Imagine if your car's windshield glass could turn from being clear to reflective, when the car was parked in the hot sun. Or perhaps if there were rooftop panels that reflected unwanted solar heat in the summer, but turned clear to let it in, in the winter. Well, such things may soon be possible, thanks to a new liquid-activated "smart glass" being developed at the University of Delaware.

The glass consists of a sheet of 3D-printed plastic, over top of which is a regular sheet of clear glass – there's a thin gap between the two.

When there's nothing but air in that gap, the pattern on the plastic causes it to be retroreflective, not unlike a bicycle reflector. Once an inexpensive fluid known as methyl salicylate is pumped in to fill the gap, however, the plastic turns transparent. This is because the fluid matches the refractive index of the plastic, essentially cancelling out its retroreflective quality.

While there is already glass that can change between clear and tinted states electronically, the researchers believe that their easily-made glass would cost about one tenth as much to manufacture. Additionally, while dark tinting may help to block the sun's rays, it doesn't reflect them. That could result in some hot windshields, if it were utilized in cars.

"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," says Keith Goossen, who led the research along with Daniel Wolfe. "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."

In its current form, the smart glass can stand up to thousands of reflective/transparent cycles without any degradation. Not all of the fluid does drain off of the plastic between cycles, however – a hydrophobic coating could help in that regard.

Goossen and Wolfe are now building an office door with a switchable privacy panel/window, to further demonstrate the applications of the technology. A paper on their research was recently published in the journal Optics Express.