Nature offers clues about staying cool or warm, like how white cumulus clouds reflect sunlight or polar bear hairs trap heat. A metasurface, a tiny engineered surface, now copies this by switching between states to control temperature. This surface can cool or heat things without using extra energy, which is useful for buildings, clothes, and security tools.
A metasurface is a layer made of tiny structures smaller than a human hair, designed to change how light and heat behave. In its white state, it bounces sunlight back. In its grey state, it absorbs sunlight to create heat. Both states stay hidden from infrared sensors, which detect heat, because the surface emits little heat itself. This is different from regular paints that glow in heat vision.
Hidden heat control
Traditional white paints, like those with titanium dioxide, scatter sunlight but only work well in shade or at night. They also show up brightly on thermal cameras because they emit heat in the 8–13 micrometer range. The new white metasurface uses disordered metallic nanostructures to scatter sunlight while keeping heat emission low, making it invisible to heat sensors even in sunlight. This is a big step forward for cooling without being detected.
The grey state of the metasurface absorbs sunlight better than black materials but avoids emitting detectable heat. This could help make smart clothes that adjust temperature or building walls that switch between heating and cooling. The surface works by using scattering, absorption, and polarizonic reflection, which are ways light and heat interact with materials.
Researchers at Aalto University created this technology, seeing it as a way to improve energy-efficient designs and security. Future plans include adding layers that change with electricity or temperature for real-time control. The researchers have described the methods and results if this study in a paper published in Advanced Materials.