Researchers at Kobe University have created a new "structural color ink" that is only 100-200 nanometers thick and displays vivid colors from a wide viewing angle without fading, while weighing less than half a gram per square meter (0.002 ounces per square foot). Common paints and pigments absorb certain wavelengths of light and reflect others, but this often degrades them at a molecular level, causing fading.

Ultra-light structural color inks: printable, rainbow-free and fade-resistant

Structural colors, on the other hand, reflect the full spectrum of light from parallel nanostructures spaced just right apart to cancel out certain wavelengths of light. It's this effect that gives butterfly wings and peacock feathers their gorgeous, shiny color. Since the light isn't absorbed, just reflected off the structure, the color doesn't fade, but the effect often depends a lot on the viewing angle, resulting in a dazzling rainbow of colors that are very beautiful in nature but a bit unorthodox for most industrial colors.

The nanospheres in the methanol suspension have a different color than the nanospheres on the surface of the monolayer. Picture/Fujii Minoru

The Kobe team researched a new way to create structural color. Instead of using parallel nanostructures, their method uses tiny crystalline silicon spheres. Through a phenomenon called "Mie resonance," these nanoscale spheres reflect certain wavelengths much more strongly than others, and these wavelengths vary as the particle size changes.

In other words, changing the size of these particles changes the color of the material. Since the sphere reflects light in all directions, there is no rainbow effect. Researchers have demonstrated that these inks can be printed. It's worth noting that the color is strongest when the nanospheres have a little space around them, rather than being tightly packed together, so less is more.

"A layer of sparsely distributed silicon nanoparticles only 100-200 nanometers thick can display vivid colors but weighs less than half a gram per square meter. This makes our silicon nanospheres one of the lightest in the world," Kobe materials engineer Hiroshi Sugimoto said in a press release.

Scanning electron micrographs of a single layer of nanospheres show nearly perfectly round particles with uniform size.

The aviation industry is one of the areas most in need of ultra-lightweight coatings. According to SimpleFlying, the weight of paint carried by passenger aircraft ranges from 272 to 544 kilograms (600 to 1,200 pounds), and anything carried by the aircraft will directly affect fuel efficiency.

"If we use ink based on nanospheres, we may be able to reduce the weight to less than 10 percent," Sugimoto said.

So wherever the plane goes, it burns a little less fuel -- about the same as it would with five or six fewer adult male passengers.

In addition, ordinary paint fades easily, requiring up to 455 liters (120 gallons) of paint to be repainted, costing $200,000. These new structural color inks are completely colorfast, so while they cost more upfront, they can be used indefinitely, resulting in cost savings over the life of the aircraft.

Commercial aircraft can often be used for decades, and once these ultra-thin structural color inks can be put into production, there will be good commercial prospects.

The research was published in the journal ACS Applied Nanomaterials, and the video below demonstrates the inks.