From hazy to clear, and back again: glass with switchable opacity

Using nanoscale grass-like structures, researchers at the University of Pittsburgh have created glass that lets through a large amount of light while still appearing hazy. This is the first time that glass has been made with such high levels of haze and light transmittance at the same time, according to the researchers.

Paul W Leu and his team developed the new glass to improve the ability of solar cells to capture light and turn it into power, working off the notion that nanostructure patterns can prevent light from reflecting off the solar cell’s surface. These structures also scatter the light that enters the glass, helping more of the light reach the semiconductor material within the solar cell.

The researchers utilised a unique pattern of nanostructures that looks much like grass, measuring anywhere from 0.8 to 8.5 µm in height. Because the structures are taller than previously used nanostructures, they increase the likelihood that light will be scattered. Although glass with the nanostructures appears opaque, tests showed that most of the scattered light makes its way through the glass.

The researchers found that shorter nanograss improved the antireflection properties of the glass while longer nanograss tended to increase the haze. Glass with 4.5 µm-high nanograss showed a nice balance of 95.6% transmittance and 96.2% haze for light with a 550 nm wavelength (yellow light, a component of sunlight). The results were published in Optica, the journal of The Optical Society.

Scattering ability of (a) flat fused silica and (b) 6 μm height nanograss glass. The scattering ability is demonstrated by shining a laser through a sample onto a target. The rings on the target are spaced 5 cm apart. The distance between the sample and target is 30 cm. Image courtesy of the study authors.

The glass additionally exhibits another remarkable quality, in that it can be switched from hazy to clear by applying water — the result of a serendipitous discovery by project lead Sajad Haghanifar.

“I was cleaning the new nanograss glass when I discovered that cleaning it with water made the glass become clear,” said Haghanifar.

“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 also showed that in addition to water, applying acetone and toluene can also switch the glass from hazy to clear.

This switchability could make the glass useful for creating smart windows that change haze or opacity to control the privacy of a room or to block glare from sunlight — it would simply require placing a piece of traditional glass over the nanograss glass. Pumps could be used to flow liquid into the space between the two glasses, and a fan or pump could be used to remove the water.

Optical images of smooth glass and glass with 2.5 and 6 μm height nanograss when placed (a) directly on paper with text and (b) about 1 cm above. The top image shows that text can be read through normal flat glass, while the glass etched with nanostructure scatters light, making the glass appear opaque. Image credit: Sajad Haghanifar, University of Pittsburgh.

The fact that the glass is highly hazy and exhibits high transmittance could also make it useful for LEDs, which work in a way that is essentially the opposite of a solar cell — by using electricity that enters a semiconductor to produce light that is then emitted from the device. The glass could potentially increase the amount of light that makes it from the semiconductor into the surroundings.

Although more work is needed to estimate the exact cost of manufacturing the new glass, the researchers predict that their glass will be inexpensive because it is easy to make. The nanostructures are etched into the glass using a process known as reactive ion etching, a scalable and straightforward method commonly used to make PCBs.

“Switchable glass available today is quite expensive because it uses transparent conducting layers to apply a voltage across the entire glass,” said Leu. “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.”

“We are now conducting durability tests on the new nanograss glass and are evaluating its self-cleaning properties,” said 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.”

Top image caption: New glass etched with nanograss structures can be switched from hazy to clear by applying water. As shown here, removing the water from the glass makes it appear hazy again. Image credit: Sajad Haghanifar, University of Pittsburgh.