Interfacial flow around a pusher bacterium

TL;DR

This study investigates the flow fields generated by pusher-mode bacteria at fluid interfaces, revealing unique asymmetries and hydrodynamic modes that differ from bulk fluid behavior, with implications for biological and biomimetic systems.

Contribution

It provides the first detailed analysis of flow fields around bacteria at fluid interfaces, decomposing them into fundamental hydrodynamic modes and highlighting the role of Marangoni stresses.

Findings

Flow fields exhibit dipolar asymmetries distinct from bulk fluids.

Decomposition reveals a force-doublet and a second dipolar mode influenced by interface stresses.

Bacterial configurations at the interface affect the flow modes and dynamics.

Abstract

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.