| Description |
Microorganism motility supports many features in the microbial world and is closely linked to their abilities to find food, mate, and colonize, which are essential to their survival. While swimming, the organisms interact with their fluid environment in a manner quite different from our common human experience in fluids, making it interesting to understand the physics of such low Reynolds number hydrodynamics. In this dissertation, I discuss four different aspects of locomotion in viscous fluids. We first study the suitable choice of computational method for swimming and pumping with a helical flagellar filament at a low Reynolds number. Comparing the most commonly used approaches to solve Stokes equations numerically, we provide a guideline for the optimal choice of the tuning parameters for a wide helical geometries range balancing the accuracy and computational time expenses. We then study the effect of helical cell bodies on the swimming speed and trajectory by comparing to rod-shaped geometries. We validate our numerical model with experiments from high-frame-rate digital tracking and image processing for both helical and straight mutant (Helicobacter pylori) swimming mucin and broth solution. We find that the helicity of the cell body makes at most a 15% contribution to the additional thrust and change in swimming speed. We also study the single-flagellated bacterial flicks and instabilities due to hook and flagellum flexibilities. We find that dynamic instability initiates bacterial flick and then flexibility of the flagellar filament plays an important role in reorientation of the swimmer. Finally, we consider hydrodynamic interactions of a swimmer with a complex biological environment and other passive particles in viscous fluid. We examine long-range hydrodynamic interactions of a simple swimmer near spherical and filamentous obstacles and find that swimming velocity fluctuation is closely related to the correlations in density and orientations of obstacles. For the hydrodynamic interactions and approach of organisms with passive particles, we find exact solutions for the approach of spherical shapes in the cases that the swimmer is driven by a localized constant force and with distributed propulsion (squirmer model). We study the feasibility of approach and find that an organism can approach any similar size or bigger target compared to the organism's size, but approaching smaller targets depends on the current strength generated by the swimmer. |
