Inspired by Water Striders, Researchers Invent a Micro Robot Capable of Skimming Across Water Surfaces

According to foreign media reports, a collaborative research team from the University of California, Berkeley, Georgia Institute of Technology, and Ajou University in South Korea has discovered that the unique fan-like propellers of water striders (Rhagovelia), which enable them to glide through rapid streams, can passively open and close like a brush, at a speed ten times faster than a blink of an eye. Inspired by this biological innovation, the team developed an insect-scale robot equipped with self-deforming fans that can mimic the agile movements of water striders. This study highlights how the adaptive morphology and function shaped by natural selection can enhance the mobility and endurance of both water striders and bio-engineered robots without adding extra energy costs.

Image source: University of California, Berkeley

Rhagovelia water striders stand out among water striders because these millimeter-sized semi-aquatic insects use special fan-shaped structures on their propelling legs, enabling them to make quick turns and bursts of speed.

"During the pandemic, when I was doing postdoctoral research at Kennesaw State University, I was deeply fascinated the first time I saw water striders," said Victor Ortega-Jimenez, an integrative biologist at the University of California, Berkeley, and the lead author of the study. "These tiny insects skim and spin rapidly across the surface of turbulent water, making them look like flying insects. How do they do it? This question lingered in my mind, and it took me more than five years of close collaboration with them to find the answer."

Prior to this, it was believed that these fan-shaped structures were entirely driven by muscle movement. However, a study published in the journal *Science* reports that the flat, ribbon-like fan structures of *Rhagovelia* can passively deform using surface tension and elasticity, without relying on muscular energy. Dr. Ortega-Jimenez stated, "For the first time, we observed that isolated fan structures almost instantly passively expand upon contact with a water droplet, which was completely unexpected."

The remarkable combination of foldability during leg retraction and rigidity during propulsion allows these insects to complete sharp turns in as little as 50 milliseconds and move at speeds of up to 120 body lengths per second, rivaling the rapid aerial maneuverability of flies.

Dr. Ortega-Jimenez left Kentucky State University (KSU) in 2020 to join Georgia Tech, where he submitted this project and the initial observations of Rhagovelia bugs to Dr. Saad Bhamla.

"I saw a discovery hidden in plain sight. We often think of science as a pursuit carried out by geniuses working alone, but this is far from the truth. At the heart of modern science are interdisciplinary teams of curious scientists who cross borders and disciplines, jointly exploring nature and designing new biomimetic machines," said Dr. Bhamla. This interdisciplinary research has lasted for more than five years, integrating experimental biology, fluid physics, and engineering design.

Inspired by water striders, creating an insect-sized robot is a significant challenge, particularly because the microscopic structure design of the fan remains a mystery. Dr. Dongjin Kim and Professor Je-Sung from Asia University captured high-resolution images of the fan using a scanning electron microscope, ultimately achieving a breakthrough and uncovering the answer to this mystery.

"We initially designed various types of cylindrical fans, which we typically thought of as being shaped like hair," said Dr. Dongjin Kim, a postdoctoral researcher at Asia University and the lead author of the study. "However, the dual function of the fan—requiring rigidity to generate thrust and flexibility to fold—could not be achieved through a cylindrical structure. After numerous attempts, we eventually overcame this challenge by designing flat, ribbon-shaped fans."

Dongjin Kim added, "We strongly suspected that biological fans might have similar forms, and eventually discovered that the Rhagovelia fan indeed possesses a previously unreported flat banded microstructure. This discovery further validated our design principles for artificial flat banded fans. Based on these insights, they were able to decipher the structural basis and function of this natural propulsion system and reproduce it in robotic form. Ultimately, they designed an elastic capillary fan weighing one milligram, capable of self-deployment, and integrated it into an insect-sized robot. This miniature robot is able to enhance thrust, braking, and maneuverability, and was verified through experiments with live insects and robot prototypes."

“Our robotic fans utilize only surface tension and flexible geometry for self-morphing—just like their biological counterparts. This is a form of mechanically embedded intelligence, refined by nature through millions of years of evolution. In the field of small-scale robotics, such efficient and unique mechanisms will become key technologies to break through the traditional limitations of robot miniaturization,” said the study's senior author, Professor Je-sung Koh.

This study not only establishes a direct link between fan microstructures and motion control in water, but also lays the foundation for the future design of compact semi-aquatic robots capable of exploring water surfaces in challenging fast-flowing environments.

The fan-shaped structure of the ripple worm folds and reopens rapidly when entering and exiting water, demonstrating an unprecedented biomechanical duality—high flexibility allows for quick deployment, while high rigidity provides thrust. This duality addresses long-standing limitations of small aquatic robots, such as low paddling recovery efficiency and limited maneuverability.

It is well known that during propulsion, aphids without fans (such as those of the family Gerridae) generate characteristic dipole vortices and capillary waves as they stroke their superhydrophobic legs across the water surface.

In contrast, each stroke of the fan-shaped Rhagovelia bug generates unique and complex vortex patterns, which are very similar to the wake produced by flapping wings in the air.

"Rhagovelia insects are like Hermes in Greek mythology, having small wings on their legs," said Dr. Ortega-Jimenez. "Future research needs to determine whether Rhagovelia insects can use their fan-like structures to generate lift-like thrust in addition to drag-based propulsion."

This possibility is intriguing because there is evidence that both the rotifer and the cormorant generate hydrodynamic lift to propel swimming through their hairy legs and webbed feet, respectively.

In addition to these vortices, Rhagovelia bugs create symmetrical capillary waves when propelling with their legs, which seem to help generate thrust, while also forming a strong bow wave at the front of the body.

Natural streams pose a real challenge for them, especially for the tiny animals living and active at their junctions. The ripple worm, which is only about the size of a grain of rice, must navigate through highly dynamic, undulating, and turbulent water flows while evading predators, capturing prey, and searching for mates.

The turbulence intensity these insects endure daily far exceeds the turbulence we typically experience on airplanes. Surprisingly, 24-hour monitoring of these insects in the laboratory revealed their extraordinary endurance.

"They paddle almost continuously day and night throughout their entire lifecycle, only stopping to molt, mate, or feed," said Ortega-Jimenez.

The unstable conditions present in streams also pose significant challenges in designing interface microrobots that can move effectively in such unpredictable waters.

When designing small robots, it is important to consider the specific environment in which they will operate—in this case, the water surface. By leveraging the unique properties of this environment, the robot's performance and efficiency can be significantly enhanced. For example, Rhagobot can quickly travel along flowing streams, thanks to its intelligent fan structure, which is driven by surface tension and water resistance," said Jesung Koh.

Finally, these findings could have broad implications for bionic robotics, particularly in the development of environmental monitoring systems, search and rescue microrobots, and devices capable of navigating disturbed water-air interfaces with insect-like flexibility.