Researchers create coating that can’t get wet

Engineering researchers at the University of Michigan announced that they’ve created a nanoscale coating that’s at least 95 percent air and repels the broadest range of liquids of any material in its class.

“Virtually any liquid you throw on it bounces right off without wetting it,” said Anish 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 and his colleagues tested more than 100 liquids and found only two chlorofluorocarbons that were able to penetrate the coating. In Tuteja’s lab, the surface repelled coffee, soy sauce and vegetable oil, as well as toxic hydrochloric and sulfuric acids that could burn skin. Tuteja says it’s also resistant to gasoline and various alcohols.

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

To apply the coating, the researchers use 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. In addition to the material’s chemistry, the coating hugs the pore structure of whatever surface it’s being applied to and 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 Van der Waals forces that normally draw the two states of matter together.

“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.”

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 the coating repels is the so-called non-Newtonian category that change their viscosity depending on the forces applied to them, which includes shampoos, custards, blood, paints, clays and printer inks.

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

The paper is titled “Superomniphobic Surfaces for Effective Chemical Shielding.” 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. The work is funded by the Air Force Office of Scientific Research.
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