Researchers from Korea and the United States have identified the leg structure and movement principles of water striders that glide on the surface of water, and have developed a miniature robot imitating this. It is expected that, as research advances, it could be used in various fields such as environmental monitoring, water quality exploration, and aquatic rescue.
Professor Ko Je-sung of Ajou University, in collaboration with researchers from the University of California, Berkeley (UC Berkeley) and Georgia Institute of Technology, announced the development of a miniature robot called 'Rhagobot,' modeled after the movement of the water strider insect Rhagovelia, on the cover of the international journal 'Science' on the 22nd.
Rhagovelia can instantaneously unfurl its fan-shaped legs at the ends, gaining propulsion even in swift currents and changing direction nimbly. It has been a mystery how a water strider can unfurl and fold its legs in such a short time.
The research team found a clue by implementing the same structure in the robot. They created a miniature device with 21 strands of artificial hair arranged in a fan shape to mimic the leg structure of Rhagovelia and applied it to the robot. As a result, they confirmed that the device opens by itself within 0.01 seconds when the robot enters the water and folds while functioning on the water's surface.
Professor Ko stated, 'I proved that the fan at the end of the water strider's leg operates automatically due to a combination of water surface tension and the elasticity of the fan, rather than muscle,' adding that 'Rhagobot can move much faster and further than existing aquatic miniature robots, and it can perform complex movements like rotation and braking just like a water strider.'
The elasticity-capillary phenomenon is the occurrence in which thin and flexible structures deform quickly due to the surface tension and elasticity of the water. Surface tension is the force that attracts water molecules to minimize the surface area. A water strider can walk on the water surface because its feet do not penetrate the water due to surface tension. The fan-shaped legs of Rhagovelia also add elasticity to the surface tension, allowing the object to return to its original form.
Professor Ko's research on water striders has been ongoing for 15 years since he began pursuing a doctoral degree under the guidance of Dr. Jo Gyoo-jin at Seoul National University's Department of Mechanical Engineering. In 2015, he demonstrated through robotic experiments the process by which water striders leap without breaking the surface tension, publishing a paper in Science. After moving to Ajou University, he expanded his research by developing the highest jumping surface leap robot in the world.
Five years ago, the research team from UC Berkeley and Georgia Institute of Technology introduced Rhagovelia to Professor Ko's team, leading the research to expand into the principles of the fan-shaped leg's movement. The research team closely observed and analyzed the movements of Rhagovelia over the past five years, resulting in the birth of Rhagobot.
Professor Ko reflected, 'It was interesting that applying too much force could actually break the water and prevent jumping,' noting that 'nature was utilizing physical laws such as surface tension and drag in the most efficient way.' This Rhagobot research is an extension of his long exploration and represents the achievement of mechanically replicating the evolved movements of water striders.
The research team believes that advancing the capabilities of Rhagobot could enhance potential applications on the water's surface for rescue operations, environmental monitoring, and precision exploration.
Professor Ko mentioned, 'To develop it for water search and environmental monitoring robots, it is necessary to increase the size and mass, as well as to incorporate a control system and sensors,' adding, 'The power issue can be resolved in various ways by harnessing natural sources such as solar energy, tides, and chemical energy.'
The technology used in this research has potential applications not only for robots but also for wearable devices. Professor Ko predicts, 'The market for new forms of wearable devices that interact directly with people, such as a lightweight glove that delivers tactile feedback like a haptic device, will grow significantly.'
References
Science (2025), DOI: www.doi.org/10.1126/science.adv2792
Science (2015), DOI: https://doi.org/10.1126/science.aab1637