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Coating of air makes liquid bounce off fabric

U. MICHIGAN (US) — A nanoscale coating that’s at least 95 percent air repels liquid and causes it to recoil from treated surfaces.

In addition to super stain-resistant clothes, the coating could lead to breathable garments that protect soldiers and scientists from chemicals, and advanced waterproof paints that dramatically reduce drag on ships.

Droplets of solutions that would normally damage either your shirt or your skin recoil when they touch the new “superomniphobic surface.”

httpv://www.youtube.com/watch?v=ICw5k1tDZSk

An uncoated tile of screen gets wet, but a treated piece remains dry. Researchers have developed a “superomniphobic” surface that can repel virtually any liquid. View larger. (Credit: Joseph Xu/U. Michigan)

“Virtually any liquid you throw on it bounces right off without wetting it. 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,” says Anish Tuteja, assistant professor of materials science and engineering, chemical engineering, and macromolecular science and engineering at the University of Michigan.

Of more than 100 liquids, only two chlorofluorocarbons were able to penetrate the coating. Chlorofluorocarbons are chemicals used in refrigerators and air conditioners. The “superomniphobic surface” repelled coffee, soy sauce, and vegetable oil, as well as toxic hydrochloric and sulfuric acids that could burn skin. The coating is also resistant to gasoline and various alcohols.

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

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

The material’s chemistry is important, but so is its texture. It hugs the pore structure of whatever surface it’s being applied to, and also creates a finer web within those pores. This structure means that between 95 and 99 percent of the coating is actually air pockets, so any liquid that comes in contact with the coating is barely touching a solid surface.

Because the liquid touches mere filaments of the solid surface, as opposed to a greater area, the developed coating can dramatically reduce the intermolecular forces that normally draw the two states of matter together. These 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,” says Tuteja, corresponding author of a paper published in the Journal of the American Chemical Society. “We’ve drastically reduced the interaction between the surface and the droplet.”

With almost no incentive to spread, the droplets stay intact, interacting only with molecules of themselves, maintaining a spherical shape, and literally bouncing off the coating.

One classification of liquid that this coating repels is the so-called non-Newtonian category, which includes shampoos, custards, blood, paints, clays and printer inks, for example. These are liquids that change their viscosity depending on the forces applied to them. They differ from the Newtonians, such as water and most other liquids, whose viscosity stays the same no matter the force applied. Viscosity is a measure of a liquid’s resistance to flow on the application of force, and it’s sometimes thought of as its thickness.

“No one’s ever demonstrated the bouncing of low surface tension non-Newtonian liquids,” Tuteja says.

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

The Air Force Office of Scientific Research funds the research.

Source: University of Michigan

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