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A structure like those found on water striders' legs keeps a robot out of the water.
Credit: Saad Bhamla
Robots can serve pizza, crawl over alien planets, swim like octopuses and jellyfish, cosplay as humans, and even perform surgery. But can they walk on water?
Rhagobot isn’t exactly the first thing that comes to mind at the mention of a robot. Inspired by Rhagovelia water striders, semiaquatic insects also known as ripple bugs, these tiny bots can glide across rushing streams because of the robotization of an evolutionary adaptation.
Rhagovelia (as opposed to other species of water striders) have fan-like appendages toward the ends of their middle legs that passively open and close depending on how the water beneath them is moving. This is why they appear to glide effortlessly across the water’s surface. Biologist Victor Ortega-Jimenez of the University of California, Berkeley, was intrigued by how such tiny insects can accelerate and pull off rapid turns and other maneuvers, almost as if they are flying across a liquid surface.
"Rhagovelia’s fan serves as an inspiring template for developing self-morphing artificial propellers, providing insights into their biological form and function," he said in a study recently published in Science. "Such configurations are largely unexplored in semi-aquatic robots."
Mighty morphin’
It took Ortega-Jimenez five years to figure out how the bugs get around. While Rhagovelia leg fans were thought to morph because they were powered by muscle, he found that the appendages automatically adjusted to the surface tension and elastic forces beneath them, passively opening and closing ten times faster than it takes to blink. They expand immediately when making contact with water and change shape depending on the flow.
By covering an extensive surface area for their size and maintaining their shape when the insects move their legs, Rhagovelia fans generate a tremendous amount of propulsion. They also do double duty. Despite being rigid enough to resist deformation when extended, the fans are still flexible enough to easily collapse, adhering to the claw above to keep from getting in the animal’s way when it’s out of water. It also helps that the insects have hydrophobic legs that repel water that could otherwise weigh them down.
Ortega-Jimenez and his research team observed the leg fans using a scanning electron microscope. If they were going to create a robot based on ripple bugs, they needed to know the exact structure they were going for. After experimenting with cylindrical fans, the researchers found that Rhagovellia fans are actually structures made of many flat barbs with barbules, something which was previously unknown.
The researchers also observed that the water striders produce vortices with each stroke as they trek across the water’s surface, which are not unlike the wakes that wings leave behind when flying. Their leg propulsion also produces waves. Knowing this, the researchers tried to replicate it.
Legs for days
This is a new frontier for robotics. Existing amphibious robots often use bulky pads, much like oars, to generate thrust, though some smaller models do employ thin hydrophobic legs modeled after another species of water strider. The thin legs do reduce surface tension that can slow them down, but still suffer from limited momentum.
Rhagobot was Ortega-Jimenez’s version of this already high-tech insect. After closely studying the structure and function of Rhagovelia legs and fans, he and his team created artificial versions that were also designed to morph when exposed to water. These were attached to the middle legs of Rhagobot. Just like their inspiration, the fans spread immediately when submerged in water and closed again once they were out. There is no need for an extra power source because the morphing of the fans is determined by the motion and speed of water.
The Rhagobot takes a step.
The team wanted to see if their artificial fans would give Rhagobot an edge. They built an alternate robot, modeled after another species of water strider that propels itself using surface tension, and pitted it against Rhagobot. Both were given the same amount of power, yet Rhagobot was able to travel farther and make sharper, faster turns than its competition because the fans gave it an edge
“Fan-induced thrust increased forward speed and allowed rapid braking as well,” Ortega-Jimenez said, adding that “The collapsibility of the fan also notably reduced the energy required for the robot to lift the leg from the water.”
In the future, Rhagobot could brave turbulent waters to be a part of environmental monitoring systems, and the researchers are excited about the potential that swarms of these bots could help with search and rescue missions during storms and floods—though adding the weight of sensors and power will be a significant challenge. It might even be able to explore places beyond Earth, such as the methane oceans of Saturn’s moon Titan. If Rhagovelia has proven anything, it’s that even the smallest of creatures can inspire huge leaps forward—on water or otherwise.
Science, 2025. DOI: 10.1126/science.adv2792
Credit: Saad Bhamla
Robots can serve pizza, crawl over alien planets, swim like octopuses and jellyfish, cosplay as humans, and even perform surgery. But can they walk on water?
Rhagobot isn’t exactly the first thing that comes to mind at the mention of a robot. Inspired by Rhagovelia water striders, semiaquatic insects also known as ripple bugs, these tiny bots can glide across rushing streams because of the robotization of an evolutionary adaptation.
Rhagovelia (as opposed to other species of water striders) have fan-like appendages toward the ends of their middle legs that passively open and close depending on how the water beneath them is moving. This is why they appear to glide effortlessly across the water’s surface. Biologist Victor Ortega-Jimenez of the University of California, Berkeley, was intrigued by how such tiny insects can accelerate and pull off rapid turns and other maneuvers, almost as if they are flying across a liquid surface.
"Rhagovelia’s fan serves as an inspiring template for developing self-morphing artificial propellers, providing insights into their biological form and function," he said in a study recently published in Science. "Such configurations are largely unexplored in semi-aquatic robots."
Mighty morphin’
It took Ortega-Jimenez five years to figure out how the bugs get around. While Rhagovelia leg fans were thought to morph because they were powered by muscle, he found that the appendages automatically adjusted to the surface tension and elastic forces beneath them, passively opening and closing ten times faster than it takes to blink. They expand immediately when making contact with water and change shape depending on the flow.
By covering an extensive surface area for their size and maintaining their shape when the insects move their legs, Rhagovelia fans generate a tremendous amount of propulsion. They also do double duty. Despite being rigid enough to resist deformation when extended, the fans are still flexible enough to easily collapse, adhering to the claw above to keep from getting in the animal’s way when it’s out of water. It also helps that the insects have hydrophobic legs that repel water that could otherwise weigh them down.
Ortega-Jimenez and his research team observed the leg fans using a scanning electron microscope. If they were going to create a robot based on ripple bugs, they needed to know the exact structure they were going for. After experimenting with cylindrical fans, the researchers found that Rhagovellia fans are actually structures made of many flat barbs with barbules, something which was previously unknown.
The researchers also observed that the water striders produce vortices with each stroke as they trek across the water’s surface, which are not unlike the wakes that wings leave behind when flying. Their leg propulsion also produces waves. Knowing this, the researchers tried to replicate it.
Legs for days
This is a new frontier for robotics. Existing amphibious robots often use bulky pads, much like oars, to generate thrust, though some smaller models do employ thin hydrophobic legs modeled after another species of water strider. The thin legs do reduce surface tension that can slow them down, but still suffer from limited momentum.
Rhagobot was Ortega-Jimenez’s version of this already high-tech insect. After closely studying the structure and function of Rhagovelia legs and fans, he and his team created artificial versions that were also designed to morph when exposed to water. These were attached to the middle legs of Rhagobot. Just like their inspiration, the fans spread immediately when submerged in water and closed again once they were out. There is no need for an extra power source because the morphing of the fans is determined by the motion and speed of water.
The Rhagobot takes a step.
The team wanted to see if their artificial fans would give Rhagobot an edge. They built an alternate robot, modeled after another species of water strider that propels itself using surface tension, and pitted it against Rhagobot. Both were given the same amount of power, yet Rhagobot was able to travel farther and make sharper, faster turns than its competition because the fans gave it an edge
“Fan-induced thrust increased forward speed and allowed rapid braking as well,” Ortega-Jimenez said, adding that “The collapsibility of the fan also notably reduced the energy required for the robot to lift the leg from the water.”
In the future, Rhagobot could brave turbulent waters to be a part of environmental monitoring systems, and the researchers are excited about the potential that swarms of these bots could help with search and rescue missions during storms and floods—though adding the weight of sensors and power will be a significant challenge. It might even be able to explore places beyond Earth, such as the methane oceans of Saturn’s moon Titan. If Rhagovelia has proven anything, it’s that even the smallest of creatures can inspire huge leaps forward—on water or otherwise.
Science, 2025. DOI: 10.1126/science.adv2792
