Next-gen coating mimics clouds to manage heat, evade detection

Researchers at Finland’s Aalto University have engineered a wafer-thin “cloud” metasurface that can flip between bright white and deep grey, shifting a surface from powerful daytime cooling to rapid solar heating while remaining almost invisible to infrared cameras. The invention, achieved without dedicated project funding, could reshape how buildings, vehicles, and textiles manage heat and hide from thermal sensors.

Unlike conventional white paints that scatter sunlight but glow hot in thermal imaging, the new white state of the plasmonic metasurface bounces sunlight back into space while keeping mid-infrared emissivity so low that heat cameras see almost nothing. 

Switch the same film to its grey state, and the material absorbs sunlight more efficiently than matte black, yet still refuses to radiate tell-tale warmth. Inspired by how cumulus clouds brighten or darken as they evolve, the dual behavior answers a growing demand for passive, energy-free solutions that can cool, heat, or camouflage on command.

Cloud physics on a chip

The metasurface owes its versatility to an ensemble of disordered metallic nanostructures that manipulate light through multiple scattering, strong absorption, and what the team calls “polarizonic reflection.” In the white state, solar photons ricochet through the labyrinth of nanoscale features and back into the sky, providing radiative cooling under full sun. 

In the grey state, those same nanostructures trap visible light and convert it to heat. Crucially, both modes keep emissivity in the 8–13 micron range, where most thermal cameras peer, well below the threshold that would reveal the material.

“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 said.

Traditional coatings face a trade-off, Elbahri’s group notes. White titanium dioxide paints cool reasonably well in shade but lose their edge under direct sun and glow brightly in thermal imaging. Black surfaces absorb but radiate heat as efficiently, lighting up infrared sensors. The new metasurface sidesteps both problems.

Cooling white, heating grey, and both are invisible

Graduate student Adel Assad, who helped fabricate the coatings, contrasted the approach with today’s best “cool white” paints. “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,” he said.

In tests, the grey mode reached temperatures higher than conventional black coatings but still emitted little infrared. “This grey surface gets hotter than black, but without sending out heat that heat sensors can see. This could be a game-changer for smart textiles, building materials, and camouflage,” added post-doctoral researcher Moheb Abdelaziz in an article on the university website.

Potential applications span zero-energy façades that swap from cooling to heating with the seasons, garments that keep wearers comfortable without electronics, and low-visibility drones or sensors that need to evade infrared detection. Because the metasurface is only a few hundred nanometres thick, it could be deposited on steel panels, polymer films, or even fibres without adding weight.

Next steps and a lesson in persistence

The team aims to integrate electrochromic or phase-change layers so users can trigger the white-to-grey transformation in real time, perhaps through a small voltage or environmental cue. The researchers also plan durability studies under UV exposure, humidity, and mechanical stress to qualify the coating for outdoor use.

Elbahri said the breakthrough rarely happened. “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,” he reflected.

If the technology moves from lab benches to factory lines, surfaces could soon act like programmable skies, reflecting, absorbing, or concealing heat as effortlessly as a passing cloud.

The findings were published in the journal Advanced Materials in June 2025.

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