(572h) Swimming in Yield Stress Fluids

We can find Microorganisms’ locomotion in various biological environments which affects several aspects of our life, including reproduction, infection and the marine ecosystem. The prime example includes swimming of Helicobacter pylori (H. pylori) in gastric mucus that may lead to ulcer. In the acidic environment of the stomach, gastric mucus exhibits a strong yield stress behavior that protects the stomach from invasion of bacteria. Motivated by a recent experimental study that showed lack of H. pylori motility at low pH in porcine gastric mucus (at which PGM is a yield stress fluid), we investigate the locomotion of a helical swimmer in a model yield stress fluid based on Carbopol solution. Our results indicate that locomotion of a helical swimmer in a yield stress fluid occur in three different steps. In the first step, the swimmer must overcome the elastic resistance of the surrounding material to rotate. This step is characterized by a critical yield strain (ε_Y). However, exceeding the first threshold is not sufficient for locomotion. Only below a critical Bingham number (Bi_c ≈ 0.6), when the rotational motion forces the material to yield far away from the swimmer, forward motion will occur. These critical thresholds do not depend on the swimmer’s geometric factors (e.g. the thickness of the helix’s filament (T_t) and the head length of the swimmer (H_L) ). Once swimming is underway in the yield stress fluid and below the critical Bingham number Bi_c, the yield stress to Newtonian swimming speed ratio is below unity at low pitch angles (12° ≤ ψ ≤ 37°). Remarkably, this speed ratio can increase well beyond one (up to 10) at larger pitch angles, indicating that yield stress may facilitate the locomotion. Flow visualizations indicated that the fluid deformation is highly localized, and the swimming speed is controlled by a balance between propulsion inside the tail and bulk deformation around the head.