A new fabric can keep wearers up to 12 °C cooler than their surroundings (ACS Photonics 2023, DOI: 10.1021/acsphotonics.3c00241). The fabric reflects most of the sunlight falling on it and also draws heat away from what it covers and sends the heat into space. The material cools more effectively and is simpler to make than others based on the same mechanisms. It’s also breathable, waterproof, and durable.
Air-conditioning and fans currently account for 10% of the world’s electricity use, according to the International Energy Agency. This share is expected to increase as the world warms. Radiative cooling, which happens when materials beam infrared heat rays at wavelengths that don’t get absorbed in the atmosphere and instead go straight into space, offers a way to bring temperatures down without using energy. Researchers have made such materials for building rooftops and facades to reduce the burden on air-conditioning systems.
Also, because cooling people directly is more efficient than cooling the air around them, a few groups have recently made flexible radiative cooling materials for clothing. While sportswear wicks away sweat to make wearers feel cool through evaporation, radiative cooling fabrics can bring temperatures down below the ambient temperature.
Fuqiang Wang of Harbin Institute of Technology and colleagues wanted to make a fabric that does both and is also easier to make than other similar materials. They took inspiration from the layered structure of human skin.
First, they made a 150 µm thick bottom dermis-like layer with a fabric of interlaced cotton and polyester fibers. They soaked that in a suspension of polyvinyl difluoride (PVDF) and microparticles of titanium dioxide, barium sulfate and silicon dioxide before heating it to dry. “The process is as simple as ordinary cloth dyeing,” Wang says.
The team used a controlled drying process to create flat slab-shaped pores in the fabric. The pores reflect sunlight, and together with the SiO2 microparticles, they transmit infrared heat into space, Wang says. The PVDF and TiO2 and BaSO4 microparticles, meanwhile, further scatter and reflect sunlight. The 0.3–1 µm wide pores also prevent liquid water molecules from passing through but let water vapor and air cross over, making the fabric waterproof yet breathable.
Then, the researchers topped the first layer with a 600 µm thick epidermis-like layer composed of cotton-polyester fiber that’s also embedded with BaSO4 and SiO2 microparticles. This time, they reduced the amount of solution and sped up the heating time to create a surface that wrinkles like human skin. That structure increases the propagation distance of light on the surface, Wang explains, boosting both sunlight reflectance and infrared radiation.
In outdoor tests between noon and 4:00 p.m., at about 40° North latitude in Dunhuang in July, the material stayed 7.9 °C, 8.6 °C, and 8.4 °C lower than white fabrics made of cotton, polyester, and linen, respectively, with a maximum drop of 12.6 °C. “This cooling effect is higher than most researchers have reported before,” Wang says. The team also made hats and masks with the radiative cooling fabrics, which kept the wearer’s heads and faces 5 °C and 5.4 °C cooler than a white cotton hat and a white polypropylene mask, respectively.
“The material exhibits state-of-the-art radiative cooling performance along with additional functionalities such as vapor permeability and waterproofness,” says Zhengmao Lu, a mechanical engineer at the Swiss Federal Institute of Technology, Lausanne (EPFL). A key benefit of the new material’s design is that its synthesis should be scalable, he says, adding that the researchers will need to do long-term real-world testing to see if the material’s promising performance stands up to environmental factors such as airborne contaminants and impacts from rain droplets.