The creature appears to be impossibly floating across the surface of water when, suddenly, it whips its four hair-thin legs toward its belly in one smooth sweep and rockets upward. Leaping with the force of 14 g's, it is airborne faster than you can blink, easily jumping as fast and as high as it could have on solid land, having never so much as pierced the surface of the water. But this is no water-strider, Jesus lizard, or any other brilliant bug. It is a tiny robot.

Engineers and roboticists led by Je-Sung Koh, at Seoul National University in South Korea have just designed the first robot to mimic jumping water insects like water striders. The robot balances on open water and leaps into the air using little more than the force of water's natural surface tension. As the scientists detail today in the journal Science, they first had to work out the mystery of how insects like the water strider manage this magic trick before they could build a bot to mimic it.

Real water strider with water strider robot Seoul National University

"Normally, jumping requires a large force to be applied to the surface that you are jumping on," Kim says. "[But when jumping] on water, legs will easily penetrate the water, and even if you can jump on water, the jumping height will [usually] be lower than jumping on ground. However, our small robot can jump on water without breaking the water surface, and can jump on water as high as jumping on land."

Using high-speed cameras, the roboticists recorded jumping water striders (Aquarius paludum) they caught in nearby ponds and streams outside Seoul. After studying their videos, the team broke down the mechanics of how the bugs avoid breaking the surface tension of water during their powerful leap.

As it turns out, one key to their leap is that water striders steadily and gradually increase their jumping force. That is, they drag out the lead-up to the jump and always keep the jumping pressure just below a level that might break the water's surface. How do they do this? By wicking their curved legs inward, across the water, keeping much of their leg in contact with the tiny divots each leg creates. This allows water striders to transfer the force generated by the water's surface tension (which is trying to make it stay flat) into their upward bound.

This new jumping robot is not the first to ever walk across the surface of water (Robostrider claims that title). The work done on previous bots, combined with the team's study of water striders in nature, helped them design their running and leaping little machine. Basically, just a like real water strider, the robot can stand on water because its light weight is spread widely across its legs, causing a force that's far less than force of water trying to retain a taught surface.

Seoul National University

The new robot jumps by using a simple, light clasp (almost the entire body of the robot) that snaps shut via a heat-reactant spring after being zapped by an outside pulse of heat. That clasp generates the force to move the legs, causing the robots four curved legs, which are generously coated with a water-repellent chemical called Every Dry, to pull inward. The researchers got the robots legs to mimic a water strider's by building them with a super-elastic nickel-titanium alloy. Amazingly, the 1 centimeter tall robot can jump to the height of 14 cm.

Dominic Vella, an applied mathematician at the University of Oxford who was not involved in creating the robot, says "the result is strikingly similar to the jumpingof the real water strider" in a essay published in Science alongside the new research. More than just an interesting toy, Kim's new robot is even more impressive, Vella says, when you consider that "being able to cope with different terrains remains a key challenge for robots." On water and in the air, he says, this jumping robot "is at home."

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