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Nanoparticles create anti-fog coating

01 Sep 2005

Silica nanoparticles could mean an end to the troublesome fog that forms on glass surfaces.

Fog on windows, spectacles and other glass surfaces could soon be a thing of the past if a nanotechnology coating developed in the US takes off. Michael Rubner and his colleagues at Massachusetts Institute of Technology (MIT) have devised a silica nanoparticle coating that causes water droplets to flatten into a thin uniform sheet rather than form the usual annoying light-scattering beads.

“With our coating we’re basically putting very tiny glass particles onto a surface using a special layer-by-layer technique that we’ve developed,” said Rubner. “And you end up creating a nanoporous thin film of glass on the surface that is super-hydrophillic [water loving].”

Surfaces covered with the coating do not fog up and retain their transparency when exposed to moisture -- a property that is highly attractive for many optical components that encounter humid conditions. “It could be used any place where fog is a nuisance,” said Rubner. “That would include the inside of car windows, eye glasses, your bathroom mirrors or double-pane windows.”

The coating is made by depositing alternating layers of a polymer called polyallylamine hydrochloride and silica nanoparticles. “What we do is first dip the substrate into a solution containing the polymer which spontaneously adsorbs onto the surface and sticks very tenaciously,” said Rubner. “Then we rinse that with some water. Then we dip it into a solution containing the glass nanoparticles. We go through that cycle 10 or 20 times and then the coating is done.”

Finally the coated glass substrate is heated to 400 to 500 °C to burn off the polymer and make the coating more durable by fusing the silica nanoparticles. Rubner told Optics.org that he is applying for a patent for the manufacturing process and is hoping to license it to a third party that is interested in commercialising the technology.

However, it appears that a team in Australia may have already got their first. As Optics.org reported last year, scientists from the University of Queensland in Australia have already demonstrated a very similar porous-silica coating (see related story) and have formed a start-up called XeroCoat to bring it to the market.

“Basically, they’re creating the same kind of structure that we are, we are just doing it in two different ways,” said Rubner. “The final coating is very similar.”

•  Details of the new MIT coating were presented at the 230th national meeting of the American Chemical Society which took place in Washington DC, US earlier this week.

Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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