Helical Locomotion in Yield Stress Fluids

We report three stages for locomotion of a helical swimmer in yield stress fluids. In the first stage, the swimmer must overcome the material's yield strain to generate rotational motion. However, exceeding the first threshold is not sufficient for locomotion. Only when the viscous forces are sufficiently strong to plastically deform the material to a finite distance away from the swimmer will net locomotion occur. Once locomotion is underway in the third stage, the yield stress retards swimming at small pitch angles. Conversely, at large pitch angles, yield stress dominates the flow by enhancing swimming speed. Flow visualizations reveal a highly localized flow near the swimmer in yield stress fluids.Received 27 March 2022Accepted 27 January 2023DOI:https://doi.org/10.1103/PhysRevLett.130.114002© 2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasComplex fluidsLocomotionNon-Newtonian fluidsRheologySwimmingPhysical SystemsMicroswimmersFluid DynamicsPolymers & Soft Matter