Creating glasses that don’t fog or freeze up could not only bring a world of comfort to millions of people, but it could also have a myriad of applications in cameras, microscopes, mirrors and refrigerated displays – to name just a few. While there have been many advancements in this field, so far, the main problem is that there is no systematic way of testing different coatings and materials to see how effectively they work under the pressure real-world conditions.
See things clearly
If such a method were developed, then it would benefit the field greatly and almost certainly lead to quick advancements – and that’s exactly what a team of MIT researchers has done. Immediately afterwards, they used it to test a coating they developed, and found that it works great in preventing foggy buildups, but also in maintaining good optical properties without distortion.
This work has been detailed in a paper published in ACS Nano. The research was conducted by Michael 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.
“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 this method has a big problem – if the thickness of the water layer varies considerably, then distortion occurs, and this is especially bad for systems where you want to have a clear sight of what’s going on (say cameras, for example). Also, if you have a layer of water and it’s cold enough outside, the water will freeze (doh), and that produces even more distortion.
For that purpose, what you need is a coating that holds water, but not too much water, and also that prevents it from freezing; in other words, you want both hydrophobic and hydrophilic traits in the same material – and this is exactly what the team accomplished.
They even coined a term for this hybrid property: Zwitter wettability. Zwitter, Rubner explains, is a German word for hybrid, and is often used in chemistry for things that exhibit two opposite properties at once. The surface itself is made through a layer-by-layer technique, alternating between layers of two different polymers — poly(vinyl alcohol) and poly(acrylic acid). The magic happens on the nanoscale, but is actually quite cheap and easy to make on a large scale.
“These are common polymers,” Rubner says. “They’re well-known and cheap, but brought together in a unique way.”
They tested this new material in extreme conditions – at minus 20 degrees Celsius, in a very humid environment. Everything worked fine, and the test was successful, but there are still limitations.
The main problem is that the layer is extremely thing, and therefore very vulnerable to mechanical impacts or even cleaning. Also, it can’t hold up when exposed to big quantities of frosty water, such as say, on an airplane wing. But this still leaves us with so many possible applications: glasses, automobile windshields, refrigerator cases, etc.
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.”
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