With nanotechnology against foggy eyes: Researchers have developed a sophisticated coating that can effectively prevent windows from fogging up with a "touch of gold" and with the help of sunlight. The gold nano-layer absorbs the infrared part of the light and converts it into heat. Visible radiation, on the other hand, is hardly affected, which means that the layer remains transparent. Manufacturing is comparatively simple, and the patent-pending concept opens up a variety of uses that go beyond glasses and windshields, say the scientists.
Everyone is probably familiar with this annoying effect: A fine film impairs vision through glasses, windshields and the like. This occurs when water condenses on the surface due to a drop in temperature or an increase in humidity. The resulting droplets then cause the clouded view. To prevent fogging, two methods have been used so far: Certain substances in anti-fog sprays are intended to prevent the formation of fine droplets. It is also known that heat can cause them to disappear. In this way, the condensation in the car is also removed: The windshield is heated by the heater fan and the rear window is brought to higher values by fine heating wires.
Nanotechnology as an alternative
However, these concepts require effort or a connection to an energy source. A research team from the Swiss Federal Institute of Technology in Zurich (ETH) has been working on an alternative concept for several years now: They want to get rid of fogging with passively heated surface coatings. The scientists have already developed the first prototypes of their system, but only now have they significantly improved their concept and thus apparently made it ready for the market. It is a coating just ten nanometers thick that can be applied to different types of panes. It consists of a wafer-thin layer of gold particles embedded between two fine layers of titanium oxide.
The special optical properties of the gold nanolayer ensure the heating effect: it selectively absorbs the infrared portion of the incident radiation and converts it into heat. This has considerable potential, because half of the energy contained in sunlight is in the infrared spectrum. The gold nanolayer, on the other hand, allows the visible part to pass through largely unhindered - this allows the layer to remain transparent, the scientists explain. According to them, the layers of titanium oxide also contribute to the efficiency of the system: due to their light-refracting properties, they increase the effectiveness. In addition, the top layer serves as a protective layer against mechanical loads.
Effectively provided perspective
As the researchers report, tests prove the efficiency of the concept: "Our coating absorbs a large part of the infrared radiation and thus heats up by up to eight degrees Celsius above the ambient temperature," says first author Iwan Hächler from the ETH. This means that windows can be effectively protected from fogging under realistic conditions, experiments have shown. "The photo-induced thermal effect prevents fogging even in cloudy conditions," the scientists write. As Hächler emphasizes, the fear that a car or rooms could heat up more in summer due to the coating is unfounded: "The coating on the pane absorbs the infrared rays of the sun, which heats up the pane in a targeted manner and thus prevents the radiation from entering the interior of the car cars or a room. As a result, the interior heats up even less than without the coating,” says Hächler.
In addition to glasses, windshields and windows, there is further potential for the system, say the developers. Ultimately, it can be used wherever there is a need for anti-fog protection of transparent systems with little effort - for example in the case of optical sensors. The development is now continuing, but the ETH Zurich has already applied for a patent for the coating. As the team emphasizes, the rather unproblematic manufacturing process is also an important positive aspect with regard to practical use: the coating is produced using methods that are already widely used in production. The layers are applied to the surface in a clean room by means of vapor deposition in a vacuum. The expensive gold is used in such small quantities that the material costs remain low. For comparison: An ordinary gold leaf is twelve times thicker than the three-layer system, the developers emphasize.
Source: ETH Zurich, specialist article: Nature Nanotechnology, doi: 10.1038/s41565-022-01267-1