Preventing glass from fogging or frosting up is a longstanding challenge with myriad applications: eyeglasses, cameras, microscopes, mirrors and refrigerated displays, to name but a few. While there have been many advances in meeting this challenge, so far there has been no systematic way of testing different coatings and materials to see how effectively they work under real-world conditions.
Now, a team of MIT researchers has developed such a testing method, and used it to find a coating that outperforms others not only in preventing foggy buildups, but also in maintaining good optical properties without distortion.
“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,” says Rubner, who is also director of MIT’s Center for Materials Science and Engineering. “So even though the water’s there, it doesn’t scatter the light.”
But there can be a problem with that approach: In applications where it’s important to get an undistorted view, such as cameras or other optical systems, the view can be quite distorted if the thickness of the layer of water varies considerably.
In addition, if the surface is cold, the water on the surface can begin to freeze, forming a frost layer that scatters light, Rubner says, adding: “If you’re going to have a sheet of water, how do you prevent it from freezing?”
For that purpose, what you really need is a coating that can absorb a lot of water in a form that cannot freeze. Indeed, in many applications it would be useful to have both hydrophobic and hydrophilic traits in the same material. That’s what the team has now done, and they’ve coined a term to describe this hybrid property: Zwitter wettability.
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 a process called 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. “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 on large scales. “These are common polymers,” Rubner says. “They’re well-known and cheap, but brought together in a unique way.”
To test the effectiveness of this material, and that of many other alternatives, the team devised a set of extreme tests. For example, they kept samples of the material at minus 20 degrees Celsius for an hour, then exposed them to a very humid environment. While untreated glass, or glass treated with conventional hydrophilic or hydrophobic coatings, quickly develops a layer of frost following such treatment, glass with the new treatment remains clear. However, it still appears to be hydrophobic in the presence of large water droplets.
To measure its performance, Lee says, the researchers photographed the glass slides under carefully controlled conditions. “We developed a protocol … [that] allows us to detect how good one coating is in comparison with another,” he says.
Previous testing typically measured the light transmitted through the glass after exposure to humidity, but failed to measure the level of image distortion caused by water condensation. “We came up with a way to measure them not just for transmission, but also distortion,” Lee says.
While the new coating outperforms others, it does have one drawback: It’s vanishingly thin, so could be vulnerable to aggressive cleaning or mechanical challenges. For this reason, it may not be useful for applications where it is exposed to harsh environments or to excessive wiping.
Another limitation is that the new coating only prevents small amounts of frost buildup; it wouldn’t work where there’s a continuous source of cold water, such as for deicing an airplane wing, Rubner says.
Still, that leaves many possible uses: the inside of automobile windshields, safe from both weather and windshield wipers; the inside of grocery stores’ refrigerator cases; and optical systems used in research or in photography. The coating could also be useful on the inner surfaces of double-pane windows, which can become fogged if even a small leak allows outside air into the sealed space.
Joseph Schlenoff, a professor of polymer science at Florida State University who was not involved in this work, says, “Everyone knows how inconvenient, or even dangerous, it is to have a cold window or lens fog up when water condenses on it. 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.”