(43a) Asymmetric Swimming Patterns of Bacteria Trapped at Oil-Water Interfaces.

Boundaries and confinement strongly alter motility of swimming bacteria. For instance, interface between two fluids affects swimming patterns of bacteria in a complex manner, which we study in this work. By analyzing motion of Pseudomonas aeruginosa, a uniflagellated bacterium, trapped at an oil-water interface, we realize their swimming behaviors differ significantly from bulk. PA01 in 3-d suspension swims along symmetric straight paths in run (forward) and reverse (backward) motions by altering the rotation direction of its flagellum to switch between pusher and puller modes. For interfacially trapped bacteria, however, we discover an asymmetric swimming path for run and reverse modes. Bacteria swim in segments of weakly curved CW paths with high translational speed and highly curved CCW paths with large rotational speed. Using physical arguments, we predict that hydrodynamically induced forces bend the flagellar hook to move the flagellum toward the interface normal in the puller mode, and toward the plane of the interface in the pusher mode. The reorientation of the bacterial flagellum regulates the translational and rotational speed of the cell which causes the complex swimming pattern.