Monday, April 02, 2007

Self-Cleaning, Fog-Free Windshields

A new adaptive polymer coating combines unusual chemical properties to help maintain a clear view.

By Kevin Bullis

See clearly: When water condenses on glass, it can form tiny drops that make the surface difficult to see through. A new coating causes such droplets to spread out, forming a transparent sheet. The coating also repels oil. Credit: Bill Longcore / Photo Researchers, Inc.

A new coating that changes its structure depending on whether it's in contact with oil or water could prevent windshields from fogging up or accumulating oily deposits. The coating was developed at Purdue University and reported at last week's American Chemical Society meeting.

Drop water on a surface treated with the coating, and it rapidly spreads out, creating a thin film. This action prevents the formation of the tiny water droplets that make up fog. Add oil, however, and the surface responds the opposite way, repelling oil and causing it to bead up. Any oil that doesn't run off on its own would be easily wiped away with water, making it unnecessary for a driver to use soap.

Oil and water's behavior on the coating is unusual in that with this material, oil beads up more than water. Typically, it's the other way around: the greater surface tension of water causes it to bead up more than oil.

Although the exact mechanisms involved have not yet been confirmed, it's clear that what makes the coating act in this unusual way is that it is effectively two surfaces in one, able to change its structure depending on whether it's in contact with oil or water. The coating is made of polymer chains chemically attached to a glass surface like bristles on a brush. These polymers are flexible and able to change shape. As they do, segments with distinct chemical properties can rise to the surface. The Purdue researchers suggest that one possibility is that these polymers rearrange themselves depending on the way different segments are attracted to or repulsed by oil or water. When oil is present, a segment of the polymer that causes fluids to bead up rises to the surface of the coating. But when water is present, this segment retreats from the surface, allowing the water to spread out.

"It's certainly a clever way to go," says Michael Rubner, a professor of materials science and engineering at MIT. The Purdue researchers' new coating, he says, "is uniquely positioned to be able to adapt to its environment."

The coating could be particularly useful on the inside of car windshields, says Jeffrey Youngblood, the professor of materials engineering at Purdue who led the work. Trace amounts of oil, such as from protective treatments on the interior surfaces in a car, cling to glass, changing the surface energy of the glass and increasing its tendency to fog. The coating would both repel the oil and prevent the water droplets of fog from forming. The coating's properties could also make it useful in filters. Applied to a porous silica, it allows water to pass through but raises barriers to oil. This could be useful for cleaning up oil spills.

To make the coating, the researchers developed a way to covalently bond Zonyl FSN-100, a material made by DuPont, to a glass surface. Although this process worked well for testing the properties of the system, it is too expensive for large-scale manufacturing. Youngblood and his colleagues are currently developing less-expensive spray- or roll-on techniques for making the coating.

Future steps will also include modifying the chemistry so that the coating works with types of oils besides the petroleum-based oil tested to date. Eventually, it may be possible to create a coating based on similar mechanisms that repels fingerprints, making it easier to keep eyeglasses and mobile-phone displays clean.