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Published on March 15th, 2016 | By: Stephanie Liverani


New technology uses geometry to control windows’ opacity—and it’s scalable

Published on March 15th, 2016 | By: Stephanie Liverani

[Image above] Credit: Benedicto de Jesus; Flickr CC BY-NC 2.0



You might be able to ditch those mini-blinds on your home’s windows in favor of something a little ‘smarter.’


Currently, we rely on shades and window treatments to block out the bright afternoon sun or provide privacy after nightfall.


But what if we could adjust the opacity of our windows instantaneously with the flip of a switch?


Tunable windows aren’t new to the materials scene. In fact, scientists have already developed smart windows based on dual-band electrochromic materials—a process that blends two materials with distinct optical properties for selective control of visible and heat-producing near-infrared light.


Other researchers have created a polymer coating for glass that can change color instantly with a small, user-controlled electrical current.


And recently we reported on research from the Massachusetts Institute of Technology, where scientists are working with a readily available transparent polymer that may be useful in the design of cheaper materials for smart windows that automatically adjust the amount of incoming light.


The thing is, most of these technologies rely on electrochemical reactions, and scaling up the manufacturing process to make these smart windows commercially viable is a challenge.


This week, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences announced that they’re taking a different approach to developing smarter windows: geometry.


David Clarke, the Extended Tarr Family Professor of Materials at Harvard, and Samuel Shian, postdoctoral fellow, developed a technique that can “quickly change the opacity of a window, turning it cloudy, clear, or somewhere in between with the flick of a switch” using geometric principles instead of electrochemical reactions, according to a Harvard news release.


To construct the tunable window, the researchers sandwiched a sheet of glass or plastic between transparent, soft elastomers sprayed with a coating of silver nanowires that are too small to scatter light on their own.


“But when an electric current is applied, the nanowires on either side of the glass are energized to move toward each other, squeezing and deforming the soft elastomer. Because the nanowires are distributed unevenly across the surface, the elastomer deforms unevenly. The resulting uneven roughness causes light to scatter, turning the glass opaque,” the release explains.



(a) Brightfield micrographs illustrating the random arrangement of silver nanowires (90 nm average diameter, 20–60 μm length) dispersed on the surface of an elastomer sheet. (b) Same area and imaging conditions when a voltage is applied with respect to a flat electrode underneath. Credit: Optics Letters


The transformation from clear to opaque happens in an instant.


Shian likens the concept to what happens when a pond freezes over.


“If the frozen pond is smooth, you can see through the ice. But if the ice is heavily scratched, you can’t see through,” Shian says in the release.


The degree of opacity is controllable by the level of electricity applied to the window. Clarke and Shian found that the higher the voltage, the rougher the elastomer surface, which equals a totally opaque window. A lighter electrical current would result in a slightly frosted look.


And this tunable technological development offers a scalable solution for smarter windows.


Current chemical-based tunable windows use vacuum deposition to coat the glass, which deposits layers of a material molecule by molecule—an expensive and tedious process, the release explains. But the technique developed by Clarke and Shian can be sprayed on or adhered to the elastomer, making the technology a more affordable option for larger-scale architectural projects.


“Because this is a physical phenomenon rather than based on a chemical reaction, it is a simpler and potentially cheaper way to achieve commercial tunable windows,” Clarke says.


Harvard’s Office of Technology Development has filed a patent application on the technology and is working with potential licensees in the glass manufacturing industry.


The research, published in Optics Letters, is “Electrically tunable window device” (DOI: 10.1364/OL.41.001289).


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