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.
