Tunable Windows for More Privacy

An applied voltage causes light to scatter, turning the glass opaque. (Source: D. Clarke / Harvard SEAS)

An applied voltage causes light to scatter, turning the glass opaque. (Source: D. Clarke / Harvard SEAS)

Resear­chers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences have 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.

Tunable windows aren’t new but most previous techno­logies have relied on electro­chemical reactions achieved through expensive manu­facturing. This techno­logy, developed by David Clarke, the Extended Tarr Family Professor of Materials, and post­doctoral fellow Samuel Shian, uses geometry adjust the trans­parency of a window.

The tunable window is comprised of a sheet of glass or plastic, sand­wiched between transparent, soft elas­tomers sprayed with a coating of silver nanowires, too small to scatter light on their own. But apply an electric voltage and things change quickly. With an applied voltage, 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 roughness causes light to scatter, turning the glass opaque.

The change happens in less than a second. “It’s like a frozen pond,” said Shian. “If the frozen pond is smooth, you can see through the ice. But if the ice is heavily scratched, you can’t see through,” he said. Clarke and Shian found that the rough­ness of the elastomer surface depended on the voltage, so if you wanted a window that is only lightly clouded, you would apply less voltage than if you wanted a totally opaque window.

“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,” said Clarke. Current chemical-based control­lable windows use vacuum depo­sition to coat the glass, a process that deposits layers of a material molecule by molecule. It’s expensive and pain­staking. In Clarke and Shian’s method, the nanowire layer can be sprayed or peeled onto the elastomer, making the techno­logy scalable for larger archi­tectural projects.

Next the team is working on incor­porating thinner elastomers, which would require lower voltages, more suited for standard electrical supplies. Harvard’s Office of Techno­logy Deve­lopment has filed a patent appli­cation on the techno­logy and is engaging with potential licensees in the glass manu­facturing industry. (Source: Harvard SEAS)

Reference: S. Shian et al.: Electrically tunable window device, Opt. Let. 41, 6, 1289-1292 (2016), DOI: 10.1364/OL.41.001289, online 15 March 2016

Link: Clarke Research Group (D. R. Clarke), A Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA

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