Abstract
Energy-efficient propulsion is a critical design target for robotic swimmers. Although previous studies have pointed out the importance of nonuniform body bending stiffness distribution (k) in improving the undulatory swimming efficiency of adult fish-like robots in the inertial flow regime, whether such an elastic mechanism is beneficial in the intermediate flow regime remains elusive. Hence, we develop a class of untethered soft milliswimmers consisting of a magnetic composite head and a passive elastic body with different k These robots realize larval zebrafish-like undulatory swimming at the same scale. Investigations reveal that uniform k and high swimming frequency (60 to 100 Hz) are favorable to improve their efficiency. A shape memory polymer-based milliswimmer with tunable k on the fly confirms such findings. Such acquired knowledge can guide the design of energy-efficient leading edge-driven soft undulatory milliswimmers for future environmental and biomedical applications in the same flow regime.
Highlights
A large number of fish species use undulatory swimming for energy- efficient and fast locomotion [1]
Because of the changing dominance of viscous forces and inertial forces in different regimes [14, 17], the fluid-structure interaction in the intermediate flow regime is distinct from the ones in the inertial flow, and it might require a different role of k in improving energy efficiency
To address such a challenge, we propose a class of untethered undulatory soft milliswimmers with uniform and nonuniform k to experimentally study the effects of different bioinspired k on their swimming energy efficiency
Summary
A large number of fish species use undulatory swimming for energy- efficient and fast locomotion [1]. Because of the changing dominance of viscous forces and inertial forces in different regimes [14, 17], the fluid-structure interaction in the intermediate flow regime is distinct from the ones in the inertial flow, and it might require a different role of k in improving energy efficiency To address such a challenge, we propose a class of untethered undulatory soft milliswimmers with uniform and nonuniform k to experimentally study the effects of different bioinspired k on their swimming energy efficiency. Kinematics analyses reveal that the uniform k can generate improved body waveforms at high frequencies, which refines the wake flow pattern through increasing the fluid circulation and decreasing the jet angle, visualized by particle imaging velocimetry (PIV) To verify these findings further, we build a robotic larval fish with an on-the-fly, temperature-based tunable k by using a shape memory polymer (SMP) material [19]. Fish-like robots adopting other on-board soft actuation methods at the similar time and length scale could use the knowledge gained in this work to achieve energy-efficient swimming for environmental monitoring and remediation applications [20]
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