Researchers from ETH Zurich have developed a new transparent gold nanocoating that harnesses sunlight to heat the lenses of glasses, helping prevent them from fogging in humid conditions.
Researchers in the group led by ETH Professors Dimos Poulikakos and Thomas Schutzius point out that their coating is fabricated with methods that are used extensively in manufacturing. In a cleanroom and using vapor deposition under a vacuum, tiny amounts of gold are deposited onto the surface of the lens. ETH Zurich has applied for a patent on the coating.
This new coating absorbs solar radiation selectively. Half of the energy contained in sunlight resides in the infrared spectrum, and the other half in the visible light and UV radiation spectrum. This new coating can help heat up the lenses by as much as 46 degrees Fahrenheit. The coating absorbs only a minor fraction of the radiation in the visible range, which keeps the coating practically transparent
The new coating takes an approach that differs from conventional antifogging methods. Anti-fog sprays and coatings traditionally, coat the lens with water-attracting (hydrophilic) molecules, which helps spread the condensation. This new method instead heats the surface, thus preventing humidity-induced condensation from forming there in the first place. It’s the same principle as is used for a car’s rear window.
Poulikakos, Schutzius, and their teams have been working on passively heated surface coatings for several years. Three years ago, the scientists published their first research paper on a gold coating that prevented transparent surfaces from fogging up. This new coating has many benefits over the first: It is made up of a significantly thinner single gold nanolayer that makes it more transparent as well as pliable. Further, it is also more transparent and efficient because it absorbs infrared light more selectively.
The new coating comprises extremely thin clusters of gold sandwiched between two ultrathin layers of titanium oxide, an electrically insulating material. Due to their refractive properties, these two outer layers increase the efficacy of the heating effect. The top layer of titanium oxide acts as a finish that protects the gold layer from wear. This whole “sandwich” is just 10 nanometres thick. By way of comparison, a common gold leaf is twelve times thicker.
The researchers will now develop the coating further for other applications. In the process, they will investigate whether other metals work just as well as gold. In addition to eyewear and windshields, this antifogging method could be used wherever objects must be both heated and transparent – such as windows, mirrors, or optical sensors.
This research project received funding from the Swiss National Science Foundation.
Hächler I, Ferru N, Schnöring G, Mitridis E, Schutzius TM, Poulikakos D: Transparent Sunlight-Activated Antifogging Metamaterials. Nature Nanotechnology 2022, doi: external page10.1038/s41565-022-01267-1
Related Research Briefing: Combating fogging with selective sunlight nano-absorbers. Nature Nanotechnology 2022, doi: external page10.1038/s41565-022-01269-z