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Weed Fills a Need for Anti-Drag Coating

Monday, November 14, 2011

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An invasive weed that clogs the world’s waterways has provided surprising inspiration for a promising new waterproof coating for boats and submarines.

 Salvinia molesta

 Photos: Ohio State University

An optical micrograph shows sticky, yet slippery, eggbeater-shaped hairs on the leaf surface of the Brazilian fern Salvinia molesta. A coating that replicates the surface could reduce drag and boost buoyancy on boats and submarines.

By recreating the slippery, yet sticky, textures that keep the Brazilian fern Salvinia molesta afloat, Ohio State University engineers have developed a plastic coating that they say can reduce drag and boost buoyancy and stability on boats and submarines.

The coating surface replicates the fern’s oddly shaped hairs, which trap air, reduce friction, and help the plant stay afloat.

Microscopic Shag Carpet

The surface is soft and plush, like a microscopic shag carpet, yet contains a sticky region at the end of each tiny eggbeater-shaped fiber that clings lightly to the water, providing stability, the team reports in "Nanoscale biomimetics studies of Salvinia molesta for micropattern fabrication," published in the Nov. 1 issue of  the Journal of Colloid and Interface Science.

 Bharat Bhushan
“We've gotten deep insight into a very simple concept,” said researcher Bharat Bhushan. “That's where the fun is.”

It's the combination of slippery and sticky surfaces that makes the texture special, said Bharat Bhushan, Ohio State professor of mechanical engineering, who led the research.

“The Salvinia leaf is an amazing hybrid structure,” said Bhushan. “The sides of the hairs are hydrophobic —in nature, they're covered with wax—which prevents water from touching the leaves and traps air beneath the eggbeater shape at the top. The trapped air gives the plant buoyancy."

On the other hand, he added, "the tops of the hairs are hydrophilic. They stick to the water just a tiny bit, which keeps the plant stable on the water surface."

Sticky Business

In tests, the coating performed just as the Salvinia hairs do in nature, the researchers said. The bases of the hairs were slippery, while the tips were sticky. Water droplets did not penetrate between the hairs, but clung instead to the tops of the eggbeater structures—even when the coating sample was turned on its side to produce a vertical surface.

Bhushan and master's student Jams Hunt compared the stickiness of their coating to that of the Salvinia leaf using an atomic force microscope. The two surfaces performed nearly identically, with the plastic coating generating an adhesive force of 201 nanoNewtons (billionths of a Newton) and the leaf generating 207 nanoNewtons.

That's a tiny force compared to familiar adhesives such as transparent tape or masking tape. But the adhesion is similar to that of another natural surface studied by Bhushan and other researchers: gecko feet.

‘“With this study, we've gotten deep insight into a very simple concept. That's where the fun is,” Bhushan said.

Gecko Feet and Shark Skins

“I've studied the gecko feet, which are sticky, and the lotus leaf, which is slippery,” Bhushan said. “Salvinia combines aspects of both.”

Salvinia is a popular plant for home aquariums and decorative ponds. It needs no dirt and lives solely in the water—even moving water, such as rivers and lakes.

At some point, the hearty plant escaped from people's homes into the wild, Ohio State reports. Now it has proliferated into commercial waterways in North America, South America and Australia, where it has become an invasive species.

Although confident in his discovery, Bhushan has no plans to commercialize the product himself.

“Besides,” he says, “I've already moved on to studying shark skin.”


Tagged categories: Marine Coatings; Research; Waterproofing membranes

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