With blinds and curtains in front of our windows, we usually have to choose between light and privacy, but inching ever closer to reality is "smart glass," which can toggle its opacity as needed. Electricity is usually the trigger, but now a University of Pittsburgh team has developed a new type of glass that can switch between hazy and clear in seconds – just add water.

Previous smart glass systems use electrodes, electrochemical reactions or silver nanowires to change their opacity or tint, but these setups can be expensive to produce, tricky to install and take several minutes to make the transition.

The Pittsburgh design, on the other hand, doesn't need any electronic help – the glass gets its cloudy look from its nanoscale structure, which resembles blades of grass. Not surprisingly, its inventors call it nanograss, and it works by scattering light that passes through, rendering the material opaque.

After experimenting with the height of the grass, the researchers settled on a height of 4.5 microns, which allowed a record 95.6 percent of light to pass through while still retaining a 96.2 percent haze, for yellow light.

The ability of the nanograss glass to turn clear and back again was actually discovered by accident, according to the team.

"I was cleaning the new nanograss glass when I discovered that cleaning it with water made the glass become clear," says Sajad Haghanifar, lead researcher on the project. "The water goes between the extremely hydrophilic nanostructures, making the nanograss glass act like a flat substrate. Because water has a very similar index of refraction to the glass, the light goes straight through it. When the water is removed, the light hits the scattering nanostructures, making the glass appear hazy."

The researchers say the versatile glass could have a number of applications. Originally, the nanostructure was designed to improve the efficiency of solar cells, allowing them to reduce reflection off the surface and guide more light into the semiconductor material. In the same way but the opposite direction, the nanograss glass could help guide more light out of an LED.

And of course, its ability to switch could be handy for smart windows. To do that, the team says a pane of normal glass could be laid over the top of the nanograss glass, and water or another liquid could be pumped in or out of the space between them, to switch the transparency on or off.

"Switchable glass available today is quite expensive because it uses transparent conducting layers to apply a voltage across the entire glass," says Paul W. Leu, leader of the research team. "Our glass would be potentially less expensive to make because its opacity can be switched in a matter of seconds by simply applying or removing liquid."

The other advantage is in the manufacturing. The nanograss structures are carved into the glass through a process called reactive ion etching, which is already in wide use to make printed circuit boards.

"We are now conducting durability tests on the new nanograss glass and are evaluating its self-cleaning properties," says Haghanifar. "Self-cleaning glass is very useful because it prevents the need for robotic or manual removal of dust and debris that would reduce the efficiency of solar panels, whether the panels are on your house or on a Mars rover."

The research was published in the journal Optica.

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