Jumping on water: Water strider shows the way
Nature, through evolution, is the ultimate problem solver. Need to kill microbes? Check out the elaborate and adaptive immune system. Need to fly? Countless thousands of critters have solved that, even including some mammals (the bats).
Need to walk on water? A few animals, including the water strider and some reptiles, can do that.
Need to jump on water? That's select territory. One of the few animals with that talent is the water strider.
Now, researchers in South Korea have figured out the strider's secret -- and used their understanding to build a life-size model that, like the strider itself, lifts off and then splashes down with a catawampus belly-flop.
The strider jumps to avoid danger. The Korean creation, designed by Seoul National University's Ho Young Kim and Kyu Jin Cho, jumps to impress humans.
And it does. Witness the article in this week's edition of the journal Science1
We have liftoff!
To jump clear of the water, the strider (and the robot) must exert a force on the water that overcomes gravity. To do this, the jumper exploits surface tension -- the elastic quality of a liquid surface resulting from the strong cohesion between water molecules.
When a strider supports its weight, it creates dimples in the water. In response, surface tension pushes upward as the water "tries" to reduce its surface area and flatten out.
Push too hard, of course, and the foot goes right through the water, and the jump fails, so the water strider has evolved to push hard enough and long enough to leave the water surface, without punching through it.
And that, in a nutshell, is what the Korean researchers have recreated with their 68 milligram wonder.
What does it take?
The physical elements the robot would need were evident in high-speed video of a water strider:
A propulsion system that gradually increases force on the legs -- to avoid punching through the surface.
A sweeping motion of the legs that extends the duration of the downward push to maximize total lift.
A curved "foot" that exploits surface tension through the entire stroke.
A water-repellant surface on the legs, so all water drops away during the launch.
A manufacturing technology based on paper-folding techniques that creates light-weight structures with ultra-low friction hinges.
As proof of their principle, the robot jumped just as high on water as on a hard surface.
According to a prepared statement by the authors, the organic water strider approaches perfection. "Based on a theoretical model of a flexible cylinder floating on liquid, the authors found that the maximum force of the water striders’ legs is always just below the maximum force that water surface tension can withstand. In fact, surface tension on the four legs was 16 times the robot's weight, giving plenty of scope for enough added force to launch the robot into the air."
By regulating the motive spring in their robot, they achieved much the same result -- although the robot strider, unlike the organic version, is not yet able to walk on water.
"This robotic technology could probably be used for building a large number of robots that can float, and jump on water for surveillance missions," the authors added. "However, this study did not begin with an intent to create a real world application. We were fascinated by the fact that insects can actually jump on water quite well, something that humans or any engineered system cannot replicate."
– David J. Tenenbaum
Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer
- Jumping on water: Surface tension–dominated jumping of water striders and robotic insects," by J.-S. Koh et al, Science, 31 July 2015. ↩