Bionics: Water Strider As A Model Mini Robot Maneuvers Across Water With Self-Deploying Fan Feet

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A research team describes how the fans on the middle legs of water striders unfold within milliseconds without muscle power and serve as paddles. The same geometry now powers an insect robot that turns, brakes, and swiftly glides over the water surface.

The study, published on August 21, 2025, closely examines nature's mechanics. Water striders of the genus Rhagovelia use fan-like structures on their middle legs that automatically spread open when submerged and close again when lifted out. This is achieved through the surface tension of the water combined with the elastic geometry of the fine, ribbon-like bristles. In the lab, isolated fans opened in about 10 milliseconds. The result: the insects "row" across turbulent surfaces, turn in an instant, and accelerate rapidly.

In experiments, researchers documented speeds of up to around 120 body lengths per second and full-body rotations of nearly 90 degrees within approximately 50 milliseconds. The key lies in the mechanical design: the flat-ribbon microstructure stiffens the fan underwater in the pulling direction while remaining flexible laterally. This creates a stable "paddle" during the pull, while reducing drag resistance when the legs are retracted.

From Biology to Machine

Based on this, researchers developed an insect robot, integrating self-morphing fans. The prototype, called "Rhagobot," weighs around 0.2 grams (0.007 ounces). The fans measure approximately 0.39 × 0.20 inches and are attached to the two middle legs. When submerged, they open automatically to form a rigid paddle; outside the water, they close into a fine tip. In tests, the robot reached about two body lengths per second and completed a 90-degree turn in well under half a second. Compared to variants without fans, thrust, braking distance, and maneuverability were measurably improved.

The approach differs from earlier water strider robots. Instead of incorporating additional motor mechanisms or complex control systems, the design leverages physical effects at the air-water interface as "built-in mechanics." Surface tension provides the energy for opening and closing, while the geometry ensures directional stiffness. This reduces energy consumption and keeps the structure lightweight—both critical factors for insect-scale systems.

What is Such An Insect Robot Useful For?

Applications include monitoring bodies of water, flood response, or accessing hard-to-reach shoreline areas. In such environments, small, widely deployable platforms could carry sensors, collect samples, or support search teams without capsizing in rough waters. At the same time, this work introduces a clear methodology for bionics: first, understanding the biomechanics and flow physics of the animals, then translating the principle into a simple, passive component in technology.

The authors attribute the results to an international collaboration between biology and engineering. Participants include the University of California, Berkeley, the Georgia Institute of Technology (Georgia Tech), and the Ajou University in South Korea. (mc)

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