CLCs display unusual colors due to their unique molecular structure and optical properties that lead to the selective reflection of light at specific wavelengths. Image: Yukikazu Takeoka and Jialei He.Researchers from Nagoya University in Japan have developed a method for processing cholesteric liquid crystals (CLCs) into micrometer-sized spherical particles.
CLCs are a type of liquid crystal that possess a helical structure, giving them unique optical properties and the ability to selectively reflect light. By combining the spherical CLC particles with commercially available pigments, the researchers were able to develop a unique anti-counterfeiting QR code that can only be displayed under a specific circular polarizer. They report their work in a paper in Advanced Optical Materials.
This work is an example of how nature can be used in engineering, as CLCs are responsible for the iridescent wings of butterflies and the glossy coating on the exoskeletons of beetles. Because of their unusual colors and properties, which lie between liquids and crystals, CLCs are now synthesized in the laboratory. They are able to display these unusual colors due to their unique molecular structure and optical properties, which lead to the selective reflection of light at specific wavelengths.
CLCs consist of long molecules that repeat themselves in the shape of a helix. In the helix, the vertical distance from where one region loops around and repeats itself is called the ‘pitch’. If the helix has repeating units that are close together, the liquid crystal has a short pitch and reflects shorter wavelengths of light, giving off blue and violet colors. CLCs with a longer vertical space reflect longer wavelengths, leading to red or orange colors.
To complicate matters further, because the molecules that make up the crystal are arranged in a helix, the color can change depending on the viewer's orientation to the helix. Therefore, an infinite number of colors are possible depending on how the liquid crystal is viewed.
To utilize CLCs more effectively, researchers produce spherical CLC particles that incorporate the helix in a 3D matrix, as this allows better control of the coloration. But size is a major problem. Current methods create 100µm spherical CLC particles, which are too large for most uses.
To tackle this problem, Jialei He, Yukikazu Takeoka and their colleagues at Nagoya University used a technique called dispersion polymerization to create spherical CLC particles with a controlled particle size of just a few micrometers. And after creating them, the researchers also needed to develop a technique for characterizing them.
“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 He. “Consequently, a considerable amount of effort was required to find an appropriate method to characterize the samples without causing any damage.”
Since the pitch of the CLCs in spherical particles of this size varies with the curvature of the particles, the researchers produced them with a uniform size distribution. This is known 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 He. “We also found that covering the spherical CLC particles with the polymer polydimethylsiloxane improved the coloration and thermal stability.”
One potential application of this research is the creation of more secure QR codes that cannot be replicated, which would take advantage of a feature of the CLCs called chirality. This refers to the property possessed by an object or molecule that cannot be superimposed onto its mirror image because of an asymmetry. CLCs are chiral and have optical activity, so an anti-counterfeiting QR code could be created by combining the color of the spherical CLC particles with commercially available non-chiral pigments. This code could only be read by a specific circular polarizer that can distinguish the chiral light of the CLC particles.
“The development of spherical CLC particles development resulting from this research will provide new possibilities for low-cost structural color functions different from those of conventional color materials,” said 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.”
This story is adapted from material from Nagoya University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.