Metamaterial coating: Swiss invention helps against fogged glasses
Swiss researchers have developed a coating that uses sunlight to heat eyeglass lenses.
With perspective: The left lens has the new anti-fog nanocoating.
(Bild: ETH Zürich)
(Hier finden Sie die deutsche Version des Beitrags)
It can be quite frustrating when lenses fog up - a problem that primarily affects spectacle wearers with mouth-nose protection, but also photographers and cameramen. Researchers at ETH Zurich have now developed a metamaterial coating that absorbs infrared radiation, warming the lens by up to eight degrees relative to its surroundings. Iwan Haechler and colleagues describe the technical details in a recent paper in Nature.
Because warm air can absorb more water than cold air, humidity condenses on cold surfaces. Due to the high surface energy of glass, warm air saturated with water vapor that hits cold eyeglass lenses finds almost perfect conditions for good adhesion of water droplets. In previous work, researchers have used nanostructuring or coatings with hydrophilic, water-repellent molecules to try to keep the contact area of the droplets on the glass as small as possible so that the droplets roll off. However, such coatings quickly become ineffective due to contamination.
A touch of gold for the glasses
The idea of absorbing the infrared rays in sunlight that are invisible to the human eye is therefore actually quite obvious. After all, photonic filters in the form of thin layers are already routinely applied to eyeglass lenses today - and what works for reflection can also be used for absorption. However, this works really well only in a narrow wavelength range.
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The Swiss researchers therefore vaporized an extremely thin layer of gold nanoparticles on a base of titanium dioxide. In the process, they allowed gold particles to form on the surface until the individual particles began to fuse together - the so-called percolation boundary. The perforated gold layer, which is about ten nanometers thick and covered with a second titanium dioxide layer, absorbs near-infrared radiation in a broad spectral range, while allowing visible light to pass through unhindered.
At a solar irradiation of 1000 watts per square meter - a value that can certainly be reached on sunny days - the coating heats lenses to around eight degrees above the ambient temperature. At 600 watts, the temperature is still about five degrees. However, it takes several minutes for the lenses to heat up sufficiently. The researchers have applied for a patent, but so far there are no companies that want to adopt this invention.
(wst)
