04-05 neon tetra

[Image above] Neon tetras shift color by changing the angle of microscopic structures on their skin. Thanks to researchers at North Carolina State University, you may soon be able to change color in a similar way! Credit: Toshihiro Gamo; Flickr CC BY-NC-ND 2.0

When I watch a movie involving science, there is a certain suspension of disbelief that I must allow so that I can enjoy the plot and not think too deeply on why most extraterrestrial species look humanoid.

But suspension of disbelief is much easier when you are not a marine biologist just minutes into watching Finding Dory and see a swarm of neon tetras passing through the coral reef.

Though the movie makers appear to have overlooked a basic neon tetra fact (they are freshwater, not saltwater, fish), it is understandable why the film’s animators may have wanted to include them—neon tetras have very colorful scales.

The reason for a neon tetra’s bright and color-shifting body comes from microscopic structures called platelets that make up a neon tetra’s skin. By changing the angle of their platelets, neon tetras can change how light is reflected from their bodies, making them look blue-green one moment and indigo the next.

While this piece of knowledge can serve as an esoteric tidbit for a fish enthusiast at a dinner party, it also can inspire scientists looking to develop new materials—like scientists at North Carolina State University. In a recent study, NC State researchers described how the color-shifting abilities of neon tetra fish inspired them to create their own color-changing material, a material they hope could serve a range of applications, from reflective displays to dynamic camouflage.

“For this proof-of-concept study, we’ve created a material that demonstrates a similar ability [to neon tetra fish scales],” says Zhiren Luo, a Ph.D. student at NC State and first author of the study, in a NC State press release. The researchers created a material embedded with nanostructured columns that work the same way as fish platelets—changing orientation changes reflection of light.

By shifting the orientation of nanostructured columns, NC State researchers could change the wavelength of light most strongly reflected by their material, effectively changing the material’s color. Credit:
Reprinted with permission from ACS Nano, Article ASAP. Copyright 2019 American Chemical Society.

The color-changing material consists of four layers. The bottom layer is a silicon substrate, and it is coated with a polymer layer embedded with iron oxide nanoparticles. A regular array of micron-wide pedestals covers this polymer layer. The next layer is an aqueous solution containing free-floating iron oxide nanoparticles, and the solution is held in place by a top layer of transparent polymer.

The NC State press release explains how the material works: “When a vertical magnetic field is applied beneath the substrate, it pulls the floating nanoparticles into columns, aligned over the pedestals. By changing the orientation of the magnetic field, researchers can change the orientation of the nanoparticle columns.” Because column orientation determines which wavelength will be most strongly reflected, this essentially changes the color of the material (or at least how the color is perceived).

The researchers were able to change their material from dark green to neon yellow, and they plan to improve color purity by fine-tuning column array geometry. Additionally, “We are also planning to work on the development of integrated electromagnets that would allow for more programmable color shifts,” says Chih-Hao Chang, an associate professor of mechanical and aerospace engineering at NC State and the paper’s corresponding author, in the press release.

If the researchers do succeed in creating dynamic camouflage with their material, I will definitely be interested in seeing how accurately that science is depicted in future films!

The paper, published in ACS Nano, is “Magnetically actuated dynamic iridescence inspired by the neon tetra” (DOI: 10.1021/acsnano.9b00822).