A South American butterfly flapped its wings, and caused a flurry of nanotechnology research to happen in Ohio. Researchers in Ohio have taken a new look at butterfly wings and rice leaves, and learned things about their microscopic¹ texture² that could improve a variety of products.

 

For example, the researchers were able to clean up to 85 percent of dust off a coated plastic surface that mimicked³ the texture of a butterfly wing, compared to only 70 percent off a flat surface.

 

In a recent issue of the journal Soft Matter, the Ohio State University engineers report that the textures⁴ enhance fluid flow and prevent surfaces from getting dirty -- characteristics that could be mimicked in high-tech⁵ surfaces for aircraft and watercraft, pipelines⁷, and medical equipment.

 

"Nature has evolved many surfaces that are self-cleaning or reduce drag," said Bharat Bhushan, Ohio Eminent⁸ Scholar and Howard D. Winbigler Professor of mechanical engineering at Ohio State. "Reduced drag is desirable for industry, whether you're trying to move a few drops of blood through a nano-channel or millions of gallons of crude oil through a pipeline⁶. And self-cleaning surfaces would be useful for medical equipment -- catheters, or anything that might harbor bacteria."

 

Bhushan and doctoral student Gregory Bixler used an electron microscope and an optical profiler to study wings of the Giant Blue Morpho butterfly (Morpho didius) and leaves of the rice plant Oriza sativa. They cast plastic replicas⁹（复制品） of both microscopic textures, and compared their ability to repel¹⁰ dirt and water to replicas of fish scales, shark skin, and plain flat surfaces.

 

Common to Central and South America, the Blue Morpho is an iconic butterfly, prized for its brilliant blue color and iridescence¹¹. Beyond its beauty, it has the ability to cast off dirt and water with a flutter of its wings.

 

For a butterfly out in nature, staying clean is a critical issue, Bhushan explained.

 

"Their wings are so delicate that getting dirt or moisture on them makes it hard to fly," he said. "Plus, males and females recognize each other by the color and patterns on their wings, and every species is unique. So they have to keep their wings bright and visible in order to reproduce."

 

The electron microscope revealed that the Blue Morpho's wings aren't as smooth as they look to the naked eye. Instead, the surface texture resembles a clapboard roof with rows of overlapping¹² shingles¹³ radiating out from the butterfly's body, suggesting that water and dirt roll off the wings "like water off a roof," Bhushan said.

 

The rice leaves provided a more surreal landscape under the microscope, with rows of micrometer- (millionths of a meter) sized grooves¹⁴, each covered with even smaller, nanometer- (billionths of a meter) sized bumps -- all angled to direct raindrops to the stem and down to the base of the plant. The leaf also had a slippery（不稳定的） waxy¹⁵ coating, which keeps the water droplets¹⁶ flowing along.

 

The researchers wanted to test how butterfly wings and rice leaves might display some of the characteristics of other surfaces they've studied, such as shark skin, which is covered with slippery, microscopic grooves that cause water to flow smoothly¹⁷ around the shark. They also tested fish scales, and included non-textured flat surfaces for comparison.

 

After studying all the textures close up, the researchers made molds of them in silicone and cast plastic replicas. To emulate¹⁸ the waxy coating on the rice leaves and the slippery coating on shark skin (which in nature is actually mucous), they covered all the surfaces with a special coating consisting of nanoparticles.

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