(408g) Swimming Bacteia at Complex Fluid Interfaces

Hydrodynamics of active colloids trapped at interfaces depend on the complex 2d fluid environment; their motion at interfaces differs significantly from motion in bulk fluid. To investigate the interaction of active colloids (in particular, swimming bacteria) with a fluid interface, we simultaneously measure interface mechanics, dynamics of colloid motion, and displacement fields of passive tracers. We study Pseudomonas aeruginosa PAO1 on or near hexadecane-water interfaces as a model system, and use two-point microrheology to measure the viscoelastic response of the interface. We find several distinct modes of swimming behaviors which are analyzed in terms of Stokes’ hydrodynamic multipoles for active colloids near or on boundaries. We also use 3d holographic imaging to analyze the motion of bacteria approaching/escaping from the interfaces, and identify behaviors that differ from motions near solid or free surfaces.