• A piece of glass that has one half with a normal flat surface, and the other with the new patterned surface, demonstrates the anti-fogging properties that the patterning provides.

    Image: Kyoo-Chul Park and Hyungryul Choi

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  • Through a process involving thin layers of material deposited on a surface and then selectively etched away, the MIT team produced a surface covered with tiny cones, each five times taller than their width. This pattern prevents reflections, while at the same time repelling water from the surface.

    Image: Hyungryul Choi and Kyoo-Chul Park

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Through a glass, clearly

MIT researchers find a way to make glass that’s anti-fogging, self-cleaning and free of glare.


One of the most instantly recognizable features of glass is the way it reflects light. But a new way of creating surface textures on glass, developed by researchers at MIT, virtually eliminates reflections, producing glass that is almost unrecognizable because of its absence of glare — and whose surface causes water droplets to bounce right off, like tiny rubber balls.

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The new “multifunctional” glass, based on surface nanotextures that produce an array of conical features, is self-cleaning and resists fogging and glare, the researchers say. Ultimately, they hope it can be made using an inexpensive manufacturing process that could be applied to optical devices, the screens of smartphones and televisions, solar panels, car windshields and even windows in buildings.

The technology is described in a paper published in the journal ACS Nano, co-authored by mechanical engineering graduate students Kyoo-Chul Park and Hyungryul Choi, former postdoc Chih-Hao Chang SM ’04, PhD ’08 (now at North Carolina State University), chemical engineering professor Robert Cohen, and mechanical engineering professors Gareth McKinley and George Barbastathis.

Photovoltaic panels, Park explains, can lose as much as 40 percent of their efficiency within six months as dust and dirt accumulate on their surfaces. But a solar panel protected by the new self-cleaning glass, he says, would have much less of a problem. In addition, the panel would be more efficient because more light would be transmitted through its surface, instead of being reflected away — especially when the sun’s rays are inclined at a sharp angle to the panel. At such times, such as early mornings and late afternoons, conventional glass might reflect away more than 50 percent of the light, whereas an anti-reflection surface would reduce the reflection to a negligible level.

While some earlier work has treated solar panels with hydrophobic coatings, the new multifunctional surfaces created by the MIT team are even more effective at repelling water, keeping the panels clean longer, the researchers say. In addition, existing hydrophobic coatings do not prevent reflective losses, giving the new system yet another advantage.

Other applications could include optical devices such as microscopes and cameras to be used in humid environments, where both the antireflective and anti-fogging capabilities could be useful. In touch-screen devices, the glass would not only eliminate reflections, but would also resist contamination by sweat.

Ultimately, if the cost of such glass can be lowered sufficiently, even car windows could benefit, Choi says, cleaning themselves of dirt and grit on the exterior surface of the windows, eliminating glare and reflections that can impair visibility, and preventing fogging on the interior surface.

The surface pattern — consisting of an array of nanoscale cones that are five times as tall as their base width of 200 nanometers — is based on a new fabrication approach the MIT team developed using coating and etching techniques adapted from the semiconductor industry. Fabrication begins by coating a glass surface with several thin layers, including a photoresist layer, which is then illuminated with a grid pattern and etched away; successive etchings produce the conical shapes. The team has already applied for a patent on the process.

Since it is the shape of the nanotextured surface — rather than any particular method of achieving that shape — that provides the unique characteristics, Park and Choi say that in the future glass or transparent polymer films might be manufactured with such surface features simply by passing them through a pair of textured rollers while still partially molten; such a process would add minimally to the cost of manufacture.

The researchers say they drew their inspiration from nature, where textured surfaces ranging from lotus leaves to desert-beetle carapaces and moth eyes have developed in ways that often fulfill multiple purposes at once. Although the arrays of pointed nanocones on the surface appear fragile when viewed microscopically, the researchers say their calculations show they should be resistant to a wide range of forces, ranging from impact by raindrops in a strong downpour or wind-driven pollen and grit to direct poking with a finger. Further testing will be needed to demonstrate how well the nanotextured surfaces hold up over time in practical applications.

Andrew Parker, a senior visiting research fellow at Oxford University’s Green Templeton College in the U.K. who was not involved in this work, says, “Multifunctional surfaces in animals and plants are common. For the first time, as far as I am aware, this paper learns a lesson in manufacturing efficiency from nature by making an optimized antireflective and anti-fogging device. … This is the way that nature works, and may well be the future of a greener engineering where two structures, and two manufacturing processes, are replaced by one.”

The research was funded by the Army Research Office through MIT’s Institute for Soldier Nanotechnology; the Air Force Office of Scientific Research; Singapore’s National Research Foundation through the Singapore-MIT Alliance for Research and Technology (SMART) Centre, and the Xerox Foundation. Park and Choi are recipients of fellowships from Samsung and the Kwanjeong Educational Foundation/STX Scholarship Foundation, respectively.


Topics: Chemistry and chemical engineering, Fabrication, Innovation and Entrepreneurship (I&E), Global, Mechanical engineering, Microfabrication, Superhydrophobicity, Supertransmissivity

Comments

Interesting work. Though application to solar panels will be difficult. the nanocones can easily erode away due to sand abrasion. the problem with these structures is they are not optically tolerant to even a single defect. Imagine if one of the nanocones breaks then the optical performance of the entire surface will drop. also weathering and UV tests should be done. were any abrasion or erosion tests conducted?
I know there are many more uses to this glass but I think to use it for scuba goggles would be just awesome. One of the pains for me while diving is to always have anti-fog in my lens every time I enter the water.
I think the cones are so close together (200nm) that they wouldn't be effected by general wear. Having them inside something like goggles so they wouldn't fog or something like that would be just awesome. I wouldn't mind scuba diving with no glare and no fog in my lens.
Just wondering if this really works to keep a fog free, and clean, clear view to the glass. Yes, the main part of the water droplet bounces on the surface. But there are definate traces where the dropplet has been. Water molecules (and dirt in the dropplet) are still catching on the surface. Over time these will build up. On a macro scale, say coating a scuba mask, does the technology actually work?
In all honesty, I reckon the place that this would be used most would be in the smartphone market. Manufacturers such as Nokia and Samsung would jump at the chance to be able to make a Phone with 0 screen reflection. They would pay a LOT to use this technology.
ofcourse they will break -- just try abrading the surface with your hand. if they are inside goggles how will it work, the outer surface will fog up.
Then you put them out the outside too..? Also I dont think you are grasping how small these things are. Touching it isnt going to topple them, youd have to take sandpaper to them.
The outer surface of scuba goggles will fog up? Think about what you just wrote for a minute.
Nature and Ben Franklin had the ability of practical applications.Simple progressions that balance the best solution call it trial and error. More needs to be done to study whats already here and used as inventions to help improve all productions.Energy transfer is probably the best start.
As I read the captions on the video, the little cirles left behind by the bouncing drops are not traces of moisture, but instead are the spots where the water has "picked up" the dust that was coating the surface. Alfred Poor HDTV Almanac
May break part of the premise in a science fiction story just out this week: "Nightfall on the Peak of Eternal Light", Richard A. Lovett & William Gleason "Analog", 2012 JULY/AUGUST (volume CXXXII, Numbers 7 & 8}, pages 10-43 A central part of the story is the need clean solar panels on the moon.
Great work from a materials science point of view! However, the use of photoresist is already prohibitive; it's not even used for silicon solar cells which are the most valuable part. The total costs for a pv module (frame, glass, encapsulation, etc.)are about $30 which includes 1 sq m of glass. So, I'm wondering how much does it costs to make this surface texture per square meter and how feasible is it to do this 30.000 times per day (this the typical capacity of a larger pv production line)?
This galss could be useful to dentist's mirrors?....I'm a dentist and this idea may be really good....
TelAztek, a company in MIT's backyard, has been making and selling these exact structures for many years to do exactly as these guys claim.
Actually, the statement: "In addition, existing hydrophobic coatings do not prevent reflective losses" is not correct. The existing hydrophobic coatings DO prevent reflective losses, which keeps the surface being reflective.
If the manufacturing process can be simplified, as suggested in the article by the use of textured rollers, then a source of reasonably-priced non-reflective sheet glass (or acrylic?) would be fantastic for the framing of artwork. "Museum glass" as available now is exhorbitantly expensive.
Since there would be little reflection from the front surface, this would probably be a good alternative to front surface mirrors. It will remmain to be seen if this finish is more robust than the fragile reflective coating of a typical front surface mirror
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