Fig. 5. Butterfly stroke–like bistable flapping-wing soft swimming robot.

(A) Schematic of the proposed bistable soft swimming robot by attaching the bistable flapping soft actuator with flexible films and extended fins. The inset shows the butterfly swimming stroke. Photo credit: Pixabay (inset). (B) The time-lapse side-view images of the representative swimming gaits in the bistable swimmer with a wingspan length of S = 150 mm during one cycle of down- and upstroke under an actuation pressure of 55 kPa and a frequency of 0.625 Hz. The insets show the schematic human’s swimming postures of the butterfly swimming stroke (32). Reproduced with permission (32). Copyright 1999, Elsevier. Scale bar, 15 mm. At t = 0 s, the floating swimmer is fully immersed in water except the wing tips with its wings flapped upward and body angled slightly. Upon inflating, the body starts to bend downward and barely moves forward until beyond the onset of snapping at t = 0.225 s, two wings quickly and simultaneously stroke downward and rotate within 46 ms (from t = 0.225 s to t = 0.271 s). Consequently, it lifts the body close to the water surface (t = 0.271 s) and propels forward. The body shows an angled posture with its head diving into the water (t = 0.817 s). Upon inflating to bend the body upward to snapping both wings back, the fast wing upstroke and rotation make the body steeply dive into the water (t = 1.073 s), propelling it forward with its head floating up (t = 1.285 s). The swimmer recovers to its initial posture at t = 1.614 s. (C) The tracked CoM motion of its soft body. (D) The velocity profile of the bistable butterfly stroke–like soft swimmer.