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We create bio-inspired robots to uncover underlying mechanisms from a mechanical engineer's perspective.
Ultrafast elastocapillary fans control agile maneuvering in ripple bugs and robots Science (2025, Cover) Link
Rhagovelia's acrobatics on water surface
Rhagovelia's acrobatics on water surface
Rhagovelia ripple bugs exhibit a remarkable manuverability in rapid flows. They can rapidly turn full-body 96° in 50 milliseconds, with forward speeds of up to 120 body lengths per second—on par with fruit fly saccades in air.
Functional duality of fan, flexibility and rigidity
Functional duality of fan, flexibility and rigidity
Such locomotory proficiency stems from a feathery fan like structure at the distal end of their middle legs. It generates high propulsion by maintaining a large surface area and resisting deformation caused by thrust. Conversly, this fan can flexibly collapse from ~180° to 0° (and vice versa) in ~10 ms as it enters or exits water.
Two questions
1. Does the fan spread and collapse only through active muscle actuation?
2. How can the fan combine flexibility for rapid deployment and rigidity for high thrust?
Capillary force-driven morphing fan
Capillary force-driven morphing fan
We discovered the isolated fan fully spread in ~10 ms upon contact with water. The fan rapidly collapsed upon removal from the water, also within ~10 ms. This confirms that the fan can act as an ultrafast capillary self-actuator without musle activation.
Artificial muscle-driven micro robot
Artificial muscle-driven micro robot
To explore the mechanism of the fan, we built a bio-inspired water-walking robot (~ 0.2 g), the Rhagobot, consisting of four SMA-based acutators and power transmission. The robot combines vertical and horizontal motions to acheive rowing.
Flat ribbon-shaped fan for directional stiffness
Flat ribbon-shaped fan for directional stiffness
Fixed-shaped rigid pads(1) and high-stiffness wire-type fans(2) cause high surface tension that impedes the leg from lifting off the water. Low-stiffness wire-type fans(3) collapse under capillary forces but deform under drag, resulting in reduced thrust.
The fan threfore requires two distinct stiffness levels—low stiffness to allow collapse by capillary force and high stiffness to resist the drag. We concluded that the flat-ribbon fan can achieve both compliance and rigidity in differnet direction.
Self-morphing engineered fan
Self-morphing engineered fan
We developed an artificial fan that folds and spreads passively through elastocapillarity. Weighing only ~1 mg, it consists of 21 flat-ribbon-shaped barbs. We strongly suspected that the Rhagovelia fan might have a similar morphology. Surprisingly, SEM imaging confirmed that the biological counterpart also adopts this morphology.
Rhagobot equipped with self-morphing artificial fan
Rhagobot equipped with self-morphing artificial fan
The artificial fan not only collapse but also maintain its shape during the thrust, increasing the robot agility. The self-morphing elastocapillary fan enables the Rhagobot to generate greater momentum by maintaining its large surface area underwater and avoiding speed reduction in the recovery phase by retracting the collapsible fan from the water. The Rhagobot exhibited agile turning and faster speed as compared with those of previously reported semiaquatic robots.
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