Dayna Baumeister, co-director of the Biomimicry Center at Arizona State University, isn’t surprised paint has so many hidden functions. “It’s a fantastic demonstration of what’s possible when we rethink our designs by asking nature for guidance,” she says.
For all of its imperfections, the painting is hard to beat. People have been using pigment for millennia, so the tips for getting the right look have been mastered by paint manufacturers. “They know exactly what additive to add to change the shine; they can make it brighter or dim – they’ve figured it all out over hundreds of years,” says Chanda.
New forms of painting must innovate beyond that, in the realm of physics, not just aesthetics. Yet members of Chanda’s lab stumbled upon their innovation by accident. They hadn’t planned to paint. They wanted to make a mirror, more specifically a long continuous aluminum mirror, built using an instrument called an electron beam evaporator. But with each attempt, they noticed small “nanoislands”, clumps of aluminum atoms small enough to be invisible but large enough to disrupt the glow of the mirror. Nano-islands appeared all over the surface of what was no longer – frustratingly – a continuous mirror. “It was really boring,” Chanda recalls.
Then came an epiphany: this disturbance was doing something useful. When ambient white light hits the aluminum nanoparticles, the metal’s electrons can be excited, they oscillate or resonate. But when the dimensions dip into the nanoscale, the atoms get even finer. Depending on the size of the aluminum nanoparticle, its electrons will only oscillate for certain wavelengths of light. This returns ambient light as a fraction of what it was: a single color. The layering of aluminum particles on a reflective surface – like this mirror they had tried to build – had amplified the colored effect.
What color? It depends on the size of the nano-islands. “Just by moving the dimension, you can actually create all colors,” says Chanda. Unlike pigments, which require a different base molecule, such as cobalt or purple snail slime— for each color, the base molecule for this process is always aluminum, just cut into pieces of different sizes that oscillate to light up at different wavelengths.
It was time to paint. The band process begins with a very thin sheet of double-sided mirror. The researchers coated each side with a transparent spacer material that helps amplify the color effect. Then they grew islands of metallic nanoparticles on both sides of the sheet. To make this material compatible with binders or oils used in paint, they dissolved large sheets of it into colored flakes about as fine as powdered sugar. Finally, once they had created enough colors for a small rainbow, they could paint a butterfly.
Because structural color can cover an entire surface with just a thin, ultralight layer, Chanda believes this will be a game-changer for airlines. A Boeing 747 needs about 500 kilograms of paint. He estimates that his paint could cover the same area with 1.3 kg. That’s over 1,000 pounds shaved off each plane, which would reduce the amount of fuel needed per trip.