Interfacial flow around a pusher bacterium

Abstract

Motile bacteria play essential roles in biology that rely on their dynamic behaviours, 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 behaviour. 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.

Swimmers at interfaces enhance interfacial transport

“…The bacteria and passive tracer colloids were trapped at interfaces of hexadecane and aqueous suspensions of bacteria in TRIS-based motility medium buffer; details are given in ref. 37. This flow field features unexpected asymmetries that do not arise for their bulk-fluid counterparts (see Fig.…”

“…This method is effective even when the tracers and source particles are Brownian and in relatively crowded conditions that would preclude the use of standard particle displacement velocimetry methods. 37,42 CDV relies on measuring the correlated displacement of the source colloid and tracer particles of a small lag time τ . In the method, a coordinate system ( x , y ) is constructed, with the source colloid at the origin and y axis corresponding to the source colloid's direction of motion.…”

“…1b, was reported in ref. 37. It represents the displacement field of colloids at the interface generated by an ensembled averaged bacterium swimming in pusher mode.…”

“…The bacteria's active diffusivities are attributed to diverse athermal stochastic or noisy processes including fluctuations in flagellar rotation, switching between pusher and puller modes and interactions with other swimmers. 37,41…”

“…We have measured the ensemble-averaged interfacial flow field generated by a PA01 bacterium moving in a pusher mode using correlated displacement velocimetry 37,42 (Fig. 1a).…”