Microscopic textures found in nature could benefit industry

The researchers from Ohio State University were reportedly able to clean up to 85 per cent of dust from a coated plastic surface that mimicked the texture of a butterfly wing, compared with 70 per cent from a flat surface.

In a recent issue of the journal Soft Matter, the Ohio State University engineers report that the textures enhance fluid flow and prevent surfaces from getting dirty — characteristics that could be mimicked in surfaces for aircraft 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 harbour bacteria.’

According to a statement, 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 replicas of both microscopic textures and compared their abilities to repel dirt and water with replicas of fish scales, shark skin and plain flat surfaces.

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 overlapping shingles radiating out from the butterfly’s body, suggesting that water and dirt roll off the wings.

Under the microscope, rice leaves revealed rows of micrometre-sized grooves, each covered with nanometre-sized bumps — all angled to direct raindrops to the stem and down to the base of the plant. The leaf also had a slippery waxy coating to keep the water droplets 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 smoothly around the shark. They also tested fish scales and included non-textured flat surfaces for comparison.

After studying all the textures, the researchers made moulds of them in silicone and cast plastic replicas. To emulate the waxy coating on the rice leaves and the slippery coating on shark skin, they covered all the surfaces with a special coating consisting of nanoparticles.

In one test, they lined plastic pipes with the different coated textures and pushed water through them. The resulting water pressure drop in the pipe was an indication of fluid flow.

For a pipe about the size of a cocktail straw, a thin lining of shark-skin texture coated with nanoparticles is said to have reduced water pressure drop by 29 per cent compared with the non-coated surface. The coated rice leaf came in second with 26 per cent and the butterfly wing came in third with around 15 per cent.

They then dusted the textures with silicon-carbide powder and tested how easy the surfaces were to clean. They held the samples at a 45º angle and dripped water over them from a syringe for two minutes. Using software, they counted the number of silicon-carbide particles on each texture before and after washing.

The shark skin is said to have come out the cleanest, with 98 per cent of the particles washing off during the test. Next came the rice leaf with 95 per cent and the butterfly wing with about 85 per cent washing off. By comparison, 70 per cent washed off of the flat surface.

Bhushan believes the rice-leaf texture might be especially suited to helping fluid move more efficiently through pipes, such as channels in micro-devices or oil pipelines. The clapboard roof texture of Blue Morpho’s wings might suit medical equipment, where it could prevent the growth of bacteria.