ºÚÁÏÍø

How does a cloud stay cool under direct sunlight –– or seem to vanish in infrared? In nature, phenomena like white cumulus clouds, grey storm systems, and even the hollow hairs of polar bears offer remarkable lessons in balancing temperature, colour and invisibility. Inspired by these atmospheric marvels, researchers have now created a nanoscale ‘cloud’ metasurface capable of dynamically switching between white and grey states — cooling or heating on demand –– all while evading thermal detection.

There is a major global push for passive, energy-efficient thermal management in building materials, wearables, sensors and defence applications. This newly invented system fits perfectly into emerging fields like radiative cooling, adaptive coatings, and thermal heating and thermal camouflage under climate and security pressures.

Much like the transformation from bright cumulus to dark cumulonimbus clouds, this metasurface uses multiple scattering, absorption and polarizonic reflection principles to modulate light and heat. In its ‘white’ state, it strongly backscatters sunlight to enable radiative cooling, while the ‘grey’ state absorbs sunlight efficiently for high-performance heating. Crucially, both states remain ‘invisible’ to infrared sensors due to low mid-infrared emissivity — something no previous surface has achieved.

‘We’ve engineered a nanoscale cloud on every surface. It can tune its colour and temperature like a real cloud — between cooling white and heating grey — while staying hidden from thermal cameras,’ Professor Mady Elbahri from Aalto University explains.

Both white and grey metasurfaces overcome limitations of traditional coatings

Typical white paints cool surfaces by scattering sunlight in all directions, but they still glow in heat vision. This new material works more like a cloud — cooling by bouncing sunlight back and staying hidden from heat sensors.

Conventional white coatings (e.g., titanium dioxide, TiO₂ based) scatter sunlight diffusively, but are only effective in shaded conditions or at night. Their high emissivity in the 8–13 μm range makes them bright in thermal infrared imaging, limiting use in thermal stealth.
‘This new white plasmonic metasurface scatters sunlight through disordered metallic nanostructures while minimising thermal emission — cooling surfaces in full sunlight and remaining thermally camouflaged. This feature makes the innovation groundbreaking,’ says Adel Assad, a PhD student in the group.

Black materials get hot in the sun but also light up thermal cameras as they emit infrared strongly. 
‘This grey surface gets hotter than black—but without sending out heat that can be seen by heat sensors. This could be a game-changer for smart textiles, building materials, and camouflage, says Moheb Abdelaziz, a postdoctoral researcher in the group.

Great potential grows from humble beginnings

The research opens new pathways in adaptive surface engineering. Potential applications span from zero-energy building facades that switch between heating and cooling to smart textiles that regulate body temperature without electronics. The discovery also presents opportunities in low-visibility sensors and devices for defence and surveillance.

The next step for the research is to explore dynamic coatings using electrochromic or phase-changing layers for real-time, user-controlled switching between states.

The researchers are proud that the remarkable findings came despite an initial project rejection. 

‘With no dedicated funding after initial setbacks, we relied on shared vision and collaboration –– especially with our partners in Germany –– to turn doubt into discovery. It's proof that science, like clouds, can rise against the odds,’ says Elbahri.

The research was published on  in June 2025.