Drag and Thrust of a Helical Swimmer in a Yield Stress Fluid

The locomotion of microorganisms is a common phenomenon in diverse biological environments and significantly influences human health. For instance, the mobility of Helicobacter pylori (H. pylori) through gastric mucus, yields ulcers and other diseases. Recent experimental studies indicate that H.pylori gets immobilized when the surrounding environment (mucus) behaves like a yield stress fluid. In our earlier study, for the first time, we characterized three stages of locomotion observed in yield stress fluids. In this study, we explore the forces involved in these swimming stages by assessing both thrust and drag forces exerted by the swimmer under different conditions like the shape of the head and surface roughness in yield stress fluids. Using a custom-made rotational Helmholtz-coil, a constant torque is imposed on the swimmer that generates the rotational motion (Ω) . Our experiments found that as Bingham number (Bi=τ_y/(ηΩ)) decreased, pressure forces on the swimmer decreased linearly, while thrust from the helical tail rose. For pitch angles (ψ) of 12-32°, the thrust was lower than pressure, implying no net motion. below critical Bi (Bi_c ≈0.6), thrust surpassed pressure. Swimmers with ψ ≥ 37° consistently exceeded pressure with thrust. Additionally, we observed significant drag forces on the head, influenced by swimmer geometry and surface roughness. The drag coefficient increased as increased and leveled off at a higher Bi .