We’ve made machines that can float on water and machines that can walk on water, but until now, robots or automata that can leap suddenly into the air from the surface of a pond have eluded us. But a group of engineers, led by a researcher at the Seoul National University, have just created a machine that can jump on water. Rather impressively so, in fact.

To patch this gaping hole in machine locomotion, the engineers studied the mechanics behind the water strider, an insect that can easily jump upwards from a pond. It’s an ability that’s poorly understood in insects in general, so trying to recreate it in machines has the beneficial side effect of improving our understanding of the insects themselves.

The trick, according to an analysis of a high-speed film of water striders, is to push down on the water with the maximum velocity that the surface tension can take. The further the insect’s leg pushes down, the greater the surface tension that builds under the leg and the better the upward jump. But if the leg pushes too far, the meniscus—the curved water surface—can’t take it and gives way, allowing the leg to sink.

To leap joyfully from a pond, then, it’s necessary to find the optimal balance: push down hard enough to make maximum use of the surface tension, but not so hard that you rupture it. The water strider seems to do this by rotating its middle and hind legs, rather than just pushing them downwards, which keeps its legs in contact with the water for longer, giving it more time to interact with the surface and build momentum.

A water-jumping machine would need legs that could mimic this rotation with tips that point upwards—like the water strider’s legs—to avoid piercing the meniscus. It would also need to be super-light with hydrophobic legs.

The machine—the researchers actually call it a robot—turned out to be capable of a healthy leap, reaching 92 percent of its theoretical maximum acceleration. The engineers made it leggier than its insect inspiration, with legs five centimetres longer than those of the water strider to maximise torque. To get the little machine insect jumping, a spring-like device in the "body" was heated using a small heating wire, causing a change in stiffness and activating the leap. (This doesn't quite classify as a "robot," in our opinion.)

Curiously, the insect-like machines jumped even better on water than they did on the ground—a counterintuitive result, the authors write, that can possibly be explained by lower levels of leg vibration on water than on land, translating to greater vertical kinetic energy.

It’s not clear whether water-jumping machines or robots could be useful for any practical purpose. But even if they’re not, the development was a fruitful exercise in confirming what had previously just been a theoretical understanding of surface tension and locomotion.

Science, 2015. DOI: 10.1126/science.aab1637 (About DOIs).