Saturday, 4 May 2013

Cicada Wings Are Self-Cleaning

(ISNS) -- As 17-year cicadas wriggle out of the ground all over the northeastern U.S. this spring, they'll be reemerging into a world that understands them a little better. Researchers now find the design of their wings can cause filth to jump right off of them with the aid of dew, findings that might help lead to better artificial self-cleaning materials. 

Scientists had known that cicada wings are super-water-repellent, or super-hydrophobic. This is different from a great many substances that are simply water-repellent, or hydrophobic — for instance, oil and water famously do not mix. But a number of surfaces such as lotus leaves can make themselves even more water-repellent by covering themselves with microscopic bumps, so water drops can float on top much as mystics can lie on beds of nails. For example, cicada wings are covered in rows of waxy cones about 200 nanometers or billionths of a meter high. In comparison, the average human hair is roughly 100 microns or millionths of a meter wide. 

When it rains the super-hydrophobic nature of cicada wings can help them get clean — droplets rolling or splashing off them can remove soil, dust, pollen and microbes. But what if there is no rain, especially in the four to six weeks adult cicadas have to live above ground before dying? 

Now scientists find rain is not necessary to keep cicada wings clean. Apparently, grime can simply leap right off them, given dew. 

Mechanical engineer Chuan-Hua Chen at Duke University in Durham, N.C., and his colleagues were investigating a number of natural and artificial super-hydrophobic surfaces when they noticed drops of water at times rapidly disappeared. They were mystified by this behavior for years until they made observations from a different angle — they used a high-speed video camera to watch the droplets from the side of these materials instead of from above. 

"That's when we saw them jumping upward," Chen recalled.

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