Ripple Bugs' Wing-Like Feet Inspire Mini Robot Propulsion

A tiny bug's unique, wing-like feet, which allow it to skim the surface of turbulent streams with amazing maneuverability, has inspired a robot that is similarly agile on the water.

A biologist from the University of California, Berkeley, and engineers from Ajou University in South Korea and the Georgia Institute of Technology report in this week's issue of Science that water striders in the genus Rhagovelia - often called ripple bugs - have feet that bloom into a fan when immersed in water, providing an effective oar for boating along the water surface.

No muscles are required to open the fan, which resembles a Japanese folding fan; the surface tension of the water and its tendency to form a taut elastic surface does all the work. When removed from water, the fan collapses like the tip of a paintbrush - also because of surface tension and the flexibility of the fan - reducing drag as the bug repositions its legs to reenter the water.

"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, a UC Berkeley assistant professor of integrative biology and lead author of the paper.

Inspired by this neat trick, engineers at Ajou University designed a similar fan-like oar-tip and attached them to the legs of an insect-sized robot. Called Rhagobot, its self-spreading passive fans significantly improved thrust, braking and turning - key factors for controlled, high-speed maneuvers - compared to robots without the fans.

Such robots could be useful in environmental monitoring systems, as search-and-rescue microrobots and in devices capable of navigating turbulent water with insect-like dexterity.

Using electron microscopy, the Ajou University team discovered Rhagovelia's secret. The bug's fan is actually a series of flat, flexible, ribbon-like strips with barbules that make them look like feathers. When fanned out underwater, they are rigid enough to make an effective oar. Rhagobot's fan-like feet work the same way.

"Our robotic fans self-morph using nothing but water surface forces and flexible geometry, just like their biological counterparts," said Je-sung Koh, a professor at Ajou University and a senior author of the study. "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."

"We learned a rule from nature: the air-water surface can act as a battery," said Saad Bhamla, a professor at Georgia Tech and a senior author of the paper. "Surface tension powers the insect's collapsible fan, and the same design powers the robot fan."

The biologists measured the turning speed of the Rhagovelia water striders and found them on par with the fastest recorded turns in animal fliers like fruit flies: able to make a 90-degree turn in about 50 milliseconds and shoot off at speeds reaching about 120 body lengths per second.

Skimming the surface

Rhagovelia are unique among water striders in having both fans and claws on the ends of their two oaring legs. It was traditionally believed that the fans were exclusively opened and closed by specialized muscles, but Ortega-Jiménez placed an isolated fan on a water droplet and found that surface tension was sufficient to do so in a remarkably short time, a mere 10 milliseconds. The muscle that controls the claw also folds the fan while underwater, but not completely.

The bugs, a mere 3 millimeters in length, are voracious predators and frequent cannibals that live in turbulent streams and coastal waters, which they must navigate while escaping predators, catching prey and finding mates. The relative levels of turbulence that these insects endure almost constantly far exceed what we typically experience during airplane turbulence, Ortega-Jiménez said.

"They literally row day and night throughout their lifespan, only pausing to molt, mate or feed," said Ortega-Jiménez, who documented this with a GoPro camera running 24/7 for several months.

Having previously studied the jumping performance of large water striders on unsteady waters, he said he "was intrigued the first time I saw ripple bugs, while working as a postdoc at Kennesaw State University during the pandemic. These tiny insects were skimming and turning so rapidly across the surface of turbulent streams that they resembled flying insects. How do they do it? That question stayed with me and took more than five years of incredible collaborative work to answer it."

After moving to Georgia Tech as a research scientist, he mentioned these observations to Bhamla, who became equally fascinated and eager to explore the behavior. Bhamla invited the Korean team to collaborate, and together they investigated the biomechanics of the fan and how to mimic it. Designing artificial fans that operate like those of the insect proved challenging, said postdoctoral researcher and also lead-author Dongjin Kim, but they eventually hit upon the idea of using ribbon-like fan blades.

"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," he said.

Rhagobot's fans measure about 10 by 5 millimeters and are attached at the ends of two of the robot's long, thin legs, each about 5 centimeters long. With the self-deploying, elastocapillary fans, Rhagobot - weighing a mere one-fifth of a gram - can propel itself at about two body lengths per second and make a 90 degree turn in less than half a second. The study lays the foundation for future design of compact, semi-aquatic robots that can explore water surfaces in challenging, fast-flowing environments.

Ortega-Jiménez, who studies how animals interact with fluids, found that fanned Rhagovelia bugs, unlike other water striders, produce a distinct and complex series of vortices or tornadoes in the water with each stroke. These vortices closely resemble the wake produced by flapping wings in air.

"It's as if Rhagovelia have tiny wings attached to their legs, like the Greek god Hermes," he said.

He is currently investigating whether the fans, like wings, also may produce lift. He has a colony of Rhagovelia in his lab, collected from a creek that runs near UC Davis.

Sunny Kumar of Georgia Tech and Changhwan Kim of Ajou University are also authors of the paper.