Water striders are neat little creatures. With seemingly no effort or cognition, they dance across the surface of still water with grace and speed. Though scientists have long been aware that the basic mechanism behind their divinely-inspired locomotion is surface tension, their range of movements required deeper analysis. Now, researchers at Harvard’s Wyss Institute for Biologically Inspired Engineering have managed to reproduce the water strider’s movements in robot form.
What perplexed researchers the most was the water strider’s ability to jump from the surface of the water. Walking is one thing, but being able to apply enough force to launch their bodies without breaking the water’s surface tension is another thing entirely.
“Water’s surface needs to be pressed at the right speed for an adequate amount of time, up to a certain depth, in order to achieve jumping,” said the study’s co–senior author Kyu Jin Cho, Associate Professor in the Department of Mechanical and Aerospace Engineering and Director of the Biorobotics Laboratory at Seoul National University. “The water strider is capable of doing all these things flawlessly.”
In their research, the biologists and engineers found that the water strider’s secret was to keep their curved legs in contact with the water’s surface as long as possible. Applying maximum force as quickly as possible, as one would do on the ground, would just break the surface tension. In order for the robot water strider to do this, engineers equipped it with a “torque reversal catapult mechanism,” which mimics the way fleas jump.
“The resulting robotic insects can achieve the same momentum and height that could be generated during a rapid jump on firm ground – but instead can do so on water – by spreading out the jumping thrust over a longer amount of time and in sustaining prolonged contact with the water’s surface,” said Robert Wood, Ph.D., who is a co–author on the study and a Wyss Institute Core Faculty member.