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Self-Cleaning Surfaces Tested that Emulate Natural Models

Morpho butteryfly (ArmandoMaynez/Flickr)

Morpho butteryfly

Engineers at Ohio State University in Columbus devised and tested material surfaces that clean themselves and reduce drag, based on models in nature such as shark skin and butterfly wings. Mechanical engineering professor Bharat Bhushan and doctoral candidate Gregory Bixler recently published their findings online in the journal Soft Matter (paid subscription required).

Materials that can clean themselves or improve the flow of liquids off their surfaces can make a sizeable difference in industrial applications. “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,” says Bhushan. “And self-cleaning surfaces would be useful for medical equipment, catheters, or anything that might harbor bacteria.”

Bhushan and Bixler examined the surfaces of animal and plant organisms for possible analogs for surfaces with the desired self-cleaning or drag-reducing properties. The researchers used an electron microscope and optical profiler — a device that measures surface patterning, such as roughness — to study the wings of the Giant Blue Morpho butterfly and the leaves of the rice species Oryza sativa.

The Blue Morpho butterly, native to Central and South America, is known for its brilliant blue color and and iridescence. The Ohio State engineers, however, were more interested in the butterfly’s ability to shake off dirt and water with a flutter of its wings, a property that makes the brilliant, shimmering blue color more visible. Bhushan and Bixler found the wings’ surface to feature rows of tiny, overlapping shingles emanating out from the butterfly’s body that suggest the shingles encourage water and dirt to roll off, like water rolls off a roof.

Likewise the researchers found the rice leaves characterized by rows of microscale grooves and even smaller nanoscale (millionths and billionths of a meter, respectively) bumps arrayed in patterns that direct rainwater to the stem and base of the plant. The leaf also has a waxy coating that encourages the flow of water off the surface.

Bhushan and Bixler cast plastic replicas of these wing and leaf surfaces and covered the plastic surfaces with nanoparticle coatings to simulate the waxy coating on the rice leaves. They then tested the wing and leaf surface properties against known natural analogs, such as shark skin and fish scales, as well as non-textured flat surfaces for comparison.

In one test, the researchers lined plastic pipes with the different coated textures and measured the drop in water pressure in the pipe as an indicator of fluid flow. For a pipe with the diameter of a cocktail straw, the shark skin texture lining with nanoparticles reduced water pressure drop by 29 percent compared to the non-coated surface. The coated rice leaf reduced water pressure by 26 percent, followed by the butterfly wing with about a 15 percent reduction in pressure.

Another test examined self-cleaning properties with silicon carbide, an industrial powder similar to common dry dirt. The surfaces were held at a 45-degree angle and the researchers dripped on the surfaces about two tablespoons of water from a syringe for two minutes. A software program counted the number of silicon carbide particles on each texture before and after washing. The shark skin surface again performed the best, washing off almost all (98%) of the silicon carbide particles, closely followed by the rice leaf surface (95%) and butterfly wing (85%).

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Photo: Armando Maynez/Flickr

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