Bacteria At Oil-Water Interfaces

Bacteria are active colloids whose collective motion is studied in the context of non-equilibrium statistical mechanics. Bacteria can also become trapped at or near fluid interfaces, impacting bacteria fate and interface mechanics. Here I study bacteria near hexadecane-water interfaces. I investigate films formed by Pseudomonas sp. P62 at initially pristine interfaces using particle tracking and pendant drop elastometry. Adhered bacteria colonize and form structures at the interface. The interface evolves via three mechanical and dynamical stages. Initially, it is covered with motile bacteria. Thereafter, it becomes viscoelastic as polysaccharides, surfactants, and bacteria accumulate, with hallmarks of soft glassy interfacial rheology. Finally, the interface is covered with a thin elastic solid film. On pendant drops covered with such films, the film wrinkles upon compression; the wavelength of these wrinkles allows estimation of the film\u27s bending modulus. I compare interfaces in contact with Pseudomonas aeruginosa PAO1 and PA14 suspensions. Film formation is species dependent. While PAO1 cells form elastic films, PA14 cells move actively without elastic film formation. PAO1 mutants lacking flagella, pili, or certain polysaccharides also form elastic films. Transcriptional profiling identifies highly induced genes including a carbohydrate metabolism enzyme, alkB2. PAO1 mutants lacking the alkB2 gene do not form an elastic layer, but form active films. This suggests that the ability to metabolize alkanes may play a role in elastic film formation. Finally, I study trajectories of passive colloids at interfaces of a suspension of a PA14 mutant selected because it forms highly motile layers. We observe three types of trajectories including diffusive trajectories, Levy walks, and curly trajectories. These latter two are superdiffusive, with prolonged correlation times and rapid displacements inconsistent with hydrodynamic interactions between an active and passive colloid. Analysis reveals that bacteria-particle adhesion gives rise to this distinct non-diffusive behavior. This work reveals the important role of interfacial mechanics in the dynamics of bacterial suspensions with free surfaces. Furthermore, films of bacteria and interfaces have implications in extraction of petroleum from reservoirs and oil spill remediation. The phenomenon of cargo-carrying bacteria, already harnessed in microrobotics, has as yet unexplored implications for micromixing in nature

Lessons Learned From the United States Ocean Observatories Initiative

The Ocean Observatories Initiative (OOI) is a United States National Science Foundation-funded major research facility that provides continuous observations of the ocean and seafloor from coastal and open ocean locations in the Atlantic and Pacific. Multiple cycles of OOI infrastructure deployment, recovery, and refurbishment have occurred since operations began in 2014. This heterogeneous ocean observing infrastructure with multidisciplinary sampling in important but challenging locations has provided new scientific and engineering insights into the operation of a sustained ocean observing system. This paper summarizes the challenges, successes, and failures experienced to date and shares recommendations on best practices that will be of benefit to the global ocean observing community

Flagellated microswimmers: Hydrodynamics in thin liquid films

The hydrodynamics of a flagellated microswimmer moving in thin films is discussed. The fully re- solved hydrodynamics is exploited by solving the Stokes equations for the actual geometry of the swimmer. Two different interfaces are used to confine the swimmer: a bottom solid wall and a top air-liquid inter- face, as appropriate for a thin film. The swimmer follows curved clockwise trajectories that can converge towards an asymptotically stable circular path or can result in a collision with one of the two interfaces. A bias towards the air-liquid interface emerges. Slight changes in the swimmer geometry and film thickness strongly affect the resulting dynamics suggesting that a very reach phenomenology occurs in the presence of confinement. Under specific conditions, the swimmer follows a “crown-like” path. Implications for the motion of bacteria close to an air bubble moving in a microchannel are discussed