A new ultra-black wool could make it desirable to be a black sheep. The specially dyed wool could be a low-cost, biodegradable material for producing potable water from saltwater using only sunlight.
The material reflects less than 0.5% of visible light falling on it and helps to purify saltwater by evaporating it. It produces water that exceeds both World Health Organization and Environment Protection Agency drinking-water standards, says Larissa Shepherd, a professor of human centered design at Cornell University. Shepherd and her colleagues reported their new ultrablack solar water evaporator in the journal Advanced Science.
Nearly half the world’s urban population is projected to face water scarcity by 2050. Conventional desalination technologies can convert seawater into potable water. But they are “too infrastructure-heavy and energy-intensive to reach the communities that need it most,” Shepherd says.
Distillation is one of the simplest ways of purifying water. It involves heating water into a vapor to separate any solid impurities such as salts, and then cooling the vapors to give fresh water. Researchers have been developing many methods and novel materials to distill water using only sunlight.
Most solar vapor generation devices that perform well, however, “rely on complex 3D fabrication or synthetic nanomaterials that are costly, hard to scale, and risk leaching into the water they produce,” Shepher says.
So she and her colleagues took commercial white merino wool fabric and dyed it with polydopamine, a light-absorbing polymer inspired by melanin. The researchers took cues from the deep-black feathers of the riflebird, a type of Bird of Paradise. They etched the dyed wool to create spiky microscopic growths that mimic the tiny structures on the bird’s feathers that help absorb most of the light that hits them. They reported this “darkest fabric ever made” last year in the journal Nature Communications.
The team has now used the material as a solar water evaporator. The researchers created three wool structures with horizontal, one-sided vertical and two-sided vertical configurations. While the tiny spike fibers trap light, the polydopamine converts near-infrared light into heat.
In tests under simulated sunlight, the two-sided vertical system produced 2.43 liters of water per square meter of fabric each hour. That is the range of high-performing 3D architectures that require far more elaborate fabrication, Shepherd says. And it is lower than more complex systems and those using microplastics or nanomaterials.
But this material is 100% natural, simple, and low cost. The material weighs about 281 grams per square meter, and costs roughly $0.62 in materials for a 20 × 20 cm piece, she says. “Our modeling suggests a single square meter could yield around 4,700 liters of vapor a year in a sun-rich location like Mokha, Yemen. That makes off-grid, textile-based water purification realistic for vulnerable communities. I see it being used as an off-grid water source for communities that conventional infrastructure doesn’t reach.”
The researchers have already tested the material outdoors on a partly cloudy day in Ithaca, New York, on water three times saltier than seawater and it performed well. They now plan to test its performance on brackish ground water and wastewater.
Source: Kyuin Park et al. Polydopamine-Wool Textiles for Sustainable Interfacial Solar Vapor Generation. Advanced Science, 2026.
Image: ©Anthropocene Magazine
