ITHACA, N.Y. – 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 University 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 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.
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 protectionand 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, assistant professor in the Department of Human Centered Design.
Shepherd, the RAD Lab’s director, is senior author of “Ultrablack Wool Textiles Inspired by Hierarchical Avian Structure,” which published in Nature Communications.
Their work included analysis of feathers from the riflebird. Its 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.”
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.
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.
For additional information, see this Cornell Chronicle story.
Cornell University has dedicated television and audio studios available for media interviews.
Media note: Pictures can be viewed and downloaded here: https://cornell.box.com/v/avianinspiredultrablack
-30-
Journal
Nature Communications