Materials engineers have created a new type of coating that attracts water yet beads oils - traits that are usually mutually exclusive - promising potential applications such as "self-cleaning" eyeglasses and antifogging car windshields and ski goggles. The material prevents fogging by not allowing water to form beads on surfaces, said Jeffrey Youngblood, an assistant professor of materials engineering at Purdue University.
The coating consists of a single-molecule-thick layer of a material called polyethylene glycol, and each molecule is tipped with a Teflonlike "functional group" made with fluorine. Water molecules pass through the Teflonlike layer, which acts as a barrier to the larger oil molecules. Then the water is attracted to the polyethylene glycol immediately adjacent to the glass surface.
"So, it repels the oil but not the water," said Youngblood, who is working with materials engineering doctoral student John A. Howarter. "This is important because oil normally sticks to surfaces that attract water, a property we call hydrophilic. However, we now have one that's hydrophilic but that oil doesn't like."
The findings demonstrate how an oily substance called hexadecane beads up on the coating while water spreads out to cover the surface instead of forming beads. Moreover, when water is attracted to the underlying polyethylene glycol layer it gets between the hexadecane and the glass, dislodging the oily film.
"If you place a droplet of oil and a droplet of water right next to each other, the water will move underneath the oil and cause it to fall off of the surface," Youngblood said.
Self-cleaning and antifogging behavior has been demonstrated in experiments using glass surfaces coated with the material.
Eyeglasses and goggles used by skiers are two obvious potential applications, along with automotive windshields. The inside of a car's windshield fogs because condensing water beads up on oily films that form on glass surfaces.
"If glass is perfectly clean, it won't fog, but glass is never perfectly clean because it picks up junk from the air and other sources," Youngblood said. "You will notice, for example, that it's difficult to remove fingerprints from eyeglasses. That's because of oils that are deposited from your skin onto the glass surface. Our material resists that junk from getting on the surface to begin with."
Unlike other "stimuli-responsive" coatings, the new material does not need to be pretreated with a solvent to achieve the desired behavior.
"It works as soon as you apply a drop of water," Youngblood said.
Eyeglasses coated with such a material would automatically shed fingerprints and other oily films when rinsed with water, eliminating the need to clean the surface with a cloth or to use soap. Such a material might be used as a permanent coating on glass surfaces and also could be added to liquid window cleaners to prevent fogging.
A patent is pending, and it will likely be several years before any products based on the technology are commercialized.
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