A switchable window – one that transforms from a clear to tinted state – is not a new invention.

What is new, however, is a “smart glass” that is low-cost.

Engineers from the University of Delaware developed liquid-activated panels that change from transparent to opaque – an achievement that the team hopes will provide a more affordable, commercial appeal to consumers.

Imagine a window that absorbs heat in the summer and reflect the rays away in the summer.

In addition to windows, Keith Goossen, associate professor of electrical and computer engineering at the university, sees the smart glass someday supporting new kinds of eco-friendly windshields, building envelopes, and roof panes.

Goossen shared his latest smart glass prototype on Monday, March 5 in a keynote address at the SPIE Smart Materials and Nondestructive Evaluation for Energy Systems IV conference in Denver.

The smart glass has a relatively simple design: two plastic sheets separated by a thin cavity. The plastic contains tiny, retroreflective, cube-shape structures, bouncing light back to its source like a bicycle reflector.

The panel, whether a windshield or window, is reflective when dry. Once the chamber is then filled with a fluid called methyl salicylate — an inexpensive wintergreen extract with similar optical properties to the retroreflective plastic — the facet reflectivity disappears, the light passes through, and the panel becomes transparent.

Two new smart-glass panel prototypes. (Image Credit: University of Delaware)

According to a 2016 paper published in the journal Optics Express, Goossen’s 3D-printed smart glass system can switch from transparent to reflective a thousand times without degrading.

So, what is preventing mainstream adoption of smart glass? Goossen spoke with Tech Briefs and looked into some smart-glass possibilities.

Tech Briefs: What are the advantages of this kind of smart glass?

Prof. Goossen: The main advantage, we feel, will be cost. While current switchable transparency glasses, such as electrochromic glasses, cost thousands of dollars per square meter, we are targeting hundreds of dollars per square meter, which is the cost of the panel itself, and the pump and fluid.

Our low cost is due the simplicity of the system. Also, our smart glass has higher reflectivity in the reflective state, and greater transparency in the transparent state, than other smart-glass technologies.

Tech Briefs: What needs to happen for “smart glass” to catch on with consumers?

Prof. Goossen: We feel the main impediment to adoption of smart glass has been the cost. Consumers are simply unwilling to pay thousands of dollars for a switchable window. Thus, we feel that our smart glass will spur adoption.

Tech Briefs: What kinds of technology work needs to be done for the system to be commercialized?

Prof. Goossen: For our smart glass to be commercialized, particular issues of our optofluidic system need to be acceptable for the application. Depending on the final fluid adopted, the freezing point will be 3-16 °F. Thus, either [the system] will be restricted to applications at greater than those temperatures, or integral heaters will need to be used. Other issues are concern of leakage of the fluid. However, our system is not under fluid pressure, and the panels are integral and sealed, so we feel that we can establish that the system will not leak.

Tech Briefs: How does your glass improve upon previous designs?

Prof. Goossen: Some of our designs are retro-reflective in the reflective state. That means that a ray of light incident upon the panel will be reflected exactly back towards its source. That can be important in building envelope applications, since it will reflect sunlight back to the sun, which 1) lowers atmospheric absorption and climate change effects, and 2) avoids scattering to the streets, which would blind people.

Tech Briefs: What inspired you to do this work?

Prof. Goossen: My career has been mostly in optics, particularly a field of optics known as optical modulators. Optical modulators are devices that alter the transmission of light with an applied voltage. They are used, for example, to encode light pulses onto optical fiber. Such devices cost thousands of dollars and alter a fiber-thin beam of light. I wondered if one could cheaply modulate square meters of light, but switching in seconds rather than picoseconds. To modulate light, something has to move, whether it is an electron cloud in a crystal, or a shutter (like on ships for sending signals with a lamp). I wondered if moving a fluid could be effective.

Tech Briefs: What's next?

Prof. Goossen: We have been manufacturing our prototypes in a 3D printer, and are limited to approximately 8x10 inches. We need to transfer this to injection molding to make large panels. We already know what material system and fluid we will use, and it should actually have better optical performance than our 3D printer prototypes.

What do you think? Will this kind of ‘smart glass’ catch on? Send us your comments and questions below.