Smart Window Switches From Clear to Reflective

A prototype smart glass that is retroreflective and becomes clear when a liquid with optical properties similar to the reflective structure is pumped into a chamber in front of the structure. (Source: K. Goossen, U. of Delaware)

Researchers have demonstrated proto­type windows that switch from reflective to clear with the simple addition of a liquid. The new switchable windows are easy to manufacture and could one day keep parked cars cool in the sun or make office buildings more energy efficient. The technology can also be used to make roof panels that keep houses cool in the summer and warm in the winter. Although glass that uses an applied voltage to switch from clear to an opaque or tinted state is commer­cially available, its high cost around $100 per square foot has hindered wide­spread use.

“We expect our smart glass to cost one tenth of what current smart glass costs because our version can be manu­factured with the same methods used to make many plastic parts and does not require compli­cated electro-optic tech­nology for switching,” said Keith Goossen, who led the research team with Daniel Wolfe of the Univer­sity of Delaware. The new smart windows contain a plastic panel with a pattern of structures that is retro­reflective. This means that rather than reflec­ting light in all directions like a mirror, it reflects light back in the direction it came from like a bicycle reflector.

The researchers demonstrate a proto­type of the new smart glass consisting of a 3D printed plastic panel covered by a thin chamber. When the chamber is filled with the fluid methyl sali­cylate – which matches the optical properties of the plastic – the retro­reflective structures become transparent. “Although we had to develop new ways to process 3D printable plastics with good optical performance, develop inex­pensive refrac­tive index-matching fluids and come up with highly reflec­tive optical structures, the inno­vation here is mostly in recognizing that such a simple concept could work,” said Goossen.

One of the most promising appli­cations for the new switchable glass may be in cars, where it could be used to change the windshield to a reflective state when the car is parked in the hot sun. “You can’t use today’s commer­cially available switchable glass for this appli­cation because in the darkened state the windshield still absorbs sunlight and becomes hot,” said Goossen. “Because our glass is retro­reflective in the non-transparent state, almost all the light is reflected, keeping the glass, and thus the car, from getting hot.”

The fact that the glass is retro­reflective means that if it were used on the outside of skyscraper, for example, it would direct light up toward the sun rather than down to the street. This reduces the building’s contri­bution to city warming, which is a problem in many urban areas. The researchers have also shown that the plastic retro­reflective panels can be used as an inex­pensive switchable roofing structure that cuts down on heating and cooling costs. In places that are warm and sunny year-round, white roofing materials have been shown to lower cooling costs by reflecting sunlight. However, in areas with cold winters, these white roofs prohi­bitively add to heating costs in the winter.

“Here in Delaware, you would like to have a white roof in the summer to keep the house cool and a dark roof in the winter to absorb sunlight and help lower heating costs,” said Goossen. “For smart roofing, our new tech­nology offers a more effective type of cool roof because it is retro­reflective while also allowing the roof to switch to dark in the winter.” For roofing appli­cations, a layer of material placed under the panels is used to absorb light when the panels are in their clear state. This helps keep the house warmer when outside tempera­tures are cold. Although the methyl salicylate used in the proto­type could freeze in very cold climates, freeze-resistant fluids could be developed.

To make the new switchable glass, the researchers started by using 3D printing to make plastic panels with repeating retro­reflective structures of various sizes for testing. They used a commer­cially available clear 3D printable material and developed post-pro­cessing steps to ensure the plastic remained highly transparent after printing and exhibited very accurate corners, which were important to achieve retro­reflection. “Without 3D printing, we would have had to use a molding tech­nology, which requires building a different mold for every different structure,” said Goossen. “With 3D printing, we could easily make whatever structure we wanted and then run experiments to see how it performed. For commer­cial production, we can use standard injection molding to inex­pensively make the retro­reflective panels.”

Once the researchers figured out the optimal size to use for the repeating structures, they performed optical testing to determine whether charac­teristics such as surface roughness or the material’s light absorp­tion would cause unex­pected optical problems. These optical tests showed that the structures worked exactly as indicated by optical simu­lations. “Importantly, we also demon­strated that the device can undergo thousands of cycles from transparent to reflec­tive without any degra­dation,” said Goossen. They did, however, find that some fluid stays on the structure instead of draining off. To solve this issue, the researchers are deve­loping coatings that will help the fluid drain off the plastic without leaving any residue. “To further demonstrate the tech­nology’s usefulness as switchable glass, we are building an office door that incor­porates the new smart glass as a switchable privacy panel,” said Goossen. “These types of panels are currently made with much more expen­sive tech­nology. We hope that our approach can broaden this and other appli­cations of smart glass.” (Source: OSA)

Reference: D. Wolfe & K. W. Goossen: Evaluation of 3D printed optofluidic smart glass prototypes, Opt. Exp. 26, A85 (2018); DOI: 10.1364/OE.26.000A85

Link: Dept. of Electrical and Computer Engineering, Univ. of Delaware, Newark, USA

 

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