Superhydrophobic Anti-Glare Glass

May 2, 2012

Glass is a versatile and inexpensive material with many admirable physical and chemical properties. Although plastic is the material of our age,[1] a cursory survey of my pantry reveals many glass food containers. The glass we've looked through most often is the windshield of our automobile, and that observation reveals some of the opportunities for improving window glass. These windows are dirty, and they produce distracting glare.

Glass technology has had a resurgence in recent years with the strict requirements of display glass. This started with the replacement of nearly all cathode ray tubes, for both entertainment displays and computer displays, with flat panel displays, and it continued with some special requirements for cellphones and tablet computers. I wrote about the glass used in the Apple iPhone, "Gorilla Glass," in a previous article (Gorilla Glass, August 11, 2010). This might not be the best example, since the iPhone glass is based on some very old technology.[2]

A team of engineers from the Massachusetts Institute of Technology has addressed some problems with glass by a surface microstructuring process to simultaneously reduce glare and improve surface cleanliness.[3-6] They used conical nanotextures to give the glass surface a large topographic roughness (see figure). When you're producing nanoscale features covering a large area, there are quite a few processing challenges. You need to maintain the nanoscale features of the structures and their periodicity with very few defects.

The immediate benefit of the nanostructure is that the tapered surface material presents a perfect refractive index gradient that matches the refractive index of the glass to air, thereby minimizing reflection (glare). Optical transmission exceeding 98% was achieved over a wide range of wavelengths and incident angles, which means that the reflection was only 2%.

Left image - The surface structure of the MIT glass. The aspect ratio of the cones is about five-to-one. Right image - The anti-fogging property of the MIT glass that results from the surface superhydrophobicity. (MIT images by Hyungryul Choi and Kyoo-Chul Park).[3]

The MIT team produced the microstructured surfaces using the photolithographic and etching techniques used in integrated circuit fabrication. The cones were made by successive application of a resist masking layer and subsequent etching.[3] The design of the surface was biomimetically inspired by the surface structure of the lotus leaf.[3]

Illustration of a lotus leaf surface. (Still image from an animation by William Thielicke, via Wikimedia Commons).

Surprisingly, at least to me, is the idea that the surface should be robust to the application of most forces, including being poked with a finger.[3] Computer simulations show that this is the case, although real experiments are always welcome. Just as for a lotus leaf, the surface is highly hydrophobic to an extent that it's described as superhydrophobicity. I wrote about such hydrophobicity in a recent article (Tiny Droplets, March 6, 2012). One of the team members, Kyoo-Chul Park, says that water droplets bounce off the surface "like tiny rubber balls."[5] Such a property is predicted by Cassie's law.

A water droplet bouncing off an MIT nanostructured glass surface. (Still images from a YouTube video.[6]

This superhydrophobicity has quite a few advantages. The surfaces are also anti-fogging, and they would resist sweat contamination in touchscreen displays.[3] There is also a self-cleaning effect, as the sessile droplets will collect dirt and roll it off the surface, a boon for my automobile windshield.[3] The greatest benefit may be in application to photovoltaic panels.[4]

Park states that photovoltaic panels can lose 40% of their efficiency in six months because of dust and dirt accumulation. The MIT glass, having a higher optical transmission at even shallow angles, would allow a greater insolation of such panels, almost doubling the harvested solar energy at certain times of the day.[3] Although the surface pattern was etched into glass, it might be possible to imprint such a texture onto plastic sheets using a heated roller.[3]

This work is described in and article in the journal, ACS Nano, authored by MIT mechanical engineering graduate students Kyoo-Chul Park and Hyungryul Choi, postdoctoral associate Chih-Hao Chang, who is now at North Carolina State University, Robert Cohen, a professor of chemical engineering, and mechanical engineering professors Gareth McKinley and George Barbastathis. This research was supported by multiple sources, notably the Air Force Office of Scientific Research and the Xerox Foundation.[3]

References:

1. Remember, for example, this famous scene from The Graduate (1967, Mike Nichols, Director):
   Mr. McGuire: I just want to say one word to you. Just one word.
   Benjamin: Yes, sir.
   Mr. McGuire: Are you listening?
   Benjamin: Yes, I am.
   Mr. McGuire: Plastics.
2. David C. Boyd, "Sodium Aluminosilicate Glass Article Strengthened by a Surface Compressive Stress Layer," U.S. Patent No. 3,778,335 (December 11, 1973).
3. David L. Chandler, "Through a glass, clearly - MIT researchers find a way to make glass that's anti-fogging, self-cleaning and free of glare," MIT News Office Press Release, April 26, 2012.
4. Kyoo-Chul Park, Hyungryul J. Choi, Chih-Hao Chang, Robert E. Cohen, Gareth H. McKinley and George Barbastathis, "Nanotextured Silica Surfaces with Robust Super-Hydrophobicity and Omnidirectional Broadband Super-Transmissivity," ACS Nano Just Accepted Manuscript, April 8, 2012, DOI: 10.1021/nn301112t.
5. Dave Smith, "MIT Researchers Invent 'Perfect Glass'," International Business Times, April 26, 2012
6. Fog-free glass, MIT News Office, YouTube video, April 26, 2012.