Climbing a Watery Slope

By Ivars Peterson

Muse, September 2006, p. 28.

Some insects can walk on water. They take advantage of water's high surface tension to skate across a pond or puddle. But at the edge of the pond, where wet meets dry, surface tension makes the water curve upward in a meniscus. For tiny, water-walking insects, scaling this slope isn't easy. If they try to stride up the slope, they simply slide back down.

So how do these insects get out of the pond? It turns out water-walking insects rely on special tricks to propel themselves upward. Surprisingly, these don't require them to move their legs back and forth in a walking motion at all, say mathematicians David Hu and John Bush of the Massachusetts Institute of Technology. The MIT mathematicians used high-speed video to capture the meniscus-climbing antics of several tiny insects.

As this water treader approaches a meniscus, its front and rear legs deform the water's surface to help it move up the slope.

Courtesy of Hu and Bush

Some water treaders, for example, have retractable claws on their front and hind legs that allow them to pull up on the water to create tiny peaks. Each peak is itself a meniscus. And, because one meniscus is attracted to another, the slope tugs on the tiny peaks. Because the insect's front legs are closer to the slope that its rear legs, its front legs are tugged more strongly, and it is propelled forward and upward. In fact, the attraction is so strong, the insect may glide forward faster than 30 body lengths per second!

The larva of the waterlily leaf beetle uses an alternative strategy to scale a slippery meniscus. A poor swimmer, this creature simply arches its back, creating a meniscus at each end. The insect then gets pulled up the slope to a nice, juicy leaf.

Courtesy of Hu and Bush

The same force is responsible for clumping breakfast cereal, such as Cheerios, in a bowl of milk. The meniscus created by a small floating object like a Cheerio attracts the meniscus produced by another nearby.

Don't lie: you've always wanted to know why the cereal ring formed in the bowl, haven't you?

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