Brazilian fern with ‘eggbeater’ texture inspires waterproof coating for boats

A study led by an Indian origin researcher has described how a floating weed that clogs waterways around the world has inspired a high-tech waterproof coating intended for boats and submarines.

In the study, Bharat Bhushan and his team of engineers from the Ohio State University have illustrated how they recreated the texture of the Brazilian fern Salvinia molesta, which resembles a carpet of tiny eggbeater-shaped fibres.

The fern has a dotted surface with oddly shaped hairs that trap air, reduce friction and help the plant stay afloat, while the sticky region at the tips of the eggbeaters clings to the water and provides stability.

“It’s the combination of slippery and sticky surfaces that makes the texture so special,” Bhushan, Ohio Eminent Scholar and the Howard D. Winbigler Professor of mechanical engineering at Ohio State, said

“The Salvinia leaf is an amazing hybrid structure. 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.

“But 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,” he said.

In tests, the coating performed just as the Salvinia hairs do in nature and the bases of the hairs were slippery, while the tips of the hairs were sticky.

Water droplets did not penetrate between the hairs, but instead clung to the tops of the eggbeater structures – even when the coating sample was turned on its side to a 90-degree vertical.

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Bhushan and his student Jams Hunt compared the stickiness of their plastic coating to the stickiness of the natural Salvinia leaf using an atomic force microscope.

They found that 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.

The study has been recently published in the Journal of Colloid and Interface Science.

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