Curvature-dependent propulsion of elastic flagella.

Soft robotic swimmers that can mimic the flagella-powered locomotion of micro-organisms are of significant interest in a broad range of applications. However, realising micro-organisms' dexterity in soft robots remains challenging without an effective mechanism to achieve bidirectional propulsion in low Reynolds numbers. Here, inspired by recent theoretical studies that suggest the possibility of intrinsically curved elastic flagella to achieve bidirectional propulsion, we experimentally investigate the propulsion behaviour of elastic artificial flagella with uniform intrinsic curvature, actuated by transverse oscillations at Re < 0.1. Our results reveal that the flagella's curvature influences the propulsion direction and magnitude, suggesting a transition between positive and negative propulsion when the flagella's central angle in the stress-free state () is between 60° and 90° at Sp = 1.5 and 1.8. We also investigate the relationship between the propulsion force and flagella oscillation with numerical simulations. These findings suggest the potential of on-demand curvature modulation during active oscillation to achieve bidirectional propulsion, enhancing the dexterity in flagella-driven artificial swimmers for a broad range of applications in microscale systems.