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A new kind of metasurface uses the sun to clear foggy screens

Misted windshields could become a thing of the past thanks to a clever material engineered on a microscopic scale.
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When a pair of spectacles fog up, it can be nuisance. But when a car windshield or an astronaut’s helmet fogs, the consequences can be fatal. That’s why cars and space suits have their own air-conditioning systems to remove or prevent fogging.

But air-conditioning is expensive, bulky, and environmentally unfriendly. So engineers and materials scientists are keen to find a way to prevent fogging more effectively.

Enter Christopher Walker and colleagues from ETH in Zurich, Switzerland, who have designed a new material with properties unlike anything found in the natural world. Their material—or metasurface, to give it its proper name—captures radiation from the sun and uses it to burn off any condensation or prevent it from forming in the first place. The result is an effective and relatively cheap way of tackling this insidious problem.

A metasurface is a material engineered to have surface properties that are not found in nature. They are often created using a repeating pattern of smaller units, such as nanoparticles.

In this case, the team create their surface by covering a silica sheet with gold nanoparticles and sealing them in place with a layer of titanium dioxide. They then repeated this process to create multiple layers.  

The nanoparticles absorb some of the sunlight that hits them, giving the glass a tint. But they also heat up, raising the surface temperature of the glass by up to 10 °C.

That’s the key to preventing fogging. The heat either prevents water from condensing or causes any water to evaporate.

An important question is whether this approach outperforms conventional anti-fogging technologies. The most common of these is to coat glass in a superhydrophilic or superhydrophobic material. These do nothing to prevent condensation, but they do change the size and behavior of the water droplets that form, often creating a continuous thin layer of water on the surface.

To find out how the metasurface compares, the team pitted it against an untreated surface and ones coated in superhydrophilic and superhydrophobic materials.

The results are persuasive. The researchers say their new metasurface significantly reduces the rate of condensation and increases the rate of evaporation compared with other materials.

And because the gold nanoparticles are protected by a titanium oxide layer, these surfaces are robust, too. “We believe this research will lead to more robust and enhanced passive antifogging and defogging surfaces,” they say.

That’s interesting work. The new metasurface is relatively easy to make, opening the way to large-scale manufacture on a wide range of materials such as glass and polymers. “This approach would result in considerable performance gains for applications such as windows, windshields, electronic displays, cameras, mirrors, and eyewear,” they say.

Of course, the material exploits sunlight, which raises the question of how it might work at night. The surface can still be wiped or cleared with fresh air. But the metasurface raises the possibility of other mechanisms, such as an artificial source of illumination. In the future, anti-fogging might as easy as switching on a light.

Ref: arxiv.org/abs/1904.02534 : Transparent Metasurfaces Counteracting Fogging by Harnessing Sunlight

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