A recent study published in Science details how the unique feet of ripple bugs, or water striders from the genus Rhagovelia, have inspired a new propulsion system for miniature robots. The research was conducted by a team including Victor Ortega-Jiménez, assistant professor of integrative biology at the University of California, Berkeley; engineers from Ajou University in South Korea; and researchers from the Georgia Institute of Technology.
The team found that Rhagovelia insects possess feet that expand into fan-like structures when submerged. These fans open without muscular effort due to water’s surface tension and collapse when removed from water, minimizing drag as the insect moves. “Observing for the first time an isolated fan passively expanding almost instantaneously upon contact with a water droplet was entirely unexpected,” said Ortega-Jiménez.
Electron microscopy revealed that each fan is composed of flat, flexible strips with feather-like barbules. When fanned out underwater, these become rigid enough to function as effective oars. Inspired by this mechanism, Ajou University engineers created similar fans for an insect-sized robot named Rhagobot. According to Je-sung Koh, professor at Ajou University and senior author on the study: “Our robotic fans self-morph using nothing but water surface forces and flexible geometry, just like their biological counterparts. It is a form of mechanical embedded intelligence refined by nature through millions of years of evolution. In small-scale robotics, these kinds of efficient and unique mechanisms would be a key enabling technology for overcoming limits in miniaturization of conventional robots.”
Saad Bhamla, professor at Georgia Tech and another senior author, noted: “We learned a rule from nature: the air-water surface can act as a battery. Surface tension powers the insect’s collapsible fan, and the same design powers the robot fan.”
Rhagovelia insects demonstrate high agility in turbulent streams—able to turn 90 degrees in about 50 milliseconds and reach speeds up to 120 body lengths per second. Unlike other water striders, they have both fans and claws at the end of their oaring legs. Research showed that while muscles help fold these fans underwater alongside claw movement, surface tension alone can rapidly open them within about 10 milliseconds.
These bugs are small predators living in fast-moving waters where they face constant turbulence far greater than typical airplane turbulence levels. Ortega-Jiménez described their activity: “They literally row day and night throughout their lifespan, only pausing to molt, mate or feed.”
Ortega-Jiménez first observed ripple bugs’ rapid movements during postdoctoral work at Kennesaw State University—a curiosity that led to over five years of collaborative research involving teams from multiple institutions.
Ajou University’s Dongjin Kim explained how designing artificial ribbon-like blades for robotic fans mirrored discoveries made about Rhagovelia’s natural microarchitecture: “We strongly suspected that biological fans might share a similar morphology, and eventually discovered that Rhagovelia’s fan indeed possessed a flat-ribbon microarchitecture, which had not been previously reported. This discovery further validated the design principle behind our artificial flat-ribbon fan.”
The resulting robot prototype uses elastocapillary fans measuring roughly 10 by 5 millimeters attached to two long legs (each about 5 centimeters), allowing it to travel two body lengths per second and execute sharp turns quickly.
Ortega-Jiménez also found that fanned Rhagovelia generate complex vortices during movement—patterns similar to those produced by flying insects’ wings—and continues investigating whether these structures might produce lift.
Other contributors include Sunny Kumar (Georgia Tech) and Changhwan Kim (Ajou University).
For more information on this study:
– Read more at Science
– Visit Victor Ortega-Jiménez’s lab website
– See Ajou University press release
