If opposites really do attract, a novel hybrid nanocoating with a love-hate relationship toward water could be just the thing to fight frost and fog on windshields, windows, optical equipment and more.
An MIT research team led by Dr. Michael F. Rubner, who directs the university's Center for Materials Science and Engineering, has developed and tested the coating, which manages to both repel and attract water and could have a wide variety of applications.
Preventing glass from fogging or frosting up is a longstanding challenge, MIT reports. Rubner's group has developed not only a novel coating that works without optical distortion, but also a systematic way of comparative testing the performance of coatings and materials under real-world conditions.
The coating could be just the thing for clearing the insides of windshields, optical equipment, double-pane windows, and more, researchers say.
The new approach is detailed in a paper in the journal ACS Nano written by Rubner, the TDK Professor of Polymer Materials Science and Engineering; Robert Cohen, the Raymond A. and Helen E. St. Laurent Professor of Chemical Engineering; doctoral student Hyomin Lee; and recent MIT graduate Maria Alcaraz.
Fending Off Frost
“When people want to tackle the fogging process, caused when microscopic water droplets condense on a cold surface and scatter light, the common way of doing it is to build a surface that’s so hydrophilic—water-loving—that the water spreads out into a sheet,” Rubner says in an article by MIT about the research.
“So even though the water’s there, it doesn’t scatter the light.”
But that's a problem in applications like cameras and other optical systems that require an undistorted view, the team notes. In addition, water on a cold surface can quickly begin to freeze, forming a frost layer that scatters light.
"If you’re going to have a sheet of water, how do you prevent it from freezing?” asks Rubner.
The team's answer: a hybrid material that combines both hydrophobic and hydrophilic traits. The researchers coined the term Zwitter wettability to describe the property.
Zwitter, Rubner explains, is a German word for hybrid, used in a number of chemistry terms to describe something that carries two opposite properties at once. In this case, it describes a surface that has the ability to behave as both hydrophobic (to water droplets) and hydrophilic (to gas-phase water molecules).
The surface is made by layer-by-layer deposition. In this case, alternating layers of two different polymers—poly(vinyl alcohol) and poly(acrylic acid)—are deposited on a glass surface.
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A video by MIT offers a glimpse at the coating's performance, compared to others.
“The magic of what we do is nanoscale processing,” Rubner explains: producing the layers so as to control their properties almost down to the level of individual molecules.
This production process appears relatively easy and inexpensive to carry out at a large scale, adds Rubner. “These are common polymers,” he says. “They’re well-known and cheap, but brought together in a unique way.”
To test the effectiveness of the new material and alternatives, the team devised a set of extreme tests, according to MIT.
In one test, samples were kept at -20 degrees C, then shifted to a very humid environment. Untreated or conventionally coated glass quickly frosts over in such cases; Rubner's coating did not, MIT said.
Slides were photographed under carefully controlled conditions, and researchers also developed a way to measure image distortion, the university said.
The work was supported by Samsung and by the National Science Foundation.
The coating still has some drawbacks. It's extremely thin and thus vulnerable to harsh environments and aggressive cleaning, the team says. Also, it can only prevent small amounts of frost buildup so far, making it unsuitable yet for, say, airplane de-icing.
Still, the developers see a variety of immediate applications, including the inside of automobile windshields, supermarket refrigerator cases, optical systems and double-pane windows.
“Everyone knows how inconvenient, or even dangerous, it is to have a cold window or lens fog up when water condenses on it," said Joseph Schlenoff, a professor of polymer science at Florida State University who was not involved in this work.
"The MIT group has devised a practical and effective method of combatting the fogging problem using a new ultrathin polymer film.”
Schlenoff adds, “Both the materials themselves and the techniques used to explore their properties are highly innovative. These MIT engineers are literally helping us to see technology more clearly.”