WASHINGTON--Imagine a window with no need for cumbersome blinds — with a flick of a switch, the glass simply turns opaque. While the concept is not new, and researchers have already developed prototypes, “tunable windows” face expensive manufacturing challenges that have slowed their widespread commercial adoption. Now researchers have developed a novel technique to make windows turn cloudy that is easier — and potentially cheaper — to implement. They describe their proof-of-principle design in the journal Optics Letters, from The Optical Society (OSA).
Most tunable windows on the market today have a thin coating of materials such as tungsten oxide, which is vacuum deposited on glass. When electricity is applied, it triggers an electro-chemical reaction that turns the material opaque.
David Clarke and Samuel Shian, Harvard University, Massachusetts, USA, have developed a new method that uses a physical mechanism rather than a chemical one.
The window is made from a stiff sheet of thin plastic or glass, which is inserted between two layers of a transparent rubbery material called an elastomer. The researchers then cover both sides with a network of silver nanowires. The nanowires, each a few microns long and 90 nanometers across, are scattered randomly, similar to a game of pick-up sticks.
The tiny nanowires don't scatter light and sparsely cover the surface, leaving the window transparent. But things change when you apply an electric voltage. The two nanowire layers act as electrodes, and the voltage causes them to be attracted to each other by Coulombic forces. The nanowire layers squeeze the soft elastomer between them. But they only squeeze where there are nanowires, creating an uneven surface where bulges form in the gaps between the wires.
This uneven surface scatters light just like how waves on a pool's surface prevent you from seeing into the water. "You're going from a flat surface to a rough one, and it's the roughness that causes the majority of the scattering that changes it from a transparent material to an opaque material," Clarke said.
The change in transparency happens quickly, transforming the color of the material in less than a second. It also has a neutral, gray color, whereas some of the chemical-based windows have a bluish tinge.
But one of the biggest advantages of using tiny nanowires is a potentially cheaper and simpler manufacturing process. For example, the nanowire layers could be sprayed on which means it's straightforward to scale the technology to any size. The elastomer sheets are manufactured in giant rolls. The chemical-based windows, on the other hand, require expensive high-vacuum technology to produce, Clarke said.
The researchers are now working on optimizing the device — for example by incorporating thinner elastomers so that the required voltages can be lower. Otherwise, Clarke said, he foresees no major obstacles to commercializing the technology.
"How competitive it's going to be in particular markets, we don't know," he said. "But this is a rather different way of making a material opaque, so it's worth exploring."
Funding: The research was supported by the National Science Foundation through grant CMMI-1333835 and in part by the MRSEC program of the National Science Foundation under award number DMR 14-20570.
Paper: D. Clarke, S. Shian. “Electrical Tunable Window Device,” Optics Letters 41, 1289-1292 (2016).
About Optics Letters
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