Helical Locomotion in Yield Stress Fluids

Microorganism’s locomotion is common in various biological environments and affects several aspects of our life, including reproduction and infection. We report experiments on the locomotion of a helical swimmer in yield stress fluids. The swimmer must overcome two thresholds to be able to swim forward. Above a critical yield strain ε_y≈10%, the swimmer is able to overcome the yield barrier and create rotational motion. 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 geometry and the fluid rheological properties. Below the critical Bi_c, and at low pitch angles (12° ≤ ψ ≤ 37°), the yield stress to Newtonian swimming speed ratio is below unity. 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.