Scientists create thermal-invisible surfaces by mimicking clouds

In the sun's midday glare, a white cumulus cloud bounces light off its surface, cooling its interior. But in infrared photography, it nearly disappears. That natural magic trick—cooling by bouncing sunlight off without disguising itself from heat sensors—has now given rise to a breakthrough in surface engineering.

Scientists have developed a material that replicates this phenomenon, shifting from a bright, cooling white to a warm, absorbing grey, but not detectable by thermal cameras.

This new nanoscale technology can change everything from energy-efficient building materials to military camouflage technology and wearable tech. The work has been carried out by researchers at Aalto University with German collaborators. It is a development in thermal regulation without the use of electricity, via color, light, and nanotechnology.

Likewise, natural clouds become gray when weather conditions change, this artificial material as well changes to reflect or absorb sunlight. It is a disordered plasmonic metasurface and is produced via a special nanofabrication method that builds tiny structures out of metals.

When it is white, the metasurface scatters sunlight in many directions, similar to how clouds diffuse light. That strong backscattering lowers the surface temperature below by up to 10°C from that of standard materials, even in direct sun. But white paint or standard cooling coatings do radiate in heat vision. That is because they emit a great deal of infrared radiation.

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

When it transforms to a light-colored surface, it reflects light instead. With a nanocomposite absorber, the coating captures sunlight quite well, warming up faster than even black surfaces. And again, it does this without emitting detectable heat. The surface temperature can rise 10°C above normal black coatings under identical conditions. That makes it useful for applications needing speedy heat-up—like outdoor sensors or cold-weather clothing.

Standard white paint illuminates surfaces by reflecting sunlight but is easily detectable with infrared. Standard coatings made from titanium dioxide operate only in the shade or at night. This new metasurface, however, operates in direct sunlight and cannot be detected with thermal cameras.

Adel Assad, one of the doctoral students involved in the research, explains: "This new white plasmonic metasurface scatters sunlight through disordered metallic nanostructures and quenches thermal emission—cooling surfaces in direct sunlight and remaining thermally camouflaged. This feature makes the innovation revolutionary."

In contrast, dark materials absorb heat well but do not emit infrared radiation well, lighting up brightly on heat-sensitive cameras. The grey state of this novel surface does the opposite. "This grey surface warms up more than black—but without sending out heat that can be picked up by heat sensors," explains postdoctoral researcher Moheb Abdelaziz. "This has the potential to transform smart textiles, building materials, and camouflage."

The product is made in a chamber using a process called sputtering, a type of physical vapor deposition. The process eliminates the use of toxic chemicals typically used in nanomaterial production. It also enables uniform, large-scale manufacturing.

With all-in-chamber nanofabrication, it is guaranteed that the surface will not change over time. That's critical for prolonged applications in buildings or outside technology. The method of fabrication also works with existing industrial equipment, so it might hit the market sooner.

The scientists were motivated by nature—clouds, polar bear hair, and aerosols in the atmosphere. In all these systems, there are special ways of dealing with heat and light to survive harsh environments. Now, those principles are guiding new technologies on Earth.

The impact of this new metasurface extends far from the laboratory. Adaptive building facades are among the possible applications. These surfaces could make buildings cool in the summer by reflecting light and warm in winter by facing inward to trap heat—all without electricity or moving components.

Another area of study of interest is smart textiles. The material could be used to make clothing that keeps wearers warm or cool, as necessary. The same is employed in tents, shelters, or even packaging materials needed to protect against temperature fluctuations.

In security and defense, the technology gives rise to thermal invisibility. Such equipment and materials coated with this coating may remain undetectable to infrared sensors and yet be able to control their own temperatures. That would help in reconnaissance, transport, and stealth operations.

Sensors required to work at certain temperatures would benefit too. A surface that holds in or out heat without having to be controlled externally would conserve energy and simplify design.

This advance wasn't a sure thing. The scientists began developing their project without specific funding after an initial rejection. Instead of giving up, however, they focused on collaboration, especially with partners in Germany. Together, they turned a vague idea into actual results.

"We had no particular funding except for early setbacks, so we depended on mutual vision and collaboration—especially with our partners in Germany—to turn skepticism into discovery," says Elbahri. "It's a sign that science, like clouds, can defy probability."

The researchers now want to explore how to invert the surface in real time.

One potential method is to utilize electrochromic or phase-changing films to allow a user to quickly switch between white and grey states. That would offer even more potential for personal or remote-operated usage. The study illustrates how meticulous attention to nature can yield robust technology.

As climate and energy concerns mount, new materials like these could characterize a cooler, more efficient world.

Research findings are available online in the journal Advanced Materials.

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