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Hungry Plant Inspires Slippery Coating

Thursday, September 22, 2011

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The crafty pitcher plant, which traps its prey by creating a slick surface, has inspired Harvard scientists to create a liquid-repellant coating that works much the same way.

The new material repels just about any type of liquid, even under harsh conditions, giving it strong potential as an anti-fouling, anti-icing or anti-graffiti coatings, the researchers say.

Slippery When Wet

“Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity” (or “Slippery When Wetted,”), published online Sept. 21 in the journal Nature, details the new research and the scientific quest for a durable synthetic surface that repels liquids.

 Pitcher plant

 Photo: Walter Federle, Ulrike Bauer and Holger Bohn

To repel water, the pitcher plant locks in a water layer, creating a slick coating on top. The fluid itself becomes the repellent surface.

For more than 10 years, researchers have drawn their inspiration from lotus leaves in developing liquid-repellent, micro-textured surfaces.

Those “surfaces are, however, still plagued with problems that restrict their practical applications,” the researchers note. Among the drawbacks: The materials have been expensive to produce and unable to self-heal.

Lube Job

The new approach, inspired by Nepenthes pitcher plants, “is conceptually different from the lotus effect, because we use nano/microstructured substrates to lock in place the infused lubricating fluid,” the authors say.

The team has created a simple, versatile lubricant that, when infused in fluid, forms a “stable, defect-free and inert ‘slippery’ interface.”

This so-called Slippery Liquid-Infused Porous Surface (SLIPS) outperforms both its natural and synthetic counterparts in its ability to repel a wide variety of liquids and solids, says principal investigator Joanna Aizenberg, Amy Smith Berylson Professor of Materials Science at the Harvard School of Engineering and Applied Sciences (SEAS).

The coating repairs itself almost instantaneously when damaged, resists ice, is easy and inexpensive to manufacture, and stands up to harsh environments, the researchers say.

When Plants Meet Ants

The research is a spin on the behavior of the pitcher plant, whose cupped leaves become virtually frictionless surfaces after a rain. “Sweet-smelling and elegant, the carnivore attracts ants, spiders, and even little frogs,” Harvard reports in an article. “One by one, they slide to their doom.”

(The Harvard coating was also tested with ants, to the ants’ detriment.)

“The effect is similar to when a car hydroplanes—the tires literally gliding on the water, rather than the road,” says lead author Tak-Sing Wong, a postdoctoral fellow in Aizenberg’s lab. “In the case of the unlucky ants, the oil on the bottom of their feet will not stick to the slippery coating on the plant. It’s like oil floating on the surface of a puddle.”

Versatility, Durability

Researchers say the effect persists even under extreme conditions: in high pressures (as much as 675 atmospheres, equivalent to seven kilometers under the sea), humidity and colder temperatures. In outdoor testing conducted after a snowstorm, SLIPS withstood the freezing temperatures and even repelled ice, they said.

 A Harvard illustration shows one step in the process of manufacturing SLIPS

Image: Peter Allen and James C. Weaver 

A Harvard illustration shows one step in the process of manufacturing slippery liquid-infused porous surfaces (SLIPS).

“The versatility of SLIPS, their robustness and unique ability to self-heal, makes it possible to design these surfaces for use almost anywhere, even under extreme temperature and pressure conditions,” says Aizenberg. “It potentially opens up applications in harsh environments, such as polar or deep-sea exploration, where no satisfactory solutions exist at present.”


Tagged categories: Antifoulants; Coatings technology; Protective coatings; Research

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