Hydrophobic materials have potential for everything from self-cleaning windscreens to bottles that dispense the last drop of syrup, but such materials tend to be on the fragile side. One possible solution is to make water repellent coatings that are a self-healing, which is what a team of scientists led by Jürgen Rühe at the University of Freiburg in Germany has done. Their superhydrophobic material sheds its outer skin like a snake to repair itself after being damaged.

Inspired by natural water-repellent surfaces, like those found on lotus leaves and water striders, a number of different superhydrophobic materials have been developed. Some of the most successful have been those that use a substrate coated with nanostructures supported by larger microstructures. But as useful as such materials can be, they are vulnerable to damage and a scratch can ruin their hydrophobic properties.

To overcome this, Ruhe's team developed a new self-healing material made of three layers set one atop the other. The top layer is of a hydrophobic film made of poly-1H,1H,2H,2H-perfluorodecyl acrylate (PFA), also known as "nanograss," which gains its water repellent properties from sharp, cone-like nanostructures that stick up from its surface. Under this is a layer of Polyvinylpyrrolidone (PVP), a water-soluble polymer used in various medical applications. And at the bottom is a superhydrophobic silicon nanograss film.

Cross section of the self-healing hydrophobic material(Credit: American Chemical Society)

The idea is that if the top layer is scratched, the material can be submerged in water that seeps into the scratch and dissolves the intermediate polymer layer. This allows the damaged top layer to come loose and slough off, exposing the fresh hydrophobic layer beneath.

According to the team, the skin-shedding material is still very much in the laboratory stage and more work needs to be done to strengthen the top layer to prevent a scratch damaging the lower layers. But if they're successful, their multilayered approach could lead to more resilient self-cleaning and water-repellent materials.

The research was published in Langmuir and the new material is outlined in the video below.

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