A common aquarium fish has given researchers the inspiration for a new type of color-shifting device (ACS Nano 2019, DOI: 10.1021/acsnano.9b00822). The technology might one day provide soldiers with camouflage or form the basis of a new type of display screen, the researchers say.
The prototype device uses magnets to bend microsized columns made of iron oxide nanoparticles, and the angle of the columns changes the wavelength of reflected light. The device can quickly change from yellow to green and back again.
“We were inspired by the neon tetra,” says Chih-Hao Chang, a mechanical engineer at North Carolina State University. The colorful fish (Paracheirodon innesi) produces iridescence through light bouncing off stacks of tiny guanine platelets in its scales. By tilting the platelets, the fish can change the spacing between them and therefore the wavelength they reflect. Chang calls this a “Venetian blind mechanism,” in which slats stay the same distance apart but rotate to open or close the gaps between them.
To build their device, Chang and his colleagues first used lithography to inscribe a pattern of holes in a piece of silicon. They used that as a mold for a siloxane copolymer containing iron oxide nanoparticles, forming a block studded with short pillars 1 µm tall spaced 2 µm apart, which the researchers say resembles a Lego brick. They then surrounded the pillars with more nanoparticles dispersed in water and topped the device with a layer of polydimethylsiloxane.
When a magnetic field was applied, the nanoparticles in the water arranged themselves into 20 µm high columns on top of the polymer pillars. Changing the angle of the magnetic field caused the iron oxide columns to tilt backward or forward by 30°, so that they act like the slats in a Venetian blind.
The Venetian blind approach is different from an accordion-style method that chameleons use to change color and that researchers have replicated in photonic crystals, materials designed with periodically spaced nanostructures that interact with light. In that method, stretching or compressing the material increases or decreases the spacing between nanostructures, changing the reflectance characteristics. With that approach, though, the structures have to be smaller than the wavelengths of light, which means less than 400 to 700 nm for visible light. The iron oxide columns in Chang’s system can be much larger, and therefore should be easier to make.
Pete Vukusic, who studies biophotonics at the University of Exeter, says even though the way the fish produce color changes is somewhat different than how this device works, “what I do like is the idea of magnetically switching the structural color. This is pretty novel, and I can’t recall it being done elsewhere.”