Why swim when you can jump? An international team of scientists led by Seoul National University has designed a tiny mechanical water-strider – a robot that can leap off the water's surface the way the pond-skimmers do in real life.
The robot, described in the journal Science, sheds light on how these insects manage this apparently gravity-defying feat and could help engineers design more versatile robots in the future.
Believe it or not, a whole menagerie of animals, from the basilisk lizard to the fishing spider, can manage the messiah-like trick of walking on water, using various techniques to do so. But jumping on water is another story entirely – it's akin to the difference between skipping a stone across a lake and trying to bounce a rock off the water's surface. The first is doable, with a certain amount of skill; the second feels impossible, and that's because a rock will just puncture the surface tension of the water and sink like – well, like a stone.
Some critters, such as the pygmy mole cricket, can manage to jump on water, by moving very fast (and thus taking advantage of the water's viscosity), but even they are clearly somewhat handicapped – they cannot jump as high on liquid as they can on land.
"Jumping is vertical propulsion, and it requires different criteria from walking on water, which is lateral propulsion," the study authors point out. "In contrast to jumping on solid ground, a large driving force and fast stroke in the jumping leg do not guarantee a high takeoff velocity on the water surface, especially for small insects."
The water strider, on the other hand, is a good performer on either medium. These little arthropods can actually jump as high on water as they can on land – and perform their most extreme acrobatics when exposed to danger.
The researchers wanted to see how high these little bugs could jump when properly motivated. So to study what they called "this amazing semi-aquatic motility," they went to a local pond, nabbed some water striders (known formally as Aquarius paludum), prodded them with a stick and then recorded their leaps in the lab using high-speed cameras.
The scientists found that, rather than using quick, jerky moments, the water striders actually pushed down on the water with as much force as they could without breaking the water's surface tension, and they pushed for an extended period of time, until their legs actually disengaged from the surface. The insect also doesn't simply push its legs downward – it rotates them in the process, to stay in maximum contact with the water as it shoves off.
The scientists designed this robot with these principles in mind, complete with the water strider's upturned feet, and also used a mechanism inspired by the high-jumping flea. They "built" the robot using a self-assembling origami-like folding technique described earlier by Harvard University researcher Robert Wood, a coauthor on this paper.
The result: A robot that can jump 14.2 centimeters on water – not bad for a bot with 5-centimeter legs.
"The result is strikingly similar to the jumping of the real water strider," Dominic Vella, a researcher at Oxford University who was not involved in the paper, wrote in a commentary. "What is more, the robot jumps on water equally well as, and sometimes better than, on land."
Taking a page out of these multi-talented animals' books may help inspire future designs of rescue robots. Researchers have been working on a range of flying, swimming, running and slithering robots inspired by nature. Disaster zones often require rescue robots with a certain amount of versatility – for example, perhaps a robot that can work just as well on water as on land.
"Being able to cope with different terrains remains a key challenge for robotics. Perhaps no two terrains are more different than land and water, and yet several animals can move equally well on either," Vella wrote.
For study coauthor Robert Wood, however, the robot's greatest immediate value lies in using such robots to better understand the physics of living organisms.
"The robot involved is very cool," Wood wrote in an email, "but it is a tool for exploring biological hypotheses in a way that would be very challenging with the animals alone."