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A team of physicists from the University of Shenzhen led by Wang Goping was inspired by a natural mechanism to solve the problem of reduction in surface temperature caused by conventional pigments that absorb visible light. Whereas traditional paints tend to retain heat, especially when exposed to sunlight, newly developed color films take an innovative approach to color transfer based on the structural features found in butterfly wings.

It is interesting that some butterflies display bright color not due to absorption, but to interference caused by the unique reflection of light from the periodic structure of their wings. In particular, the Morpho menelaus butterfly demonstrates an incredible ability to produce a highly saturated blue hue at wide viewing angles, thanks to its layered and irregular components.

Based on these natural phenomena, the researchers used reflectivity. titanium oxides (TiO2) and silicon (SiO2) in a multilayer arrangement. This composition was placed on top of a layer of ground glass on a reflective silver surface. By strategically optimizing the thickness of the top layers, the team achieved selective reflection of yellow light while allowing blue light to enter the structure. This resulted in a bright, rich blue that remained visible at an angle of ±60 degrees.

Color cooling film modeled from the wing of Morpho menelaus / Photo Optica

Why it is necessary?

The practical significance of this innovation is significant. When tested under harsh conditions, the newly developed film demonstrated its cooling ability, reducing the surface temperature by two degrees Celsius compared to the surrounding environment. This level of cooling efficiency is compatible with existing colorless cooling films that use silver and polydimethylsiloxane (PDMS).

To confirm their findings, the researchers conducted field experiments putting samples of cooling film on roofs and vehicles. Compared directly to traditional blue paint, the film outperformed it by keeping surfaces 45 degrees cooler in direct sunlight at 27 degrees Celsius air temperature. Such a significant drop in temperature demonstrates the film’s potential in a variety of applications, from buildings to cars.

The practical implications go beyond direct temperature reduction. In a comprehensive analysis, the researchers estimated that replacing the standard blue dye in the city of Shenzhen with this cooling alternative could save about 1,377 megajoules of energy per square meter in one year. The potential to further increase the cooling effect remains promising as the team continues to refine the structure of the film, including exploring alternatives to silver.