Scientists have developed a method for generating unique anti-counterfeiting QR codes using micrometer-sized spherical cholesteric liquid crystals (CLCs), paving the way for more secure, non-repeatable QR codes.

Revolutionary anti-counterfeiting: QR codes based on cholesteric liquid crystals

From Nagoya University’s Institute of Engineering, Dr. A research team led by Jialei He has developed a method to process cholesteric liquid crystals (LCCs) into micrometer-sized spherical particles. CLCs exhibiting a helical structure have unique optical properties and can selectively reflect light. The team combined spherical CLC particles with readily available pigments to develop a non-fake QR code that can only be seen under a specific circular polarizer. The results of the research were published July 9 in the journalism. Advanced Optical Materials.

Using the power of nature in engineering

Cholesteric liquid crystals are an example of how we can draw inspiration from nature for engineering purposes. The iridescent wings of butterflies or the shiny coating on the exoskeletons of insects are examples of extraordinary phenomena that can produce CLC. Once identified, CLCs that mimic the color-producing units of insect exoskeletons are synthesized in the laboratory because of their unusual color and properties that lie between liquids and crystals.

Optical properties of cholesteric liquid crystals

The optical properties of the CLC are particularly useful. Their unique molecular structure and optical properties allow them to selectively reflect light at specific wavelengths, resulting in extraordinary colors. CLCs consist of long molecules that repeat in the form of a helix. In this spiral, the vertical distance where an area completes a full cycle and begins to repeat itself is known as the “slope”. If the helix has repeating units close to each other, the liquid crystal has a short pitch and reflects light with shorter wavelengths, emitting blue and violet colors. However, those with more vertical area have longer wavelengths, resulting in red or orange colors.

In addition, due to the helical arrangement of molecules in the crystal, the perceived color can vary depending on the helical orientation of the viewer, presenting an almost endless color palette.

Creation of spherical CLC particles

To use CLC more efficiently, the researchers produce spherical CLC particles. These particles enclose the helix in a three-dimensional matrix, giving scientists better control over their coloration. However, size poses a serious problem. Current methods produce 100 micron spherical CLC particles that are too large for most applications. To solve this problem, researchers Jialei He and Yukikazu Takeoka of Nagoya University and colleagues used a solvent mixture to create micrometer-sized spherical CLC particles using a technique known as dispersion polymerization.

This new technique was difficult to find due to the soft nature of the samples at room temperature. Dr. “Sample testing was a particularly challenging period because of the softness of samples at room temperature, which is a hallmark of CLC,” He said. “Therefore, it took a great deal of effort to find a suitable method to characterize the samples without causing any damage.”

Creation of monodisperse spheres

Because the pitch of cholesteric liquid crystal in spherical CLC particles varies with the curvature of the particles, the team focused on producing particles of uniform size, also known as monodisperse spheres.

“During the experiment, we unexpectedly discovered that the size of the microsphere particles significantly affects the resulting structural color. We can produce different colors according to the size of the particles,” he said. “We also found that coating the spherical CLC particles with the polymer polydimethylsiloxane improves color and thermal stability.”

Anti-fraud practices and future prospects

An important potential application for this research is the creation of more secure, non-repeatable QR codes. By taking advantage of CLC’s chirality (an asymmetric property that prevents superimposition of a mirror image of an object or molecule), researchers can generate non-fake QR codes. These codes combine the color of spherical CLC particles with commercially available non-chiral pigments and can be read using a special circular polarizer that only transmits non-chiral light but blocks the chiral light of the QR code.

“The development of spherical CLC particles from this research will provide new opportunities for low-cost structural color functions that differ from conventional colored materials,” said Takeoka. “In addition to its use as a special non-fake functional pigment, it can also be used for other applications that take advantage of the circular polarization of structural color with very little angle dependence.” Source