April 14, 2024

Dr. Jialei He, from Nagoya University’s Graduate School of Engineering, and his research group have achieved a groundbreaking development in the field of liquid crystals. Liquid crystals with a helical structure, known as cholesteric liquid crystals (CLCs), possess unique optical properties, enabling them to selectively reflect light. Inspired by nature, where iridescent wings of butterflies and glossy coatings on beetles’ exoskeletons showcase the capabilities of CLCs, the researchers devised a method to process CLCs into micrometer-sized spherical particles.

The team combined these spherical CLC particles with readily available pigments to create an innovative anti-counterfeiting QR code, visible exclusively under specific circular polarizers. Their impressive results were published in the esteemed journal Advanced Optical Materials.

CLCs stand as a remarkable example of how nature can inspire engineering endeavors. By synthesizing CLCs that mimic the color-generating units in beetle exoskeletons, scientists have harnessed their unique properties, bridging the gap between liquids and crystals.

The optical properties of CLCs are particularly advantageous, leading to their fascinating display of colors. These properties arise from their distinctive molecular structure, which selectively reflects light at specific wavelengths. The helical arrangement of long molecules within CLCs determines their pitch, i.e., the vertical distance where one region loops around and repeats itself.

Short pitch CLCs reflect shorter wavelengths, producing blue and violet colors, while longer pitches result in red or orange colors due to the reflection of longer wavelengths. Moreover, the color displayed can vary depending on the viewer’s orientation to the helix, leading to a vast array of possible colors.

To enhance the utilization of CLCs, researchers have devised spherical CLC particles that incorporate the helix within a 3D matrix, providing better control over coloration. However, a key challenge lies in their size, as the current methods produce particles that are 100 micrometers in diameter, which is often too large for practical applications.

One potential application for this research is the creation of more secure QR codes that cannot be replicated. Credit: Yukikazu Takeoka and Jialei He

In a collaborative effort led by Dr. Jialei He and Dr. Yukikazu Takeoka from Nagoya University, researchers successfully addressed the challenge of particle size in cholesteric liquid crystals (CLCs) to develop a groundbreaking technique. Through dispersion polymerization, they used a mixture of solvents to create micrometer-sized spherical CLC particles with controlled sizes.

The experimentation process was arduous due to the soft nature of CLCs at room temperature, making sample testing difficult without causing damage. However, the team’s perseverance paid off, and they achieved uniform-sized spherical particles, known as monodisperse spheres, with a fascinating range of colors determined by the particle size.

To enhance the coloration and thermal stability, the researchers coated the spherical CLC particles with the polymer polydimethylsiloxane. This finding opens up new possibilities for utilizing these particles in various applications, including the creation of highly secure QR codes that are resistant to counterfeiting.

CLCs possess a unique property called chirality, indicating their inability to be superimposed onto their mirror images due to asymmetry. By combining the color of spherical CLC particles with non-chiral pigments, researchers can design anti-counterfeiting QR codes that require a specific circular polarizer to be read. This process allows non-chiral light through while blocking the chiral light of the QR code, ensuring enhanced security.

Dr. Takeoka highlights the potential of spherical CLC particles as low-cost structural color materials with various functional applications. Aside from anti-counterfeiting measures, these particles can be employed for their circularly polarized structural color in applications where minimal angle dependence is desirable. The development resulting from this research represents a significant advancement in the field, promising exciting prospects for future technological innovations.

Source: Nagoya University

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