The color "ultrablack" - defined as reflecting less than 0.5% of the light that hits it - has a variety of uses, including in cameras, solar panels and telescopes, but it's difficult to produce and can appear less black when viewed at an angle. Now, a Cornell lab has devised a simple method for making the elusive color.
Taking cues from nature - in particular the striking black of the magnificent riflebird - researchers in the Responsive Apparel Design (RAD) Lab, in the College of Human Ecology (CHE), dyed a white merino wool knit fabric with polydopamine, followed by etching of the material in a plasma chamber to create nanofibrils - spiky nanoscale growths. These features were made to mimic the light-trapping capabilities found on the riflebird's ultrablack feathers, which absorb most of the light that hits them.
Co-author Kyuin Park, M.S. '23, a doctoral candidate in fiber science and apparel design, works in the Responsive Apparel Design Lab.
The researchers' two-step approach produced the darkest fabric currently reported; it's also easy to manufacture, scalable, wearable and not angle-dependent. The researchers have applied for patent protection with the Cornell Center for Technology Licensing (CTL) and hope to form a company around their process, which can be used on natural materials, including wool, silk and cotton.
"From a design perspective, I think it's exciting because a lot of the ultrablack that exists isn't really as wearable as ours. And it stays ultrablack even from wider angles," said Larissa Shepherd, M.S. '13, Ph.D. '17, assistant professor in the Department of Human Centered Design (CHE).
Shepherd, the RAD Lab's director, is senior author of "Ultrablack Wool Textiles Inspired by Hierarchical Avian Structure," which published Nov. 26 in Nature Communications. Shepherd's co-authors are doctoral students and RAD Lab members Hansadi Jayamaha and Kyuin Park, M.S. '23.
Their work included analysis of feathers from the riflebird, a member of the bird-of-paradise family found in New Guinea and Australia. Shepherd and her team obtained riflebird feathers from the Cornell Lab of Ornithology with the help of Mary M. Ferraro, bird collections manager, and Vanya Rohwer, curator of birds and mammals.
The riflebird's striking black plumage comes from melanin pigment combined with tightly bunched barbules that serve to deflect light inward, absorbing nearly all of it. This renders the bird extraordinarily black, but only when viewed straight on; at an angle, its plumage appears shiny.
This same coloration is evident in other creatures, including fish and butterflies. Shepherd said her group's choice of polydopamine for their dye was intentional.
"Polydopamine is a synthetic melanin, and melanin is what these creatures have," she said. "And the riflebird has these really interesting hierarchical structures, the barbules, along with the melanin. So we wanted to combine those aspects in a textile."
Co-author Hansadi Jayamaha, a doctoral candidate in fiber science and apparel design, works in the Responsive Apparel Design Lab, led by Larissa Shepherd, M.S. '13, Ph.D. '17, assistant professor in the Department of Human Centered Design.
It wasn't enough to simply coat the surface of the wool: The researchers had to have the polydopamine penetrate into the fibers of the fabric, so every bit of it became black. That's because the plasma etching process removes some surface material of the outermost fibers, leaving behind spiky nanofibrils, which are a key component.
"The light basically bounces back and forth between the fibrils, instead of reflecting back out - that's what creates the ultrablack effect," Jayamaha said.
Analysis revealed that the group's fabric had an average total reflectance of 0.13%, making it the darkest fabric yet reported. And it remained ultrablack across a 120-degree angular span, meaning it appears the same at up to a 60-degree angle either side or straight on, superior to currently available commercial materials.
According to Park, their ultrablack fabric has potential in many solar thermal applications, converting and utilizing absorbed light into thermal energy. "We could actually use the ultrablack fabric for thermo-regulating camouflage," he said.
Last fall, Zoe Alvarez '25, a fashion design management major (CHE), created a black strapless dress inspired by the riflebird that incorporated as its centerpiece ultrablack material with a splash of iridescent blue. Images of the dress were used to confirm ultrablack's true "blackness": When the image's contrast, hue, vibrance or brightness were adjusted, all the other colors changed, but ultrablack remained the same.
Shepherd said her team has applied for provisional patent protection through CTL, and is looking to apply to its Ignite Innovation Acceleration program to push their idea closer to market.
Analysis of the material was conducted at the Cornell Center for Materials Research and Human Centered Design shared instrumentation facility.
