(111e) Active Surface Agents: Active Colloids at Fluid-Fluid Interfaces

We study the bacterium Pseudomonas Aeruginosa (PA01) as a model swimmer at fluid interfaces. Fluid interfaces are highly non-ideal, complex domains that impose constraints that fundamentally alter swimming behavior. Bacteria become trapped at interfaces with pinned contact lines where the interface intersects the cell body. The swimming behavior depends on these trapped configurations, which fix the angle of the cell body at the interface, and hence constrain the arrangement of the flagellum. Interfacial stresses further constrain the interfacial flow via diverging Marangoni stresses-surface tension gradients- that dictate divergence-free motion or incompressibility in the interface.

We focus on PA01 at aqueous- hexadecane interfaces. The bacteria can swim adjacent to the interface, or they can adsorb directly and swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatio-temporal implications. To understand their impact on interfacial transport, we visualize the interfacial flows generated by PA01 in pusher modes and find flow fields with unexpected symmetries that differ significantly from their bulk fluid counterparts. Analysis reveals that these flow fields can be decomposed into two dipolar hydrodynamic modes associated with incompressible interfaces. The relative importance of these modes is determined by the cell bodies’ trapped configurations.

Hydrodynamic theory allows us to understand this flow field fundamentally and to explore its implications on mixing in the interface. Our aim is to advance the concept of an Active Surface Agent, an active colloid trapped at fluid interfaces whose motion and trapping state can be designed to promote mixing and structure formation. This concept represents an important and largely untapped degree of freedom for interfacial engineering. By understanding how biological swimmers move at fluid interfaces, we can develop design rules for artificial biomimetic systems to promote transport at fluid interfaces with broad implications in chemical engineering processes.