In ancient Roman mythology, Janus was the two-faced god of transitions. Similarly, so-called Janus balls are microspheres that have two sides with distinct properties.
The team from KAIST, led by Shin-Hyun Kim, wanted to make Janus balls out of two unmixable resins: one that contained magnetic nanoparticles, and another that contained silica particles. The magnetic side of the ball would also contain carbon black, causing that hemisphere to appear dark, whereas the silica particles on the other side of the ball would self-assemble into a crystalline lattice, producing structural colours. The result would be tiny balls that normally have their black sides facing up, except when a magnetic field causes them to flip to their colourful sides.
To make Janus balls, the researchers used a microfluidic device to unite drops of the two resins, with a surfactant added to stabilise the joined drops into a spherical shape. Because the silica-containing coloured side of the drops was heavier than the black magnetic side, the force of gravity caused the black side to spontaneously face upward when the balls were placed in water.
Then, the researchers permanently aligned the magnetic nanoparticles in the balls in the same direction. By applying a magnetic field in the opposite direction, they could flip the balls to their coloured sides.
The researchers made red and green Janus balls by using different sizes of silica particles, with their magnetic nanoparticles aligned in opposite directions. By changing the direction of the applied magnetic field, they could change the colours of 3D-printed chameleon and butterfly shapes.
Using different colours and orientations of Janus balls in inks could produce sophisticated, user-interactive anti-counterfeiting tags, the researchers say.