Researchers uncover the biomechanics behind ripple bugs’ fan-like propellers and translate them into insect-scale robotics

Nature-Inspired Innovation

A team of researchers from the University of California, Berkeley, the Georgia Institute of Technology, and Ajou University (South Korea) has uncovered how the unique fan-like propellers of Rhagovelia water striders enable them to maneuver rapidly across turbulent streams.

These millimeter-sized insects, also known as “ripple bugs,” use specialized ribbon-shaped fans on their legs that passively open and close like a paintbrush, ten times faster than the blink of an eye. Inspired by this, the team engineered a robotic version that incorporates self-morphing elastocapillary fans to replicate the insects’ agility without requiring extra energy input.

How the Fans Work

Unlike earlier assumptions that Rhagovelia’s fans relied solely on muscles, the study—published in Science—reveals that the fans instead morph passively using surface tension and elastic forces.

“Observing for the first time an isolated fan passively expanding almost instantaneously upon contact with a water droplet was entirely unexpected,” said Dr. Victor Ortega-Jimenez, an integrative biologist at UC Berkeley and lead author of the study.

This passive mechanism enables sharp turns in just 50 milliseconds and speeds up to 120 body lengths per second—on par with flying insects.

Collaboration Across Disciplines

The project was born when Dr. Ortega-Jimenez, intrigued by the ripple bugs during his postdoctoral work, joined Georgia Tech in 2020 and presented his findings to Dr. Saad Bhamla. Together, they expanded the collaboration to include Professor Je-Sung Koh’s group at Ajou University.

“I saw a real discovery hiding in plain sight. Often, we think science is a lone genius sport, but this couldn’t be farther from the truth. Modern science is all about an interdisciplinary team of curious scientists working together, across borders and disciplines to study nature and engineer new bioinspired machines,” said Dr. Bhamla.

Engineering the “Rhagobot”

Designing an insect-scale robot proved challenging until Ajou University researchers used high-resolution electron microscopy to decode the bugs’ fan structure.

“We initially designed various types of cylindrical-shaped fans… After numerous attempts, we overcame this challenge by designing a flat-ribbon-shaped fan… We eventually discovered that the Rhagovelia fan indeed possesses a flat-ribbon micro architecture, which had not been previously reported,” said Dr. Dongjin Kim, postdoctoral researcher at Ajou University and co-lead author.

This breakthrough led to the creation of the “Rhagobot”—a one-milligram insect-scale robot equipped with self-morphing fans capable of enhanced thrust, braking, and maneuverability.

“Our robotic fans self-morph using nothing but water surface forces and flexible geometry—just like their biological counterparts. It is a form of mechanical embedded intelligence refined by nature through millions of years of evolution,” said Professor Je-Sung Koh, senior author.

Unique Biomechanics

The ripple bugs’ fans combine collapsibility during recovery with rigidity during propulsion, overcoming long-standing limitations in small-scale robotics. Their strokes create complex vortices on water, resembling the wakes of flapping wings.

“It’s as if Rhagovelia have tiny wings attached to their legs, like the Greek god Hermes,” said Dr. Ortega-Jimenez.

Facing Turbulent Waters

Ripple bugs must endure highly turbulent streams while feeding, mating, and escaping predators. Despite their rice-grain size, they row tirelessly day and night.

“They literally row day and night throughout their lifespan, only pausing to molt, mate, or feed,” said Dr. Ortega-Jimenez.

For robotics, this endurance highlights the potential of fan-based designs in environments where turbulence challenges conventional machines.

“When designing small-scale robots, it’s important to account for the specific environment in which they will operate… The Rhagobot, for instance, can travel quickly along a flowing stream thanks to its intelligent fan structure, which is powered by surface tension and drag forces,” said Professor Koh.

Broader Implications

The study not only connects Rhagovelia’s fan microstructure to locomotion but also lays the groundwork for bioinspired microrobots capable of:

• Environmental monitoring
• Search-and-rescue operations
• Navigation of turbulent water-air interfaces
  

Summary

• Rhagovelia water striders use ribbon-shaped fans that passively morph using surface tension and elasticity.
• Fans enable extreme agility: sharp turns in 50 ms and speeds of 120 body lengths/second.
• Researchers replicated this mechanism in “Rhagobot,” an insect-scale robot with self-deploying fans.
• Collaboration between UC Berkeley, Georgia Tech, and Ajou University drove the discovery.
• Findings could advance aquatic robotics for environmental and rescue missions.

Original Publication
Authors: Victor M. Ortega-Jimenez, Dongjin Kim, Sunny Kumar, Changhwan Kim, Je-Sung Koh and Saad Bhamla.
Journal: Science
DOI: 10.1126/science.adv2792
Method of Research: Experimental study
Subject of Research: Animals
Article Title: Ultrafast elastocapillary fans control agile maneuvering in ripple bugs and robots
Article Publication Date: 21-Aug-2025