Encoding spatiotemporally asymmetric motions in bioinspired magnetoactive propulsors enhances fluid pumping performance*

Many organisms use the sequential beating of multiple propulsors to swim or pump fluids (i.e., metachronal coordination). Often, these propulsors have temporally and spatially asymmetric beating patterns. Increasing the relative duration of the power stroke vs. recovery stroke increases temporal asymmetry. Extending the propulsor during the power stroke (enhancing thrust), and collapsing it during the recovery stroke (reducing drag) increases spatial asymmetry. The parameter space of propulsor shape and kinematics is vast, yet we know little about how key changes in these properties can alter the flows produced.

We explore the role of spatiotemporally asymmetric motions in fluid pumping performance using a soft robotic platform. This platform consists of magnetoactive propulsors, actuated by translating magnets on an underlying timing belt. We present a technique to encode spatial asymmetry in these propulsors by casting them with a curved shape, then placing them in a secondary mold to magnetically pole them under strain (flattened and tilted). This fabrication technique produces an interplay between elastic forcing and magnetic forcing which increases the spatial asymmetry of the propulsors during a beat cycle.

We characterize flows produced by these propulsors using Particle Image Velocimetry (2D2C PIV) across a range of beat frequencies and phase lags. Compared to a nearly reciprocal flat propulsor shape, the curved, spatially asymmetric propulsors move considerably more fluid. Additionally, curved propulsors that are convex with respect to the power stroke direction enhance pumping performance relative to a concave orientation. These experiments demonstrate that nuanced differences in propulsor kinematics can greatly influence the flows produced. Our platform enables the parametric exploration of metachronally coordinated propulsors and their kinematics, informing the bio-inspired design of fluid pumping devices and swimming robots.