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Nanocoating Gives Liquids the Bounce

Friday, January 18, 2013

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A new nanoscale coating that bounces liquids—from coffee to acids to custard—from its surface could eventually protect everything from military uniforms to ship hulls, according to its developers.

“Essentially, nothing sticks on these coatings. Everything just rolls or slides right off,” University of Michigan Assistant Professor Anish Tuteja explains in a video presentation of the work he is leading.

'Superomniphobic Surface'

At least 95 percent air, the coating “repels the broadest range of liquids of any material in its class, causing them to bounce off the treated surface,” according to a release from the university, where the coating has been developed with funding from the U.S. Air Force Office of Scientific Research.

Superomniphobic nanocoating
Photos: Joseph Xu / UM Engineering Communications & Marketing

“Essentially, nothing sticks on these coatings,” University of Michigan Professor Anish Tuteja explains in a presentation. "I wouldn't have believed that you could actually make coatings like this."

The coating creates a “superomniphobic surface” that causes droplets of potentially damaging liquids to “recoil,” the engineering researchers say.

“I’ve been working in this area for about five years now, and the first two years, I wouldn’t have believed that you could actually make coatings like this, where you can repel all these range of liquids, especially very, very low surface tension liquids like we’ve been able to do so,” said Tuteja.

Reducing Drag, Repelling Chemicals

The team says the coating could lead to breathable garments to protect soldiers and scientists from chemicals, and advanced waterproof paints that could dramatically reduce drag on ships.

“Virtually any liquid you throw on it bounces right off without wetting it,” said Tuteja, assistant professor of materials science and engineering, chemical engineering and macromolecular science and engineering.

“For many of the other similar coatings, very low-surface-tension liquids such as oils, alcohols, organic acids, organic bases and solvents stick to them, and they could start to diffuse through—and that's not what you want."

Tuteja is the corresponding author of "Superomniphobic Surfaces for Effective Chemical Shielding,” a paper on the coating published in the current issue of the Journal of the American Chemical Society.

Superomniphobic Nanocoating

A high-speed camera captures a droplet resting on the surface of a new super-repellant surface developed at the University of Michigan. The coating causes droplets to bounce off the surface.

Doctoral student Shuaijun Pan and postdoctoral researcher Arun Kota, both in materials science and engineering, are the first authors of the paper. Also contributing is Joseph Mabry, in the rocket propulsion division of the Air Force Research Laboratory.

Acid and Soy Sauce

Tuteja and his colleagues tested more than 100 liquids and found only two that were able to penetrate the coating, the university said. Both were chlorofluorocarbons (CFCs), the chemicals used in refrigerators and air conditioners.

On the other hand, in a demonstration in Tuteja's lab, coated surfaces repelled coffee, soy sauce and vegetable oil, as well as toxic hydrochloric and sulfuric acids that could burn skin. Tuteja says the surface also is resistant to gasoline and various alcohols.

How it Works

The coating is a mixture of rubbery plastic particles of polydimethylsiloxane (PDMS) and liquid-resisting nanoscale cubes that contain carbon, fluorine, silicon and oxygen. The cubes were developed by the Air Force.

The material's texture is as important as its chemistry. The coating “hugs the pore structure of whatever surface it's being applied to” and “creates a finer web within those pores.” The upshot is a coating that is between 95 and 99 percent air pockets, so any liquid that comes in contact with it is barely touching a solid surface.

Superomniphobic Nanocoating

An uncoated tile of screen (left) is wetted by liquids, but a superomniphobic-treated piece remains dry.

To apply the coating, researchers use a technique called electrospinning that uses an electric charge to create fine particles of solid from a liquid solution. So far, the team has coated small tiles of screen and postage-stamp-sized pieces of fabric.

Keeping Their Distance

Because the liquid touches mere filaments of the solid surface, researchers say, the coating can dramatically reduce the intermolecular forces that normally draw the two states of matter together. These so-called Van der Waals interaction forces are kept at a minimum.

"Normally, when the two materials get close, they imbue a small positive or negative charge on each other, and as soon as the liquid comes in contact with the solid surface, it will start to spread," Tuteja said. "We've drastically reduced the interaction between the surface and the droplet."

Prevented from spreading, the droplets stay intact and interact only with molecules of themselves. They thus maintain a spherical shape and literally bounce off the coating, the team says.

Paint and Custard

The coating even has been found to repel paints, blood, custards, shampoos, clays, printer inks and other so-called non-Newtonian liquids, which change their viscosity depending on the forces applied to them.

Water and other Newtonian liquids maintain the same viscosity, regardless of the force applied.

"No one's ever demonstrated the bouncing of low-surface-tension non-Newtonian liquids," Tuteja said.

   

Tagged categories: Chemical resistance; Coating chemistry; Marine Coatings; Nano and hybrid coatings; Water repellents

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