A common aquarium fish, the neon tetra, has given researchers inspiration for a new type of color-shifting device (ACS Nano 2019, DOI: 10.1021/acsnano.9b00822). The colorful fish (Paracheirodon innesi) produces iridescence through light bouncing off stacks of guanine platelets in its scales. By tilting the platelets, the fish can change the spacing between them and the wavelength of light they reflect. Chih-Hao Chang of North Carolina State University and colleagues built a device that mimics this mechanism. First they molded a siloxane copolymer containing iron oxide nanoparticles into a block studded with 1 µm tall pillars spaced 2 µm apart. They then surrounded the pillars with more nanoparticles dispersed in water and topped the device with a layer of poly(dimethylsiloxane). When they applied a magnetic field, 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 tilted the columns backward or forward up to 30°, quickly switching the color from yellow to green and back again. The technology might one day provide soldiers with camouflage or form the basis of a new type of display screen, the researchers say.