Passive coating generates heat to defog glasses and goggles

Just a few nanometres thick, the coating developed at ETH Zurich is made of gold nanoparticles embedded in non-conductive titanium oxide.

“Our coating absorbs the infrared component of sunlight along with a small part of the visible sunlight and converts the light into heat,” said Christopher Walker, a doctoral student in ETH Professor Dimos Poulikakos’s group and lead author of the study. This heats the surface up by three to four degrees Celsius, which prevents fogging.

Efstratios Mitridis, also a doctoral student in Poulikakos’s group, said: “Normally, it’s dark surfaces that absorb light and convert it into heat, but we’ve created a transparent surface that has the same effect.”

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, 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.