Locomotion Dynamics in Yield Stress Fluids

The movement of microorganisms through thick, gel-like materials plays an important role in many real-life situations. For example, Helicobacter pylori swim through the sticky mucus lining the stomach, contributing to ulcers and cancer. Other bacteria move through food gels and medical hydrogels, which can lead to contamination. Even tiny worms called nematodes burrow through wet soil, helping to improve its fertility. Despite the importance of these biological processes, the mechanics behind movement in such resistant, gel-like environments remain poorly understood. This award will fill that gap by combining innovative experiments using a helical corkscrew and self-propelled robotic swimmer with advanced three-dimensional simulations to uncover the physical principles governing locomotion in gel-like materials. Project outcomes could lead to non-antibiotic methods to prevent Helicobacter pylori infections in human, improve microrobot design for safe drug delivery, and optimize bacterial hydrogels for filtration and sensing. Additionally, the developed models will support innovations in 3D printing, drilling, and natural hazard prediction. Finally, the project will contribute to workforce development by building a collaborative fluid dynamics research and education program at FAMU-FSU College of Engineering. Yield-stress fluids behave like solids below a critical stress threshold, creating a mechanical barrier that organisms must overcome to initiate motion. Although this threshold is fundamental to locomotion, the conditions that trigger movement in yield-stress materials and the resulting swimming dynamics remain poorly understood. It is hypothesized that locomotion is governed by a critical yield strain and non-dimensional numbers such as the Bingham, Oldroyd, and Deborah numbers. To investigate this, a modular approach will be adopted. First, the fluid dynamics of the swimmer components will be studied, focusing on drag, thrust, and yield surface analysis. Using Carbopol-based fluids, well-controlled viscoplastic and elasto-viscoplastic environments will be studied to isolate rheological effects. Next, the locomotion of a self-propelled robot will be examined to identify critical thresholds for movement in viscoplastic fluids. Finally, the impact of elasto-viscoplastic properties on swimming performance will be studied. Experimental results will be compared with simulations using viscoplastic and elasto-viscoplastic fluid models. A novel prism flow analysis will further elucidate the governing fluid mechanics behind swimming in yield-stress media. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.