Materials

Vibrantly colored film could massively passively cool cars and buildings

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A new thin film, inspired by the vibrant colors of butterfly wings, could help passively cool cars and buildings
Wanlin Wang, Shenzhen University
A new thin film, inspired by the vibrant colors of butterfly wings, could help passively cool cars and buildings
Wanlin Wang, Shenzhen University
A diagram illustrating the structure of the film, containing alternating layers of titanium dioxide and aluminum dioxide (multilayer) followed by a layer of rough frosted glass (disorder) and finally a layer of silver (reflector)
Wanlin Wang, Shenzhen University

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.”

A diagram illustrating the structure of the film, containing alternating layers of titanium dioxide and aluminum dioxide (multilayer) followed by a layer of rough frosted glass (disorder) and finally a layer of silver (reflector)
Wanlin Wang, Shenzhen University

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.

Source: Optica

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6 comments
Chase
Knowing the basic people that buy most vehicles these days, even if they had the option to get something vibrant that also cools their vehicle they will still buy anything but an actual color instead, like white, gray, silver, black, and other-white. I'd happily buy a car that color of blue, or similar style green or yellow, but unfortunately I'm the weird one.
rgbatduke
Yeah, but butterfly wings are also known for being incredibly fragile. Here you have a multilayer nanostructure. Will just "touching" it essentially punch a thermal hole in it? How hard is it? How expensive will it be, to coat e.g. a car or rooftop with it? They are a long way away from showing that this is an economically viable solution to anything at all (as much as I do appreciate the physics behind it).

If you're going to go to this much trouble, why not make it into a solar cell as well, or instead? Instead of reflecting away all wavelengths, how about turning all that energy into electricity? Finally, how does it compare to just using white-white reflective paint? What about UV and IR -- it isn't all about just the visible part of the spectrum? How toxic is it? How much energy goes into its construction? We could -- and probably should -- all be driving white cars on white roadways with white upholstery and living in houses with white shiny roofs and white paint on their sides (everywhere that isn't already covered by solar panels, that is) but -- so far, not happening.

We COULD be taking the unused FEDERAL land in the usually big strip in between lanes of US highways and the interstate system and covering it -- literally -- with hundred-mile long arrays of solar cells feeding into the grid everywhere a major transmission line comes close enough to the roadway. But we aren't.

The real question is: why not? And the real answer is: because way too many corporate interests make way too much money actively blocking these things. So don't hold your breath expecting this to actually make it to any market, even if it works.
Kpar
The tough part will be transferring this tech to a painting process.
TechGazer
A simple federal bill mandating roof reflectivity would probably make a big difference in energy consumption. Is having a non-white roof really that important to a person's well-being? When I replaced my asphalt-shingle roof with metal, I preferred the sky blue option, but it was 30-40% less reflective, so I chose white, and that made a huge difference in how hot my home gets in summer. I definitely recommend white (not asphalt, since even white reveals a lot of black surface). If Butterfly Blue was an option that wasn't much more expensive than white, I might consider it, but I'm certainly not sad that my home is white on top.
Muiz
Great news. However unless I've completely misunderstood what is being represented in the last sentence of the 6th paragraph — there's something *very* wrong with the Celsius/Fahrenheit numbers in phrase "more than 15 °C (27 °F) in winter and a whopping 35 °C (63 °F) cooler in summer." If the Celsius numbers are correct 15 °C = 59 °F (NOT 27 °F) and 35 °C = 95 °F (NOT 63 °F). OR if the Fahrenheit numbers are correct ... 27 °F = -2.8 °C (NOT 15 °C) and 63 °F = 17.2 °C (NOT 35 °C).
Michael Irving
@Muiz Those temperature conversions aren't talking about specific temperatures, they're differentials. Because celsius and fahrenheit work on different scales, the change between two figures will vary for each scale. So for example, 10°C is 50°F, and 50°C is 122°F – so that's an increase of 40°C and 72°F respectively. But obviously, the specific temperature of 40°C does not equal the specific temperature of 72°F