Researchers from the Korea Advanced Institute of Science and Technology (KAIST) have developed a new anti-counterfeiting technique using tiny “Janus balls.”

Taking their name from the two-faced god of transitions in Roman mythology, the Janus balls are microspheres featuring two sides, each with their own distinctive properties.

Composed of two different resins that resist each other, the Janus balls feature one side that contains magnetic nanoparticles featuring carbon black, which darkens the hemisphere, while the second side is composed of silica particles that form a crystalline lattice configuration, resulting in structural colors. With the introduction of a magnetic field, the tiny balls, which would typically have their black sides facing up, would flip, revealing their colorful sides.

To construct the Janus balls, researchers employed a microfluidic device to combine the two resins and a surfactant stabilized the resins, shaping them into spheres. The silica side, featuring the structural colors, was heavier than the black magnetic side, with gravity forcing the black side to face up when submerged in water. The team then aligned the magnetic nanoparticles in the balls to face the same direction so that when a magnetic field was applied in the opposite direction, the balls could be flipped, revealing the colored sides.

The team envisions that the Janus balls could be incorporated into the inks used in anti-counterfeit tags on luxury products — shoes and handbags for instance — which are often subject to counterfeiting. Verification of the product's authenticity could be confirmed using a magnet.

The Janus balls are detailed in the journal ACS Nano and in the accompanying video that appears courtesy of the American Chemical Society.