Researchers from Nagoya University’s Graduate School of Engineering have developed a method to create anti-counterfeiting QR codes using micrometer-sized spherical cholesteric liquid crystals (CLCs).
The team developed a method for processing CLCs into micrometer-sized spherical particles. CLCs were chosen as they exhibit a helical structure, possess unique optical properties, and can selectively reflect light. The spherical CLC particles were combined with readily available pigments to develop an anti-counterfeiting QR code that is only visible under a specific circular polarizer.
To fully utilize CLCs effectively, the team produced spherical CLC particles, which encapsulate the helical molecules in a 3D matrix, thus providing improved control over their coloration. As the size of the particles is a significant issue, they used a mixture of solvents to create micrometer-sized spherical CLC particles using a technique known as dispersion polymerization.
“During the experiment, we unexpectedly discovered that the particle size of the microspheres significantly influenced the resulting structural color. We could produce a variety of colors depending on particle size,” said team leader, Dr. Jialei He. “We also found that covering the spherical CLC particles with the polymer polydimethylsiloxane improved the coloration and thermal stability.”
An important potential application for this research is the creation of more secure, unreplicable QR codes. By making use of the chirality of CLCs – an asymmetrical property that prevents an object or molecule from being superimposed onto its mirror image – they could create anti-counterfeiting QR codes. These codes combine the color of spherical CLC particles with non-chiral, commercially available pigments, and could only be read with a specific circular polarizer that permits non-chiral light but blocks the chiral light of the QR code.
“The development of spherical CLC particles resulting from this research will provide new possibilities for low-cost structural color functions different from those of conventional color materials,” said Dr. Yukikazu Takeoka. “As well as being used as a special functional pigment for anti-counterfeiting, it can also be used for other applications that take advantage of the circularly polarized structural color with little angle dependence.”