Microswimmers and microfluidics: Understanding and manipulating the locomotion of undulatory microswimmers

Undulatory microswimmers, such as nematodes, are of great importance to agriculture, animal and human health, and fundamental biological research. The nematode Caenorhabditis (C.) elegans is widely used as a model organism for medical studies. My work focuses on studying the locomotion of nematodes; their interactions with surfaces, fluid flow, and each other; and developing new tools to manipulate their motion for diverse applications. In the first half (chapters 2-4) of this dissertation, I investigate experimentally and theoretically the effects of flat solid surfaces, external channel flow, and other swimmers on the swimming dynamics of undulatory microswimmers. I discovered that 1) when swimming in close proximity, undulatory microswimmers synchronize their swimming gait. This synchronization is facilitated by direct collisions among the swimmers, rather than by long-range hydrodynamic interactions or deliberate actions of the swimmers; 2) undulatory micro-swimmers have a tendency to accumulate near and swim along surfaces. This behavior does not require touch sensation ability of the swimmers, and can be explained by a short-range hydrodynamic interaction between the swimmers and adjacent surface; 3) undulatory microswimmers exhibit positive rheotaxis (upstream swimming behavior) near solid surfaces. This behavior is induced by the combination of a hydrodynamic surface attraction effect and the velocity gradient of external flow near solid surfaces. These findings help explain certain intriguing behaviors of undulatory microswimmers, highlight the diverse roles of hydrodynamic forces in microswimmers\u27 life cycles, and lay the foundations for novel microswimmer manipulation methods for fundamental biological research and clinical applications. In the second half (chapters 5-7) of this dissertation, I present the design, fabrication, characterization, and applications of a few engineering devices/methods for dynamic trapping, motility measurement, motility-based sorting, and directing the motion of microswimmers. These new devices/methods enabled many studies that would be impossible or impractical with conventional methods