Reported January 2006

BACKGROUND: MIT researchers have developed a new anti-fog glass coating that can transform water droplets into smooth transparent sheets of water. The coating can be used on everything from car windows, bathroom mirrors, eyeglasses, ski goggles, underwater masks, and inside car headlights to prevent fogging.

HOW FOG FORMS: Fog is caused when steam condenses on a cool surface and then forms miniscule water droplets because of the water's surface tension. Water molecules are more attracted to each other than to air molecules, so they form a spherical shape to maximize contact with other water molecules, leaving as few as possible exposed to air. It's impossible to stop water from condensing on a surface, but water molecules are also attracted to glass. If this attraction is enhanced, it can overcome the surface tension. Decreasing the water's surface tension flattens the naturally formed water droplets (fog) and creates a thin see-through layer of water instead.

HOW THE COATING WORKS: The new coating is "superhydrophilic:" It really loves water. It's made of a 3-D matrix of water-loving polymer chains mixed in with glass nanoparticles and tiny air bubbles. The edges of the tiny glass particles come in contact with many droplets of water and the water droplets flatten and join up to form sheets. The glass nanoparticles and air bubbles also can act like the holes in a sponge, sucking the droplets downward to wick away water.

OTHER USES: The MIT researchers can also tailor the coating to be "superhydrophobic" by adding a second thin layer of water-repelling molecules. Then the large surface area created by the roughness of the surface has the opposite effect, increasing the repulsion between water and glass, causing the water to form droplets. These could be used to form self-cleaning surfaces, where such big water droplets are formed that they roll off the surface and take the dirt with them.

If you would like more information, please contact:

Dr. Michael Rubner
MIT Center for Material Science and Engineering
rubner@mit.edu
http://web.mit.edu/cmse/www

FACTOID...

The lotus leaf, or water lily, is considered sacred in Asian religions because of its ability to stay dry and clean just like the new coating. When water drops on the leaf, it beads up and rolls off the waxy surface, washing away dirt at the same time.