Vibrantly colored film could massively passively cool cars and buildings
It’s common knowledge that lighter colors reflect more light than darker ones, which can limit the practical palette choices for your clothes, car or house in a warming world. Now, scientists have developed a new material, inspired by butterfly wings, that can produce vibrant colors while reflecting 100% of the light that hits them, to keep them cooler.
A surface’s color comes from the specific mix of wavelengths of light that it absorbs and reflects. Usually, the darker the color the more light is absorbed, and that energy ends up heating the material. That’s why the interior of a black car feels hotter than a white car when both are left in the sun.
But for the new study, scientists from Shenzhen University and Shanghai Jiao Tong University developed a film that doesn’t absorb any light at all, while still producing a vibrant color. This could help keep a building, vehicle or other object much cooler while also reducing the cost and environmental impact of air conditioning.
“In buildings, large amounts of energy are used for cooling and ventilation, and running the air conditioner in electric cars can reduce the driving range by more than half,” said Guo Ping Wang, lead researcher on the study. “Our cooling films could help advance energy sustainability and carbon neutrality.”
The key to the new film’s abilities is its nanoscale structure. Intricate patterns on surfaces like butterfly wings or peacock feathers diffuse certain colors of light across a wide area, giving them the appearance of that color. The team’s film was made up of alternating layers of titanium dioxide and aluminum dioxide, above a thicker layer of rough frosted glass, which has a disordered texture to scatter the light. The bottom layer is a reflector made of pure silver, ensuring that no light at all is absorbed.
By tweaking the specific properties of the layers, the team created films that appeared blue, yellow or colorless. They then tested samples of the different-colored films on building roofs, cars, cloth and cell phones, during the day in both summer and winter, and measured their temperature. Sure enough, they found that the films remained substantially cooler than the surfaces they were placed on – more than 15 °C (27 °F) in winter and a whopping 35 °C (63 °F) cooler in summer.
“Thanks to the layered structure we developed, we were able to extend the passive cooling method from colorless objects to colorful ones while preserving color performance,” said Wang. “In other words, our blue film looks blue across a large range of viewing angles and doesn’t heat up because it reflects all the light. In addition, high saturation and brightness can be achieved by optimizing the structure.”
In future work, the team plans to investigate swapping the silver layer for aluminum, which would make the material less expensive and easier to manufacture. They also intend to optimize other properties to make it more robust.
The research was published in the journal Optica.