November 8, 2012
How Butterfly Wings Can Inspire New High-Tech Surfaces
Alan McStravick for redOrbit.com — Your Universe Online
Can technology benefit by going green? I´m not talking about retro-fitting buildings for solar power or setting up grey water collection systems. In the recent issue of the journal Soft Matter, a research team out of the Ohio State University is looking at organic materials to aid in the resurfacing of existing technologies and possibly for the manufacture of new products and materials.
The researchers contend that these items, and their structure specifically, enhance fluid flow and work to prevent surfaces from collecting dirt and dust. Their study, should the structure be able to be mimicked in high-tech surfaces, could be a substantial benefit to the manufacturers of air and watercraft, pipelines and medical equipment.
“Nature has evolved many surfaces that are self-cleaning or reduce drag,” said Bharat Bhushan, Ohio Eminent 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 pipeline. And self-cleaning surfaces would be useful for medical equipment — catheters, or anything that might harbor bacteria.”
Bhushan, along with doctoral student Gregory Bixler, studied the wings of the Giant Blue Morpho butterfly (Morpho didius) and leaves of the rice plant Oriza sativa. After viewing these two items under an electron microscope and an optical profiler, they cast plastic replicas of both of the microscopic textures. Once completed, they compared their ability to repel dirt and water to replicas of fish scales, shark skin and plain flat surfaces.
Considered to be one of the largest butterflies seen, the Blue Morpho, commonly found in Central and South America, has a wingspan that measures between 5 and 8 inches in width. These majestic wings are a brilliant blue color that is also iridescent. With the brown underside that is dotted with many eyespots, when the Blue Morpho flies, a beautiful effect is created as the colors seem to flash. But the researchers weren´t so concerned with the beauty of its wings. They were more interested in its ability to cast off dirt and water with a single flutter of its wings.
As Bushan explained, for a butterfly out in nature, staying clean is a critical issue.
“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.”
What observation under the electron microscope was able to reveal was that the Blue Morpho´s wings aren´t as smooth as they look, when viewed with the naked eye. The team found that the surface texture more resembled a clapboard roof with several rows of overlapping shingles that start at the body of the butterfly and fan out from there. This, Bushan suggests, means that the water and dirt roll off the wings “like water off a roof.”
What they found when observing the rice leaves under the microscope was far more surreal. They found it was comprised of rows of micrometer-(millionths of a meter) sized grooves, that each had a covering of even smaller, nanometer-(billionths of a meter) sized bumps. The construct of the rice leaves has these grooves and bumps angled to direct raindrops to the stem and down to the base of the plant. Add to this the fact that the leaf also has a slippery waxy coating. This coating helps to keep the water droplets in constant movement.
While the researchers had already studied surfaces like shark skin and fish scales, they wanted to test how the butterfly wings and rice leaves might compare. They were interested to see what, if any, of the characteristics each of these surfaces might share. Shark skin is covered with slippery, microscopic grooves that cause water to flow smoothly around the shark.
Once the researchers had studied each of the textures close up, they made silicone molds and then cast plastic replicas. In order to recreate the waxy coating on the rice leaves and the slippery coating on the shark skin, the team covered them with a special coating that was comprised of nanoparticles.
To test the textures of each, they lined plastic pipes in order to push water through. The intent was to measure the water pressure. This water pressure measurement was an indicator of fluid flow through the pipe. The lower the water pressure was, the greater that particular texture was at assisting with the movement of water through the pipe.
The pipe they used was no bigger than your typical cocktail straw. In the test, they found that the thin lining of shark skin texture coated with nanoparticles reduced the water pressure by 29 percent, as compared to the shark skin texture with no nanoparticle coating. The coated rice leaf saw a drop in water pressure of 26 percent. And our friend, the Blue Morpho, came in third place with a drop of around 15 percent.
The second test involved dusting the textures with a silicon carbide powder. Silicon carbide powder is a common industrial powder that resembles natural dirt. The intent of this test was to see how easy the individual surfaces were to clean. By holding the samples at a 45-degree angle and using the equivalent of two tablespoons of water, delivered over two minutes via syringe, the team wanted to determine which surfaces were practically self-cleaning. To determine this, overall, they used software that was able to count the number of silicon carbide particles on each texture both before and after the water was applied.
Once again, the shark skin fared best in this test, with 98 percent of the particles being washed off. The rice leaf, at 95 percent, came in second. And once again, the butterfly wing brought up the rear, washing off only about 85 percent of the particles applied to it. By comparison, the flat surface that was a sort of control, only washed off 70 percent of the particles.
With the testing completed, Bushan offered his thoughts on the practical uses for these two new surfaces. He believes the rice leaf texture would probably be especially well-suited to helping fluid move at a better rate of efficiency through pipes, such as channels in micro-devices or even oil pipelines.
Our Blue Morpho, however, with its clapboard roof structure that is effective at keeping the butterfly clean and dry, suggests to Bushan a suitability for medical equipment. He believes this structure could help to prevent the growth of bacteria.
Funding for this research was provided by the National Science Foundation.