Kobe University researchers have created a new "structural color ink," just 100-200 nanometers thick, that shows bright colors from wide viewing angles, without fading, while weighing less than half a gram per square meter (0.002 oz per square foot).
Regular paints and pigments absorb certain wavelengths of light and reflect others, but this tends to degrade them at the molecular level, leading to fading.
Structural colors, on the other hand, reflect the full spectrum of light from parallel nanostructures, set just the right distance apart to cancel out certain wavelengths. It's the effect that gives butterfly wings and peacock feathers their gorgeous, shimmering hues. Because light isn't being absorbed, just reflected off structures, the colors don't fade – but the effect is often highly dependent on viewing angle, leading to dazzling iridescence that's beautiful in nature, but a little out-there for most industrial colorings.
The Kobe team has investigated a new way of creating structural colors. Instead of using parallel nanostructures, their method uses tiny, crystalline silicon spheres. Through a phenomenon called Mie resonance, these nano-scale spheres reflect certain wavelengths much more strongly than others – and these wavelengths vary with the size of the particles.
In other words, change the size of those particles and you change the color of the material. There's no iridescence effect, because the spheres reflect the light in all directions. The researchers have proven these inks can be printed. And remarkably, the color is strongest when the nanospheres have a little room around them, rather than being tightly packed – so less is more.
"A single layer of sparsely distributed silicon nanoparticles with a thickness of only 100–200 nanometers shows bright colors but weighs less than half a gram per square meter," says Kobe materials engineer Sugimoto Hiroshi in a press release. "This makes our silicon nanospheres one of the lightest color coats in the world."
One area that's crying out for super-lightweight paints is the aviation sector. Airliners, according to Simple Flying, carry between 272-544 kg )600-1,200 lb) of paint – and everything an aircraft carries has a direct impact on fuel efficiency.
"If we use our nanosphere-based ink," says Sugimoto, "we might be able to reduce the weight to less than 10% of that."
So wherever the aircraft flies, it'll burn a little less fuel – about the same as it might if it was carrying five to six fewer adult male passengers.
What's more, regular ol' paint tends to fade, requiring resprays to the tune of up to 455 L (120 gal) of paint and US$200,000. These new structural color inks won't fade at all, so while they'll certainly be more expensive up front, they'll last indefinitely, saving money over the service life of the aircraft.
These machines tend to be operational for several decades – provided they don't have "Boeing," "737" and "Max" written on them – so there could yet be a strong business case for these ultra-thin structural color inks once they're ready for production.
The research is available in the journal ACS Applied Nano Materials, and the inks are shown in the video below.
Source: Kobe University
