Commentary: New robot mimics insect water-walkers

NEW YORK, Aug. 8 (UPI) -- Scientists are attempting to unlock the secrets of how water-striding insects skim aquatic surfaces and in the process have created a robotic equivalent named Robostrider.

"It's clear the natural water striders is much more elegant, but these insects have been evolving in nature for millions of years, and our robot water striders evolved over a few weeks," researcher John Bush, an applied mathematician specializing in fluid dynamics at the Massachusetts Institute of Technology in Cambridge, told United Press International. "Our Robostrider works, so we're happy with it."

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Brian Chan, a student of Bush's, designed Robostrider, a 3.5-inch, 350-milligram mechanical replica with an aluminum body and stainless steel wire legs less than one-hundredth of an inch thin.

"If you put a motor on it, it would be too heavy, so we used a spring-loaded pulley device hooked up to an elastic thread that runs the length of the body instead to drive the legs," Bush said.

In ponds, lakes and slow-moving streams across the globe, the graceful strokes of water striders -- taxonomic family name Gerridae -- create shimmering ripples as the slender-legged insects forage on top of still waters. Some can even grow to eight inches long.

"It's something you see whenever you travel in, say, the Northeast, in Cape Cod. When I was in Japan last year, I saw them there, and I saw them in a pond in England yesterday," Bush said from Britain.

For some time, Bush wondered how water striders managed to stay atop the water's surface without the aid of webbed footpads or air-filled sacs on its feet. Animals such as the basilisk lizard can, with the aid of its webbed feet, run across the surface of water, giving it the nickname, "Jesus lizard." The problem is the reptiles sink if they stop moving.

Water striders, on the other hand, can stand still on the surface of water. Scientists knew the insects' long legs are covered in tiny, waxy, water-repellant hairs, "and the fact they're light means they can use the water surface like a trampoline," Bush said.

Still, exactly how water striders actually transit from standing on water to gliding effortlessly across it has left researchers perplexed. Many thought the insects moved forward by sweeping their legs to create tiny surface ripples.

The problem with that theory is it conflicts with something known as Denny's paradox, developed by biomechanics expert Mark Denny of Stanford University in California: The minimum speed at which any surface wave must travel is roughly 10 inches per second. Adult water striders easily can generate such waves with their long legs, but the legs of baby water striders are too short to create such velocity and therefore they should be incapable of walking on the surface. But as Denny himself observed, the babies skim along just as easily as their parents. Clearly, water striders do not just buoy themselves by wave motion.

As they reported in the Aug. 7 issue of the British journal Nature, Bush and colleagues think they have found the answer.

A water striders' legs work like oars, but they do not break the surface, Bush said. "Imagine you put an elastic tarp on a pool's surface -- they row, but don't dip beneath."

Bush and his team took adult water striders, each weighing some 10 milligrams, and filmed them with high-speed video cameras at 500 frames per second. They placed individual insects in water sprinkled with pearly beads a hair's-width-wide or less, made of plastic or ground-up fish scales. Each bead had about the same density as water, making it neutrally buoyant, neither sinking nor rising when immersed.

When the team analyzed the video frames, they discovered "the only thing causing the particles to move are the motions generated by the striders," Bush said.

The researchers found the striders' legs made swirling vortexes in the water that propel the insects forward. In this way, they function very much like birds, insects, bats, fish -- and rowboats. When a pigeon flaps its wings, for instance, it creates a doughnut-shaped vortex of air, like a smoke ring, which shoots air backwards through the center of the ring and propels the bird in the opposite direction.

Rowboats produce vortexes in their wakes after each oar stroke as well. "And here you see the wakes are very similar between rowboats and water striders," Bush said. "It's just that water striders don't break the surface, they just press against the surface and create dimples under the water."

By looking at the reflective beads and measuring the size and speed of the flow patterns created by the vortexes, the researchers found the thrust generated was large enough to explain the insect's forward motion.

The head of Office of Naval Research's biorobotics office in Arlington, Va., Promode Bandyopadhyay, said the agency "would certainly welcome" the chance to develop the Robostrider further.

"I would have a swarm of these things, with many of them working in a coordinated fashion over a large area," Bandyopadhyay told UPI. They could be used to snap thermal pictures at night, "or perhaps monitor for vibrations from underground or underwater explosions over a large area without getting detected," he added. "They are stealthy, and it wouldn't be possible to shoot these swarms down."

Bandyopadhyay said the work by Bush's team was "a beautiful piece of research. I consider this somewhat of a preliminary work that opens up a new direction -- no question about it -- but that new direction raises a number of other new and interesting questions." He said he would like to see better analyses of fluid viscosity and motion concerning water strider travel. So Bush said his team is now using computers to track the motion of each particle to produce the kind of flow analyses Bandyopadhyay requested.

Bandyopadhyay said before there could be a practical application of the robot, researchers need to overcome obstacles such as power supply. "Something like solar power, maybe," he suggested, coupled with nanosensors, which are very small, and thermal sensors to capture images. "We are always welcome to new ideas," he added. "We are very curious about it."

Bush said he now plans to investigate the differences between the many varieties of water striders.

"There are a number of other means of propulsion across the water's surface," he said.