Scientists invent glass that can turn from clear to opaque in less than a second

The glass, developed at Harvard, can transition between transparent, translucent and opaque with the application of an electric current.

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Demonstration of the glass.

David Clarke/Harvard SEAS

Do drapes get you angry and blinds make you morose? What you need is a window that offers clear views or privacy with the touch of a switch.

One alternative is tunable glass, which goes from transparent to opaque through the use of nanocrystals and metallic nanostructures. These are, however, somewhat costly and difficult to manufacture.

Professor David Clarke and postdoctoral fellow Samuel Shian at Harvard John A. Paulson School of Engineering and Applied Sciences claim that their new method could be much cheaper to manufacture. The researchers have described it in a paper published in the journal Optics Letters.

Their technique uses a sheet of glass sandwiched between two sheets of clear, soft elastomer. These have been sprayed with a coating of silver nanowires. This is what makes the manufacturing so simple. Previous methods of creating tunable windows are manufactured using a technique called vacuum deposition, which painstakingly deposits molecular layers of material.

"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 said in a statement.

On their own, the nanowires are too small to catch the light in a way that makes them visible to humans. However, if you run an electric current through the window, the nanowires will all move, trying to get closer to each other.

This causes the elastomer sheet to deform in such a way that it scatters the light, rendering the surface cloudy in less than a second. With a strong enough current, the window becomes completely opaque, the researchers said.

Shian compared the deformed elastomer surface to a coating of ice on a pond.

"If the frozen pond is smooth, you can see through the ice," he explained. "But if the ice is heavily scratched, you can't see through."

The university has filed a patent on the technology, and Clarke and Shian are working toward developing an even thinner elastomer films, which would lower the voltage needed to make the glass opaque.

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