A team of biologists and engineers has developed a new propulsion system for miniature robots, inspired by the unique feet of water striders in the genus Rhagovelia, also known as ripple bugs. The collaboration involved researchers from the University of California, Berkeley, Ajou University in South Korea, and the Georgia Institute of Technology.
The findings, published in Science, detail how these insects possess feet that unfold into fan-like structures when submerged. These fans act as effective oars for moving along water surfaces. The expansion and collapse of the fans are controlled not by muscles but by water’s surface tension and the flexibility of the fan structure.
“Observing for the first time an isolated fan passively expanding almost instantaneously upon contact with a water droplet was entirely unexpected,” said Victor Ortega-Jiménez, assistant professor of integrative biology at UC Berkeley and lead author.
Electron microscopy revealed that each fan is made up of flat, flexible strips with feather-like barbules. When spread out underwater, these structures become rigid enough to propel the insect efficiently.
Inspired by this mechanism, engineers at Ajou University created a robotic equivalent called Rhagobot. This robot features self-spreading passive fans on its legs that improve thrust, braking, and turning compared to robots without such adaptations. “Our robotic fans self-morph using nothing but water surface forces and flexible geometry, just like their biological counterparts,” said Je-sung Koh, professor at Ajou University and senior author. “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.”
Potential applications for this technology include environmental monitoring systems and search-and-rescue microrobots capable of navigating turbulent waters.
Saad Bhamla from Georgia Tech highlighted another insight: “We learned a rule from nature: the air-water surface can act as a battery,” he said. “Surface tension powers the insect’s collapsible fan, and the same design powers the robot fan.”
Researchers measured Rhagovelia’s agility in turbulent streams and found that they can make 90-degree turns in about 50 milliseconds and reach speeds up to 120 body lengths per second—similar to some flying insects.
Ortega-Jiménez noted that previous assumptions held muscles responsible for operating these fans exclusively. However, his experiments showed that surface tension alone could open them rapidly within about 10 milliseconds.
Rhagovelia bugs are predators living in fast-moving streams or coastal waters where turbulence levels are much higher than typical human experiences during airplane travel. According to Ortega-Jiménez: “They literally row day and night throughout their lifespan, only pausing to molt, mate or feed.”
After observing ripple bugs’ movements while working at Kennesaw State University during the pandemic, Ortega-Jiménez continued investigating their biomechanics with colleagues at Georgia Tech before involving collaborators from South Korea. Dongjin Kim from Ajou University explained how their artificial flat-ribbon fan design was validated after discovering similar microarchitecture in Rhagovelia’s natural fans.
Rhagobot itself measures one-fifth of a gram; its elastocapillary fans allow it to move at two body lengths per second and turn sharply on water surfaces.
Ortega-Jiménez also observed that fanned Rhagovelia produce complex vortex patterns with each stroke—similar to wing wakes produced by flying insects—and suggested possible future research into whether these fans generate lift like wings do.
Other contributors include Sunny Kumar (Georgia Tech) and Changhwan Kim (Ajou University).
Further information can be found through Science magazine’s article on ultrafast elastocapillary fans, Victor Ortega-Jiménez’s lab website, or an Ajou University press release.
