Taking a Walk on the Wet Side

Designing a robot is always a matter of balancing the pros and cons of a set of trade-offs. Many of these trade-offs arise from an inherent issue with virtually all untethered robots — energy is a very limited resource. The goal may be to build a quadcopter drone to support a particular application, but if that drone needs to travel long distances its rotors will be far too inefficient to get the job done. In cases like this, multimodal designs are often considered.

In the past we have seen wheel-legged robots that can save energy by rolling along on smooth terrain, and flying wheels that can take to the sky when the going gets too tough. More recently, a group of engineers at the University of California, Berkeley took a different approach that allows robots to walk on water. Inspired by the water strider, their solution could prove to be ideal for drones flying over water that need to conserve energy.

First, the team had to find out what made water striders so good at zipping around on the surface of water. Electron microscope imaging revealed the secret. The key to their success is unique fan-like structures found at the tips of their middle legs. This allows the bugs to row with incredible agility, even in turbulent streams. The fans collapse when lifted from water but instantly spread open upon contact with the surface, thanks to the elastic forces of surface tension. This passive mechanism requires no muscular input, yet transforms the leg into a rigid paddle able to push strongly against the water.

This discovery led the researchers to build a robotic version called Rhagobot. Just a fraction of a gram in weight, this insect-scale machine is equipped with artificial ribbon fans that mimic the passive, water-powered deployment of the biological version. The fans significantly improve thrust, braking, and sharp turning, enabling the robot to skim the surface with agility far beyond that of earlier water-walking robots.

Tests showed Rhagobot can turn 90 degrees in under half a second and propel itself at about two body lengths per second. While not as blisteringly fast as its biological counterpart, which can reach 120 body lengths per second, the robot demonstrates how surface tension can act as a kind of built-in “battery,” powering motion without onboard motors or complex controls.

Potential applications for this technology include environmental monitoring, microrobots for search and rescue, or compact vehicles able to scout fast-moving waters. More broadly, the project highlights how carefully observing nature can uncover entirely new design principles for robotics. The energy-efficient mobility that today’s robots need may be hiding in plain sight.