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Nature Inspires Slippery Glass Coating

Thursday, August 8, 2013

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An ultra-slick new bio-inspired coating could make windows, solar panels and other glass surfaces more durable and self-cleaning without diminishing clarity, Harvard University researchers report.

The team, from the Harvard School of Engineering and Applied Sciences (SEAS) and Harvard’s Wyss Institute for Biologically Inspired Engineering, reported its advance July 31 in the journal Nature Communications.

Solar photovoltaic array
USDA

The slick new transparent coating could be used on solar panels and glass, researchers report.

The coating builds on an earlier innovation and could be used on windows, solar panels, medical diagnostic devices and eyeglasses, according to principal investigator Joanna Aizenberg, Ph.D., a core faculty member at the Wyss Institute, Amy Smith Berylson Professor of Material Science at SEAS, and a professor of Chemistry and Chemical Biology.

The researchers say they produced the transparent coating by building a glass honeycomb-like structure with craters, coating it with a Teflon-like chemical that binds to the honeycomb cells to form a stable liquid film. The film repels droplets of both water and oily liquids.

Because it's a liquid, it flows, which helps the coating repair itself when damaged, according to the researchers.

Building on Previous Technology

The coating builds on an award-winning technology that Aizenberg and her team pioneered in September 2011 called Slippery Liquid-Infused Porous Surfaces (SLIPS), the slipperiest synthetic surface known, the team reported in a press release.

The new glass technology is equally slippery, but more durable and fully transparent, the researchers say.

pitcher plant
Kate Ter Haar / Flickr

Slippery Liquid-Infused Porous Surfaces (SLIPS) coatings were inspired by the carnivorous pitcher plant. The plant lures ants and other insects onto the slippery surface of its leaves, where they slide to their doom.

“These advances solve longstanding challenges in creating commercially useful materials that repel almost everything,” according to the researchers.

SLIPS was inspired by the slick strategy of the carnivorous pitcher plant, which lures insects onto the ultraslippery surface of its leaves, where they slide to their doom, the team says. Unlike earlier water-repelling materials, SLIPS repels oil and sticky liquids like honey, and resists ice formation and bacterial biofilms.

First SLIPS

While SLIPS was an important advance, it was also “a proof of principle”—the first step toward a commercially valuable technology, said lead author Nicolas Vogel, a postdoctoral fellow in applied physics at SEAS.

“SLIPS repels both oily and aqueous liquids, but it’s expensive to make and not transparent,” Vogel said.

The original SLIPS materials also need to be fastened somehow to existing surfaces, which is often not easy.

Ice skating rink
Dionisius Purba / Wikimedia Commons

SLIPS’ thin layer of liquid lubricant allows liquids to flow easily over the surface, much as a thin layer of water in an ice rink helps an ice skater glide.

“It would be easier to take the existing surface and treat it in a certain way to make it slippery,” Vogel explained.

Thus, the team sought to develop a coating that accomplishes this and works as SLIPS does.

How it Works

In order to create the new coating, the researchers enclosed a collection of tiny spherical particles of polystyrene, the main ingredient of Styrofoam, on a flat glass surface, like a collection of Ping-Pong balls.

The scientists then poured liquid glass on the particles until they were more than half buried in glass. After the glass solidified, the team burned away the beads, which resulted in a network of craters that resemble a honeycomb.

The researchers coated the honeycomb with the same liquid lubricant used in SLIPS to create a tough, but slippery, coating.

Harvard research
Nicolas Vogel / Harvard University

Researchers create the ultraslippery coating by creating a glass honeycomb-like structure with craters (left), coating it with a Teflon-like chemical (purple) that binds to the honeycomb cells to form a stable liquid film. The film repels droplets of water and oily liquids (right). Because it's a liquid, it flows, which helps the coating repair itself when damaged.

"The honeycomb structure is what confers the mechanical stability to the new coating," said Aizenberg.

By adjusting the width of the honeycomb cells to make them much smaller in diameter than the wavelength of visible light, the scientists kept the coating from reflecting light, the team said.

Repelling Liquids, Characteristics

In trials, the coated glass slides repelled a variety of liquids, just as SLIPS does, including water, octane, wine, olive oil and ketchup.

And, like SLIPS, the coating reduced the adhesion of ice to a glass slide by 99 percent.

“Keeping materials frost-free is important, since adhered ice can take down power lines, decrease the energy efficiency of cooling systems, delay airplanes and lead buildings to collapse,” the research team said.

Importantly, the honeycomb structure of the SLIPS coating on the glass slides confers “unmatched mechanical robustness,” the team reports.

Harvard research
Nicolas Vogel

The tiny, tightly packed cells of the honeycomb structure, shown in this electron micrograph, make the SLIPS coating highly durable.

The coating withstood damage and remained slippery after various treatments that can scratch and compromise ordinary glass surfaces and other popular liquid-repellent materials, including touching, peeling off a piece of tape, and wiping with a tissue.

Ongoing Development

“We set ourselves a challenging goal: to design a versatile coating that’s as good as SLIPS but much easier to apply, transparent, and much tougher—and that is what we managed,” Aizenberg said.

The team is now honing its method to better coat curved pieces of glass as well as clear plastics such as Plexiglas, and to adapt the method for the rigors of manufacturing.

Durability + Design’s sister publication PaintSquare News has reported on previous SLIPS developments in several news articles, including “Nano Materials Stop Water from Icing,” “New Harvard Coating Gives Ice the SLIPS,” and “Slime Fighter: Coating Evicts Biofilms.”

   

Tagged categories: Biomimicry; Coating chemistry; Coatings technology; Durability; Glass coatings; Research; Solar

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