Helical Swimming Through a Viscoplastic Fluid

The motility of microorganisms exerts profound effects on various life processes, encompassing reproduction, infection, and marine ecosystem dynamics. This talk presents experiments on a 3D-printed helical swimmer in yield stress fluids using Carbopol. In our previous study, three distinct stages controlling helical swimmer locomotion in viscoplastic fluids were identified [Nazari et al., 2023]. This talk focuses on the impact of drag and propulsion on swimming stages in viscoplastic fluids through variation of helix filament thickness (T_t), swimmer head length (H_L), and cylindrical head cross-section (D). The study reveals that some of these geometric factors significantly influence the third locomotion stage by modulating drag and thrust forces experienced by the swimmer. Our results indicate that the swimmer shape does not affect the critical yield strain (ε_y) to initiate rotational motion. Forward motion occurs only when rotational motion induces material yielding far from the swimmer, specifically occurring below a critical Bingham number (Bi_c). Intriguingly, remains independent of geometric factors, T_t, H_L, and D. Subsequently at the third stage of swimming, (Bi〈Bi_c), at low pitch angles (12° ≤ ψ ≤ 37°) and below the critical Bi_c, the yield stress to Newtonian swimming speed ratio remains below unity. However, at larger pitch angles, this ratio can exceed one (up to 10). Notably, an optimal T_t is associated with the highest swimming speed. Moreover, an increase in D results in a hindrance to the swimming speed, while surprisingly, in this stage, swimming speed exhibits no significant dependency on H_L. Flow visualizations depict highly localized fluid deformation, with swimming speed governed by a balance between tail propulsion and bulk deformation around the head. These findings offer valuable insights into helical swimmers' locomotion in yield stress fluids, with implications in complex biological environments.