Ultrafast elastocapillary fans control agile maneuvering in ripple bugs and robots

Abstract Millimeter-sized ripple bugs in the genus Rhagovelia exhibit exceptional agility and rapid maneuvers in fast, unsteady streams, comparable to animal fliers. Their remarkable interfacial transit and turning skills stem from a specialized fan structure on their middle legs. While researchers have suggested active fan actuation, the role of capillary forces and unique microstructure in self-spreading remains unclear. We reveal that Rhagovelia’s fans possess a flat-ribbon architecture with directional stiffness, enabling ultrafast elastocapillary morphing for passive actuation in under 10 ms, independent of muscle control, while producing high-thrust momentum through unsteady vortical wakes. These self-morphing fans allow Rhagovelia to execute ∼90 ° turns at a rate of ∼4200 °/s, in ∼50 ms, with speeds reaching ∼120 BL/s – on par with the fastest recorded turns in animal fliers like fruit flies. Inspired by these, we develop an ultralight, ultrafast elastocapillary robotic fan (∼1 mg, ∼100 ms opening/closing time) and integrate it into an insect-scaled robot (Rhagobot, ∼0.2 g). The engineered fans passively balance surface tension and water drag through stiffness anisotropy, enabling the robot to achieve high agility with speeds up to ∼2 BL/s and turning rates of 206 °/sec. Experiments with both insects and robots, with and without fans, show that a self-spreading passive fan significantly improves thrust, braking, and turning – key factors for controlled, high-speed maneuvers. This elastocapillary innovation enables ripple bugs to survive and thrive in turbulent streams and offers new insights for agile aquatic robotics.