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The Sciences

How to Walk on Water

By Fenella SaundersNovember 10, 2003 6:00 AM


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Courtesy of David Hu/MIT

Water-striding insects skim across the surface of a pond with no effort at all, but a team of MIT mathematicians had to work hard to figure out how they do it. John Bush and his graduate student David Hu set up vats of dyed water and high-speed video cameras to catch the striders in action. Using this setup, Bush and Hu could see that they row with their center legs, even though surface tension keeps the insects from dipping into the water. “It’s sort of like the breaststroke, but they don’t break the surface. When they push down on the water, they create a small valley, called a meniscus, and that acts like an oar,” Hu says. The middle legs press against the back wall of the meniscus, pushing the insect forward and transferring momentum to a series of vortices in the water (see below). To prove this result, MIT mechanical-engineering graduate student Brian Chan built a four-inch-long robot that uses the same technique. It water-walked successfully, if not quite as elegantly as its living counterpart.

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