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Aug 31, 2023

Optics & Photonics News

Stewart Wills Researchers at Shenzhen University, China, have reportedly created a “cooling film,” based on the model of the Morpho butterfly, that can produce a consistent blue color, while still

Stewart Wills

Researchers at Shenzhen University, China, have reportedly created a “cooling film,” based on the model of the Morpho butterfly, that can produce a consistent blue color, while still achieving radiative cooling 2°C below ambient temperatures. [Image: Wanlin Wang, Shenzhen University]

Amid increasing worries about climate change, interest has grown in radiative cooling—materials-based approaches that allow passive cooling of objects to below ambient temperatures, without energy-sapping air-conditioning systems. But a world of radiatively cooled objects promises to be a rather bland one. Most passive-cooling materials, not surprisingly, tend to be white in color, to absorb as little as possible of the solar spectrum.

A number of research groups are looking at ways to circumvent this logic and create radiatively cooling materials that also feature a bit of color. In one recent example, a team at Shenzhen University, China, has reportedly created a film in rich blue that, according to the team, can reduce the temperature of coated objects to a full 2°C below ambient levels (Optica, doi: 10.1364/OPTICA.487561).

The film achieves this feat by combining so-called structural color with control of the material’s absorption and emission profile across a range wavelengths. The bag of tricks includes, crucially, the use of a silver mirroring surface at the base of the structure that staves off heating from the sun by reflecting virtually all of the solar spectrum. And the team’s modeling suggests that that, by tweaking the materials further, the researchers could achieve passive cooling to more than 10°C below ambient temperatures—with no air conditioner in sight.

As classically conceived, radiative cooling takes advantage of the transparency of Earth’s atmosphere to radiation at mid-infrared (MIR) wavelengths between around 8 and 13 µm. By engineering materials to emit preferentially at these wavelengths, thermal energy can in principle be shunted through the atmosphere to radiate into the coldness of outer space—the ultimate heat sink. But a workable system requires careful control of other wavelengths, too. In particular, the system should absorb as little as possible of visible and near-infrared (NIR) light from the sun, to avoid externally heating the object.

The mechanics of radiative cooling have resulted in some innovative materials—including, recently, a two-layer “Janus” thermal cloak that reportedly can keep objects cool in the summer and warm in the winter. Yet in this case and many others, the radiatively cooling material is available only in basic white, to optimize the reflection of visible and NIR radiation from the sun.

To overcome this monochromatic picture, the Shenzhen University team, led by Guo Ping Wang, looked to create a radiatively cooling material with a dash of structural color. Such coloration schemes, seen in natural examples ranging from peacock feathers to beetle shells, achieve brilliant, saturated coloration through scattering and interference of light due to micro- and nanostructures embedded in scales, feathers and other features.

Like many researchers working in structural color, the Shenzhen team cites in particular the example of the blue Morpho butterfly. In contrast to structural-color examples such as peacock feathers, which offer a play of different, iridescent colors as the viewing angle changes, the Morpho’s wings glimmer in a brilliant, saturated blue that is consistent across a range of viewing angles. Previous work has tied these characteristics to a combination of a multilayer or grating structure in the insect’s wings, to create the structural color, and a sufficient degree of disorder or randomness to eliminate the color’s directional dependence.

The three-layer structure developed by the Shenzhen team includes a top multilayer to create structural color, a ground-glass mid-layer to diffusely reflect the color and reduce its angle dependence, and a bottom silver reflective layer to eliminate absorption of solar wavelengths and thus stave off solar heating. [Image: Wanlin Wang, Shenzhen University] [Enlarge image]

With those parameters in mind, the Shenzhen team set to work designing a material that could produce both a Morpho-like blue and cooling below ambient temperatures.

The configuration the researchers came up with is a three-layer sandwich. The top slice consists of a Morpho-style TiO2/SiO2 multilayer that produces the structural color. The researchers used numerical modeling to optimize the multilayer for high transmissivity of the desired blue light, and complete reflection of undesired yellow light.

The multilayer sits atop a frosted-glass layer, which diffusively reflects the blue structural color and, analogous to the Morpho wing, introduces enough disorder to reduce the color’s angle dependence. The bottom slice of the sandwich—a silver film that is highly reflective across visible and NIR wavelengths—provides a radiative-cooling kicker, by virtually shutting down heating from absorption of incoming solar radiation.

Top: In daytime measurements, the blue cooling film (2) achieved temperatures 2°C below ambient (Air) temperatures, up to 26°C below a conventional blue paint (1) and slightly above measured temperatures for a similar yellow cooling film (3) and a colorless radiative-cooling film (4). Middle and bottom: Equipped with a lower silver mirror layer, the blue film effectively reflected incoming solar radiation across the full visible and NIR spectrum, while still maintaining high absorptivity (and thus high emissivity for radiative cooling) in the mid-IR atmospheric window. [Image: W. Wang et al., Optica, doi: 10.1364/OPTICA.487561]

In tests of the three-layer material, the team found that, at an illumination angle of 10 degrees, the film’s color remained a steady, saturated blue even as the viewing angle changed across a range from 0 to 60 degrees. On the thermal side, the material absorbed virtually no energy across the visible to NIR solar spectrum, while still exhibiting high emissivity in the 8-to-13-µm, mid-IR atmospheric window for radiative cooling.

The team tested the cooling film in separate daytime measurements on a Shenzhen rooftop in December 2021 and July 2022, using a thermocouple and infrared camera to acquire real-time temperature measurements. The researchers found that, in addition to producing a rich blue color, the film had a temperature 2°C below the ambient level—clear evidence of radiative cooling at work.

Even more strikingly, the temperature was a full 26°C below that of a conventional blue paint, which lacked the cooling film’s ability to eliminate heating due to incoming NIR solar radiation. In light of those tests, the Shenzhen team calculates that the film it demonstrated “can save 1377 MJ/m2 of energy annually” relative to conventional blue paints.

But the team believes it can go further than that. As noted, the demonstrated cooling film achieved temperatures 2°C below ambient levels. The researchers’ numerical modeling, however, suggests that if the silver reflective layer were replaced with an optimized dielectric multilayer “for thermal management,” the film could cool objects to more than 10°C below ambient temperatures. That, the authors conclude, suggests “great potential for colorful objects to be used for energy sustainability and carbon neutrality.”

Publish Date: 07 August 2023