ETHZ develops gold coating to prevent fogging of glasses

The coating developed by ETH Zurich could be used on eyeglass lenses and car windows, among other things. The researchers in the groups led by ETH professors Dimos Poulikakos and Thomas Schutzius emphasize that they are using a technically simple coating method that is widely used in industry. In this process, tiny amounts of gold are vapor-deposited onto the surface in a clean room under vacuum. What's special about the new coating is that it selectively absorbs solar radiation. "Our coating absorbs a large proportion of infrared radiation and thus heats up - by up to eight degrees Celsius," explains ETH doctoral student Iwan Hächler, who played a key role in driving the development. Radiation in the visible range, on the other hand, it lets through. This is the reason why the coating is transparent. The new coating uses a different approach than conventional anti-fogging methods such as anti-fog sprays: Conventionally, surfaces are often coated with water-attracting (hydrophilic) molecules. This causes condensed water to spread evenly over the surface. The new method, on the other hand, heats the surface, preventing moisture from condensing on the surface. The new coating is passive and requires no additional energy when the sun shines.

ETH professors Poulikakos and Schutzius and their teams have been working for several years on surface coatings that heat passively. The scientists published an initial research paper on a gold coating that prevents transparent surfaces from fogging three years ago. The coating now presented has several advantages over the first: It is made up of fewer layers and is significantly thinner. This makes it more transparent and flexible. It is also more efficient and transparent because it selectively absorbs infrared. The researchers emphasize that although gold is expensive, so little of it is needed that the material costs are nevertheless low. The coating is based on the sandwich principle. Very small and extremely thin clusters of gold are sandwiched between two layers of titanium dioxide, an electrically insulating material. Due to their light refraction properties, these two layers increase the efficiency of heat recovery. In addition, the upper titanium dioxide layer serves like a varnish to protect the gold layer from abrasion. The entire sandwich coating is only ten nanometers thick. By comparison, gold leaf is about twelve times thicker. The individual gold clusters touch each other slightly at the sides. This makes the gold layer electrically conductive. Without sunlight, it would therefore still be possible to heat the coating with electricity. The researchers will now further develop the coating for applications. In doing so, they will also investigate whether other metals are just as suitable as gold.