January 1, 2006 When moisture condenses on a cool surface, droplets can form that are the right size to scatter light, fogging up glass. A new polymer coating draws droplets into nanopores and transforms them into a transparent sheet, improving vision.
BOSTON--If you're all fogged up, a new discovery may have you seeing more clearly. Fog is not just a weather nuisance for drivers; it can cause problems just about everywhere. Now, a new anti-fog glass coating is clearing the way for consumers.
Your bathroom mirror, eyeglasses, and your car windshield all fall victim to fog. It can happen anywhere moisture condenses on a cool surface.
Michael Rubner, a materials chemist and professor of Polymer Material Science at Massachusetts Institute of Technology in Boston, says, "When they condense they are just the right size to scatter light. If light gets scattered you can't see through those glasses anymore."
Michael created a polymer coating, made from different materials that transform the opaque droplets of water into a transparent sheet. "All of the process that we use to create these coatings is done with water," he says.
The process begins by dipping the glass into a solution of negatively charged tiny glass particle. After, it's dipped into another solution with positively charged polymers. Michael says: "We're forming what we call nanopores. The pores are so small that you can't see them with your eyes. They don't scatter light. But they're large enough so that when you put a drop of water onto the surface, it's drawn into those pores and spread across the surface instantaneously."
The effect allows you to continue seeing clearly through the piece of coated glass. This not only helps you at home, but even the military is looking for fog-free glass. Thomas Long, from Fosta-Tek Optics in Leominster, Mass., says, "The soldiers are faced with either having a foggy field of vision or taking those glasses off to improve their vision, but then being vulnerable to fragments of shrapnel ending their eyesight."
Michael says his coating promises to be long lasting for soldiers out in the field and to drivers just trying to see their way home. He's trying to find a cost-effective way to mass market the polymer coating and hopes it will be available in the next few years.
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: MIT's new coating is "superhydrophilic": it really loves water. It's made of a three-dimensional 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.