  Motile bacteria play essential roles in biology that rely on their dynamic
behaviors, including their ability to navigate, interact, and self-organize.
However, bacteria dynamics on fluid interfaces are not well understood.
Swimmers adsorbed on fluid interfaces remain highly motile, and fluid
interfaces are highly non-ideal domains that alter swimming behavior. To
understand these effects, we study flow fields generated by Pseudomonas
aeruginosa PA01 in the pusher mode. Analysis of correlated displacements of
tracers and bacteria reveals dipolar flow fields with unexpected asymmetries
that differ significantly from their counterparts in bulk fluids. We decompose
the flow field into fundamental hydrodynamic modes for swimmers in
incompressible fluid interfaces. We find an expected force-doublet mode
corresponding to propulsion and drag at the interface plane, and a second
dipolar mode, associated with forces exerted by the flagellum on the cell body
in the aqueous phase that are countered by Marangoni stresses in the interface.
The balance of these modes depends on the bacteria's trapped interfacial
configurations. Understanding these flows is broadly important in nature and in
the design of biomimetic swimmers.
