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Spongy nanocoating makes for fog-free glass

August 30, 2005 By Andreas von Bubnoff This article courtesy of Nature News.

Easy technology could soon be on car windscreens.

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Imagine glasses that never fog up or reflect light. US scientists think they have the technology to make this happen, along with fogless, reflection-free ski goggles, windscreens, bathroom mirrors and more.

Michael Rubner of the Massachusetts Institute of Technology in Boston and his team have developed a nanoparticle coating that reflects just 0.2% of light hitting its surface. That's much less than the 2-3% that is reflected by current antireflective coatings, he says.

Rubner likes to call them molecular diapers.
The coating also soaks up the tiny water droplets that cause fogging. It consists of multiple layers of polymer fibres and glass nanoparticles that form a network filled with tiny gaps. Water is sucked into these spaces, which act like a sponge, says Rubner, who presented the work on Monday at a meeting of the American Chemical Society in Washington DC. The result is a thin film of water, rather than a number of droplets, which doesn't scatter light and cloud the glass, he says.

Rubner likes to call them 'molecular diapers' says Christopher Barrett, formerly a postdoc in Rubner's lab and now at McGill University in Montreal, Canada. The particles involved are just 7 nanometres in diameter (a hundred times smaller than the wavelength of visible light) and this keeps the coating transparent.

Bake it away

As a bonus, the process is so simple that the team got local high-school students to whip up the coatings.

The scientists soak the surface of choice in a series of solutions, which alternate between being positively and negatively charged to help glue the layers together. They then bake the coating at 500ºC to make it harder and more scratch resistant.

This step means that, at the moment, the coating can be applied only to surfaces, such as glass, that can withstand high temperatures. "We are still working to apply this to low melting-point materials like plastics," Rubner says.

Lotus position

Rubner has used similar technology to create a coating that has the opposite effect: it repels water extremely well. This material is covered in an additional layer of wax-like polymers, mimicking the surface of the lotus flower. The lotus repels water so strongly that droplets roll off, taking dirt with them.

Mixtures of these two materials might come in handy, Rubner says. The carapace of a beetle from the Namib Desert (Stenocara), for example, has parts that attract water and others that repel it. This helps the bug to attract tiny droplets from the air, concentrate them, and direct them to its mouth to drink.

So what's next for Rubner? Commercialization of course. Rubner says he has had visits from three car manufacturers interested in the technology.

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