It is well known that white causes the least heat – other colors, on the other hand, absorb light and cause objects to heat up. To get around this problem, researchers have now been inspired by the color concept of the wings of the morpho butterfly. The special optical properties of your film can produce brilliant colors and still provide a passive cooling effect. The concept could thus enable colorful facades or cars without increasing the cooling effort, say the developers.
As is well known, when the summer sun burns down from the sky, cooling is required: shades, fans or air conditioning systems have to protect objects and people from overheating. The effort is often high and cooling systems consume a lot of energy and resources. In order to limit the need, surfaces should therefore be designed from the outset in such a way that they heat up as little as possible. As is well known, the color white has proven its worth. Because it is based on a broad reflection of the light spectrum, which limits the heating of surfaces. Unfortunately, this is not the case with bright colors: When traditional pigment colors produce blue, for example, only the corresponding wavelengths are emitted – others, on the other hand, are absorbed, which leads to heating.
Morpho butterfly role model
To circumvent the problem, the Chinese research team led by Guo Ping Wang from the University of Shenzhen has now explored the potential of another form of color generation in nature. Instead of pigments and the absorption of certain wavelengths, structural colors are based on the optical influence of light through fine surface structures. The concept has already been used for effective passive thermal management strategies. But it mostly stayed with the color white. "A strategy for cooling colored objects while maintaining excellent color properties with high saturation and from many viewing angles remains a major challenge," the researchers write.
With their new development, they have now oriented themselves to the structures of the wings of a particularly deep blue butterfly. In the case of the morpho butterfly, special nanostructures on the scale-covered wings lead to this colour, which also appears strong from different viewing angles. The team has now managed to artificially mimic this concept. They converted it into the form of a bio-inspired foil made of layered structures. At the top are several layers of titanium dioxide and aluminum dioxide material. This is followed by a layer of structured glass with special optical properties. At the bottom, the foil is closed off by a wafer-thin layer of silver. It provides a basic reflection and thus a severely limited light absorption.
As the researchers explain, the color of the film is determined by how the titanium dioxide and aluminum dioxide layers and the structured glass layer respond to incident light. According to them, by making slight adjustments to the system, different colors can also be generated. To bring out blue, similar to the morphofold model, the multi-layer titanium dioxide and aluminum dioxide material are arranged to reflect yellow light over a very narrow range of angles. The glass structures, on the other hand, scatter the blue light components over a wide area. The bottom line is that this results in a rich blue, explains the team.
Successfully stayed cool
"Thanks to the layered structure we developed, we were able to extend the passive cooling method from colorless objects to colored objects while guaranteeing color intensity at the same time," Wang sums up. “In other words, our blue film looks blue from a wide range of viewing angles and hardly heats up because it reflects all the light. In addition, we were able to achieve high saturation and brightness through the design of the structures,” says the scientist.
To document the thermal protection effect of the technology, the researchers created differently colored films that they placed outdoors on surfaces such as roofs, cars, fabrics and mobile phones. With the help of thermocouple sensors and infrared cameras for temperature measurement, they were able to prove that the coatings heated up significantly less than comparable materials and were therefore also able to protect objects from heating up. For example, the blue version of the film was found to stay around 26 degrees Celsius cooler than conventional blue car paint in the sunshine.
“In buildings, large amounts of energy are used for cooling and ventilation, and running the air conditioning in electric cars can significantly reduce the range. In the future, our cooling foils could therefore help to promote energy sustainability and CO2 neutrality,” says Wang. He and his team now want to devote themselves to this goal by further optimizing their system. For example, replacing the silver with aluminum could save costs, and the researchers now want to further improve the mechanical properties of their “cool” material.
Source: Optica, specialist article: Optica, doi: 10.1364/OPTICA.487561