Interfacial flow fields generated by a pusher bacterium*

Motile bacteria are model active colloids that navigate, interact, and self-organize, guided in part by hydrodynamic interactions (HI). While HI are well known to alter micro-swimmers’ behavior near solid surfaces, bacteria motion near fluid interfaces is far less understood, despite the importance of interfaces in nature and the complexity of these highly anisotropic domains. We study Pseudomonas aeruginosa PA01 in the pusher mode at aqueous-hexadecane interfaces. The bacteria become trapped with their bodies spanning the interface with pinned contact lines. Surfactant-associated interfacial incompressibility further constrains the flow. Analysis of correlated displacements of tracers and swimmers reveals flow fields generated by the bacteria motion with unexpected asymmetries. These fields can be decomposed into an expected force-doublet mode corresponding to propulsion and drag in an incompressible interface, and a second dipolar mode, associated with forces exerted by the flagellum in the aqueous phase. The balance of these modes depends on the bacteria’s trapped configurations. The implications of these flows on enhanced transport and swimmer pair interactions are explored.