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Thin-film modelling of complex fluids and bacterial colonies

Abstract : Bacteria colonise interfaces by the formation of dense aggregates. In this thesis, we develop and analyse simple models to clarify the role of passive physico-chemical forces and processes - such as osmosis, surface tension effects and wettability - in the spreading of bacterial colonies at solid-air interfaces. The models are based on a hydrodynamic description for thin films of liquid suspensions that is supplemented by bioactive processes.We first focus on the osmotic spreading mechanism of bacterial colonies that relies on the generation of osmotic pressure gradients. The bacteria secrete a polymeric matrix which acts as an osmolyte and triggers the influx of nutrient-rich water from the moist substrate into the colony. We find that wettability crucially affects the spreading dynamics. At low wettability, the lateral expansion of the colony is arrested, albeit the colony is biologically active. However, a small reduction of the surface tension and the resulting improvement of the wettability suffices to induce continuous spreading. This can, e.g., result from the production of bio-surfactants by the bacteria.Next, we study passive liquid films covered by insoluble surfactants before developing a model for the surfactant-driven spreading of bacterial colonies. In this spreading mechanism, Marangoni fluxes arising due to a non-uniform surfactant concentration at the edges of the colony drive cooperative spreading and may cause an instability of the circular colony shape. We find that variations in wettability and surfactant production suffice to reproduce four different types of colony growth, namely, arrested and continuous spreading of circular colonies, slightly modulated front lines and the formation of pronounced fingers.In the final part, we take a first step towards the incorporation of active collective bacterial motion in the employed thin-film framework and present a phenomenologically derived model for active polar films.
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Submitted on : Monday, September 30, 2019 - 10:27:09 AM
Last modification on : Tuesday, October 6, 2020 - 12:44:29 PM
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  • HAL Id : tel-02301013, version 1




Sarah Christine Trinschek. Thin-film modelling of complex fluids and bacterial colonies. Soft Condensed Matter [cond-mat.soft]. Westfälische Wilhelms-Universität, 2019. English. ⟨NNT : 2019GREAY012⟩. ⟨tel-02301013⟩



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