, rotating the bacterium about the y axis to the upstream direction. The weathervane effect enhances this rotation as the cell pivots about the anchoring point. c Now the vorticity pushes the flagella onto the wall and lifts the body up. d Subsequently the body is advected faster, rotating the swimmer back to the downstream direction. This cycle is repeated, leading to oscillatory trajectories. Note that this is a simplified picture and all surface and flow effects (Fig. 2) contribute to the dynamics at any one time