Fog Be Gone: Nanocoating clarifies the view

10:09am, August 31, 2005
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A new coating that prevents fogging and reflection could one day clear the world of misty mirrors, glaring glasses, and cloudy camera lenses. Researchers described their innovation, which relies on porous layers of nanoscale particles, at the American Chemical Society's meeting this week in Washington, D.C.

Fogging occurs when water droplets from moist air condense on a cool surface and scatter light there. Most of today's antifogging options are spray-on polymers that, once applied, flatten the droplets into nonscattering shapes. However, these treatments quickly wear off.

Michael F. Rubner, a materials scientist at the Massachusetts Institute of Technology (MIT), says that his foray into fog busting was inspired by the lotus plant. Lotus leaves are so water repelling, or superhydrophobic, that when raindrops hit their surface, the drops remain spherical. After working on coatings that mimicked the lotus leaf's surface, Rubner, MIT chemical engineer Robert E. Cohen, and their coworkers investigated the opposite extreme: coatings that are extremely water loving, or superhydrophilic.

The coatings woo water so well because they are composed of tiny, hydrophilic glass particles packed irregularly, leaving minuscule spaces between them that instantaneously suck overlying water toward the interior. This spreads out the water, maximizing its contact with the surface. Unlike the rounded droplets on a fogged surface, the resulting film of water can't scatter light.

The porosity of the glass-and-air coating brings with it a bonus: It prevents glare. Nearly 100 percent of light can travel through one of Rubner's nanocoated glass surfaces, whereas 97 to 98 percent passes through currently available antireflection coatings and only 92 percent passes through untreated glass, he says.

"You get two useful properties" from one novel structure, says Joseph B. Schlenoff, a chemist at Florida State University in Tallahassee.

Rubner's group makes its nanocoatings by alternating layers of a positively charged polymer with negatively charged glass particles that are 7 nanometers in diameter. The scientists build the layers in such a way that the glass nanoparticles don't pack efficiently, resulting in an ultrathin coating that holds many air pockets.

Svetlana A. Sukhishvili, a polymer chemist at the Stevens Institute of Technology in Hoboken, N.J., says that "this is basically the strongest work" she's seen, in terms of controlling the development of pores at the nanoscale level.

To increase the durability of the nanocoating, Rubner's group heats it to 500°C, a step that burns away the polymer and fuses the glass nanoparticles. The coating thus becomes much more scratch resistant but maintains its Swiss cheese structure. Glass slides with coatings prepared this way a year ago are as fog resistant as ever, Rubner reports.

He notes that only materials that can withstand high heat, such as glass, can be coated at this point. "If you want to go with lower-temperature substrates, like plastics, we're still working on solving that problem," he says.

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