Anti-fogging nanoparticles

Those of you who don’t wear glasses don’t know how lucky you are. I’ve been a contact-wearer now for many years, but from the time I was about five until I was almost thirty I wore glasses, and I the most annoying thing about them was their inclination to fog up the minute you came inside from outside on a cold day.

And, of course, fogging isn’t just a problem for eyeglass wearers. It also plagues automobiles, bathroom mirrors, camera lenses, ski goggles…the list goes on and on.

But now researchers at the Massachusetts Institute of Technology may have come up with a solution: a coating made of nanoparticles that, they say, can create surfaces that never fog.

“Nano” means very, very small. (More precisely, a nanoparticle is a particle that is less than 100 nanometres in diameter. A nanometre is a billionth of a metre, or, if you prefer, a millionth of a millimeter.)

Glass (and other surfaces) fog when they’re cooler than the surrounding air. The moisture in the air condenses onto the cool surface. Because water molecules are more attracted to each other than air molecules, they run together to form drops, fogging the surface.

There have been lots of attempts to come up with a solution. You can buy anti-fog sprays, but they have to be constantly reapplied.

Fogging wouldn’t be a problem if you could make the surface more attractive to water, so the water would spread out in sheets instead of forming vision-obscuring droplets. Coating glass with titanium oxide increases its attractiveness to water-but the coatings have to be charged by ultraviolet light. That means they don’t work for very long in the dark. As well, they tend to stop working completely after just three months.

The MIT team, led by Michael Rubner, a materials science researcher, believes it has come up with a permanent method of making surfaces more attractive to water.

Their “superhydrophilic” (super-water-loving) coating consists of a three-dimensional matrix of nanoparticles of glass, a polymer called polyallylamine hydrochloride, and tiny bubbles. The nanoparticles and polymer are relatively cheap to manufacture, and Rubner, who has applied for a patent, thinks the coating could be ready for consumer products in just two to five years; he says he already has interest from the military and two major car manufacturers.

The roughness of the coating increases the surface area of glass in contact with the water; this helps reduce the surface tension and flatten the water droplets so they form up into continuous sheets. The bubbles actually act rather like a sponge, sucking some water downwards, again flattening the droplets.

Rubner tested his team’s concoction with the simplest of experiments: he took a piece of glass, half coated, half not, into a steamy bathroom. Sure enough, the coated side remained clear, while the uncoated side fogged up.

Interestingly, the coating could easily be reengineered to act in exactly the opposite fashion, repelling water, instead of attracting it, causing the water to form larger droplets. That could be used to develop a self-cleaning surface-any water that fell on it would snap up into large droplets that would then roll away, taking dirt with them.

Many other nanoparticle-based coatings are being developed by other researchers around the world. Some emerging applications include fireproof wood doors, stain-resistant jeans, key-stratch-resistant clear coats for cars (Mercedes already applies nanoparticle clearcoat paint to many of its models), and even bacteria-killing bandages. Ceramic nanoparticles can be melted onto metal to create a thin, hard coating resistant to corrosion.

Even Rubner’s nanoparticle coating could have more uses than just preventing fogging. Nanoparticle coatings on glass don’t obstruct vision because the particles involved are smaller than the wavelength of visible light-which means they’re invisible. In fact, the coatings can be engineered to reduce glare and maximize the amount of light passing through glass-which could be valuable for greenhouses and solar cell panels.

Alas, because the current coating just keeps water from forming droplets, it probably won’t help prevent frosting: the water will still freeze to the window, and will still have to be scraped off.

So keep working, Professor. Your little (literally) invention may impress them in Massachusetts, but here in Saskatchewan, you’ve still got your work cut out for you.

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