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How butterflies stay dry

Slightly bumpy surfaces reduce water drops’ contact time

WATERPROOF  As a water droplet hits the wing of a butterfly, a raised vein (vertical line, shown) splits the splash, and sends the water bouncing away more quickly than a smooth surface would.

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So much for rain slickers. Slightly bumpy surfaces, like those of butterfly wings, repel water better than completely smooth surfaces do, researchers report November 20 in Nature.

James Bird of Boston University and colleagues dripped water onto silicon wafers with different textures and filmed the droplets’ splashes. Drops of water striking smooth surfaces flattened into a Frisbee shape before bouncing away. But droplets hitting ridged wafers flared into a splatter that skimmed surfaces only briefly before rebounding.

Ribbed textures force droplets to skip off surfaces quickly, cutting the contact time between water and wafer, the authors found. The results help explain why the veined wings of Morpho butterflies are so good at staying dry.

SURFACE SPLASH  Water droplets take longer to rebound from a smooth surface (top) than one with raised ribs (bottom). This ridged texture reduces the length of time that water touches a surface, helping it stay dry. The first clip shows the droplets moving at normal speed. The second clip shows the video at a quarter of its original speed.

Courtesy of James C. Bird, Rajeev Dhiman, Hyuk-Min Kwon and Kripa K. Varanasi

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