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Licentiate seminar

On the behavior of motile microbes in Fluid Flow


Defendant Main Advisor Extra Advisor Date
Cai-Juan Zhan Luca Brandt Anders Dahlkild 2014-06-13

Opponent
Toschi Federico , Technische Universiteit Eindhoven

Evaluation committee

Abstract

In this thesis, we focus on the behavior of motile microbes in laminar and turbulent shear flows. The microscopic coupling between the flow structure in ocean and the motility of its inhabitants, such as bacteria and phytoplankton, determines the macroscopic marine landscape. We have carried out numerical simulations of microorganisms, which are modeled as prolate spheroids, in fluid flows. An individual motile microorganism's swimming trajectory is determined by the action of its cilia or flagella and by the advection by the bulk fluid. The motility is crucial for microorganisms seeking food or escaping predators and is a main trait distinguishing them from passive particles. Depending on different external stimuli, the motility of microorganism can be categorized, so-called taxis. Among many different types of taxis, we mainly focus on gyrotaxis and chemotaxis. We investigate how the motility affects the swimmers' distribution by performing simulations with motile and non-motile swimmers. As turbulence characterizes the life of microbes in water supply systems, ocean and bioreactors, we consider the influence of turbulent flow on aggregations of motile microbes. Two different types of turbulence, homogenous isotropic turbulence and Kolmogorov turbulence, are discussed. We note that large-scale patchiness, which can be observed in laminar flows, linear shear and vortex flows are significant decreased by the turbulent flow. Our results show that motile microorganisms with one preferential swimming direction (e.g.\ gyrotaxis) show significant small-scale clustering in a turbulent flow if spherical in shape, whereas prolate swimmers remain more uniformly distributed. The adaption to the flow environment has also implications for the strategies used to find nutrients as also documented in this work. Microorganisms are very diverse and one of the fundamental features of microbial life is morphology. Their shapes alter their orientation and influence their distribution. When microbes encountering shear flow, their aspect ratios are crucial since elongate particles could be more sensitive to the vorticity and rate of strain of the fluid. We therefore examine the clustering or patchiness of swimmers with different slenderness. Our results show how purely hydrodynamic effects can alter the ecology of microorganisms that can vary their shape and their preferential orientation.
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