Beetle-inspired liquid crystals: Are these our future secure QR codes?

These materials possess a distinct molecular arrangement that forms a helix, which interacts with light in a way that selectively reflects specific wavelengths, resulting in brilliant and varied colors.

Simply put, short-pitch CLCs reflect blue and violet, while longer pitches produce red and orange hues. Moreover, color variation occurs based on the viewer's perspective, offering an infinite color spectrum.

While scientists are already lab-producing CLCs, existing methods produce particles 100 micrometers in diameter, which is too large for most applications. 

To overcome this challenge, a team of researchers from Nagoya University, led by Jialei He and Yukikazu Takeoka, developed a solution using a mixture of solvents and a dispersion polymerization technique. 

This innovative approach allows them to create much smaller spherical CLC particles with a more precise size control, measuring only a few micrometers in diameter.

"The sample testing was a particularly challenging time due to the softness of the samples at room temperature, which is a property inherent to CLCs," said Dr. He in a press release.

"Consequently, a considerable amount of effort was required to find an appropriate method to characterize the samples without causing any damage."

To ensure consistent coloration, the researchers made the CLC particles spherical with a uniform size distribution, which is referred to as a monodisperse sphere. 

"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 Dr. He. 

"We also found that covering the spherical CLC particles with the polymer polydimethylsiloxane improved the coloration and thermal stability."

This research has potential applications in creating more secure QR codes that are difficult to replicate. These could utilize a property of CLCs called chirality, which refers to an object's inability to be superimposed onto its mirror image due to its asymmetry.

Combining the color of spherical CLC particles with non-chiral pigments can produce an anti-counterfeiting QR code, which can only be read using a specific circular polarizer that allows certain light to pass through while blocking the rest.

The complete study was published in Advanced Optical Materials on July 9.

Study abstract:

Cholesteric liquid crystals (CLCs) are becoming increasingly popular due to their unique chiral structural color. Unlike ordinary CLCs materials, CLCs particles exhibit angle-independence, making them particularly noteworthy. However, currently, there are limited effective methods for controlling the structural color of CLCs particles, other than adjusting the concentration of chiral dopants or introducing stimuli-responsive groups. Here, a scalable and cost-effective process for preparing monodisperse CLCs particles via dispersion polymerization is reported. By making CLCs into micrometer-sized monodisperse spheres, the helical pitch of CLCs can be varied according to its particle size, and the resulting structural color hue due to Bragg reflection can also be changed. Covering the CLCs particles with polydimethylsiloxane results in the formation of a polymer dispersed liquid crystals–like structure, which enhances the structural color appearance and thermal stability of the CLCs particles. Additionally, a simple strategy to produce chiral anti-counterfeiting QR codes is developed. By combining CLCs particles with commercially available pigments, an anti-counterfeiting QR code that can only be displayed under a specific circular polarizer is created. This approach and resulting CLCs particles expand the modulation of CLCs structural color and enrich the application of structural color in the field of chiral optical anti-counterfeiting.