Low-cost smart glass developed that switches between opaque and transparent

Eco-friendly smart glass technology that can switch from transparent to opaque has been developed at the University of Delaware and is a fraction of the cost to produce compared to competing products.

This glass could be utilised in eco-friendly windows, windshields, roof panes and building envelopes, absorbing light and heat in the winter and reflecting it away in the summer.

Associate professor Keith Goossen, who led development on the project, said that although he wasn’t the first scientist to make smart glass, his team’s invention is about one-tenth the price of other versions.

It is also more transparent in its transparent state and more reflective in its reflective state than competitors.

The principles behind this smart glass technology are surprisingly simple. It starts with two sheets of plastic separated by a thin cavity. The plastic contains tiny cube-shaped structures that make the material retroreflective, meaning that it bounces light back to its source, like a bicycle reflector does.

Then the chamber is filled with a fluid called methyl salicylate - an inexpensive wintergreen extract that is the same active ingredient as found in some over-the-counter pain relief creams.

This liquid has optical properties, or interaction with visible light, that match the optical properties of the retroreflective plastic. When combined, the light can pass through and the system becomes transparent. This is called refractive index matching.

Goossen’s smart glass system can switch from transparent to reflective a thousand times without degrading.

He recently demonstrated an improved design which, instead of utilising cubes, relies on the total internal reflection of one-dimensional structures layered perpendicularly. It is highly reflective at up to a 60-degree angle of incidence, an improvement over the previous prototype.

“It performed better than we thought it would based on our theoretical understanding,” he said.

Goossen uses 3D printing to make his prototypes, but this technology could eventually be manufactured at a high volume and low cost using injection moulding.

He is now testing his system over a wide range of temperatures to see how it performs, especially as it approaches temperatures that could cause the fluid within to freeze, which will be between -16°C to -8°C, depending upon the eventual fluid that is used.

“There is a lot of interest in the capability this might represent,” he said.

In 2016, British researchers developed self-cleaning smart glass that can limit glare by utilising principles found in the eyes of moths. 

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