Anyone who skis, wears glasses, uses a camera, or drives a car is familiar with the problem: Coming into a humid environment from the cold causes eyewear, camera lenses, or windshields to quickly fog up. Researchers have developed a new transparent material coating that greatly reduces this effect. A few nanometers thick, the durable coating is made of gold nanoparticles embedded in non-conductive titanium oxide.
The coating absorbs the infrared component of sunlight along with a small part of the visible sunlight and converts the light into heat. This heats the surface up by 3 to 4 °C. It is this difference in temperature that prevents fogging.
Condensation occurs on a surface whenever there is a sudden drop in temperature or increase in humidity, forming tiny droplets of water that disperse incident light in different directions in much the same way as atmospheric fog. As an alternative to using heat to prevent fogging, susceptible surfaces can be coated with hydrophilic agents. Because they attract water, these agents ensure that the condensation forms an even, thin film of liquid over the surface rather than separate droplets. Anti-fog sprays for glasses usually work on this principle.
Tests have shown that when exposed to sunlight, fogged surfaces coated with gold nanoparticles and titanium oxide clear four times faster than surfaces treated with a normal anti-fog agent. Spray treatments often lose their effect after a while because the anti-fog film dries up or becomes unevenly distributed.
The coating has a range of potential applications including car windshields and rear-view mirrors as well as ski goggles and diving masks. Heat is also the answer to the problem of fogging on car windows. Warm air from the in-vehicle heating system heats the front windshield, while the rear window is fitted with a grid of electrical heating elements. But unlike these methods, the new coating works passively. Since the only energy source required is the Sun, the coating is especially suitable for wearable items such as glasses and goggles.