Date of Award

2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

Kathleen J. Stebe

Abstract

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's 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.

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