Nature is the easiest source of inspiration for scientists. But copying what nature has mastered over billions of years of evolution is another matter altogether. Sometimes, though, we can do it.
Je-Sung Koh of Seoul National University and his colleagues have copied the amazing water strider to build a robot that can jump on water, without sinking. The spring-loaded, ultra-light robot manages to get to about 14cm above water.
To achieve this feat, Koh collected water striders from a pond and captured their jump on a high-speed camera. He found that, apart from having hydrophobic legs that allow it to float on water, during the jump the water strider rotates its legs. This curious motion allows the strider to transfer its force gradually without ever breaking the surface of the water.
The water strider is able to achieve this feat because of a property of liquids called “surface tension,” which is the result of water molecules liking other water molecules more than molecules of air. The cohesion between water molecules makes them want to hold on to each other, at least till there is enough force to break them apart.
Water’s surface tension is among the highest in liquids, because of its unique chemical structure. In water molecules, oxygen atoms are slightly negative and hydrogen atoms are slightly positive. Opposing charges attract—creating “hydrogen bonds—which make water molecules hold onto each other more strongly than molecules of most liquids.
Koh’s robot weighs a mere 68 milligrams (compared to a water strider at 40 milligrams). The robots legs—coated with a superhydrophobic material—are twice the length of a water strider (160mm vs 80mm). The two legs are held together with a tiny body and a spring. When the spring is released, the twisted legs rotate and the robot jumps just like a water strider.
Throughout the process, the total force that the legs put onto the surface of the water is never more than the reactive force that water’s surface tension produces, which enables it to jump instead of sinking. But relying on surface tension alone makes the robot sink in the water after the jump, unlike the water strider which “lands” on the water smoothly and devours a dead bee.
A 2012 robot built by researchers in China can not just jump on water but also float on water after the jump. It uses a different mechanism to make the jump. Its four feet are attached to big sheets of hydrophobic material which help it float, then two paddles splash into the water to give it the jumping force.
Nevertheless, Koh’s achievement is not to be undermined. It’s a simpler design and makes use of much less force to make the jump. More importantly, though, Koh can be proud that he managed to do in just a few years what took nature millions of years.