A research team of Dr. Jialei He from Nagoya University‘s Graduate School of Engineering has developed a method for converting cholesteric liquid crystals (CLC) into micrometer-sized (Water crystals mimic the shell color units used to create a more secure form of QR code) spheres. CLC is a type of liquid crystal that has a helical structure, giving them unique optical properties and the ability to selectively reflect light. By combining circular CLC particles with commercially available pigments, researchers have developed an anti-counterfeit QR code that can only be scanned under a single circular polarizer.
The results were published in the journal Advanced Optical Materials. CLC is an example of how creativity can be used in engineering. If you’ve ever seen the wings of butterflies or the shiny coating of exoskeleton beetles, you’ve seen what CLC can do. Once discovered, the CLCs that mimic the units that produce the color of the exoskeleton are used in the laboratory because of the color and their unusual properties, which are between liquid and crystal.
The optical properties of CLC are particularly useful. They exhibit unique colors due to their unique cell structure and optical properties that lead to the production of light in long wavelengths. CLC is composed of long molecules arranged in a helix. In a helix, the distance from the loops is called “pitch”.
If the helix has units of frequency close to each other, the liquid crystal has a small pitch and reflects a short wavelength of light, giving blue and purple. However, those with vertical space have longer wavelengths, leading to a red or orange color. To put it further, since the cells that make up the crystal are arranged in a helix, the color can change depending on the viewer’s rotation in the helix.
Therefore, an infinite number of colors are possible depending on how the liquid crystal is viewed. In order to use CLC effectively, researchers are making CLC components around. These objects are circular and include a helix in a 3D matrix so that scientists can control their color more precisely.
However, one key issue is size. Current methods produce spherical CLC particles of 100 micrometers, which are large enough for many uses.
To solve this problem, doing research Jialei He (he/she) and Yukikazu Takeoka (he/she) from Nagoya University and their colleagues use a mixture of solvents to produce spherical CLC that controls the particle size. a few micrometers to use a process called dispersion polymerization.
As the sample is taken at room temperature, finding the new method is difficult. “Testing the sample is a particularly difficult time because of the softness of the sample at room temperature, which is a property of CLC,” Dr. He said. “Therefore, a lot of effort is needed to find the right method to identify those samples without doing anything wrong.”
Since the pitch of the cholesteric liquid crystal of Spherical CLC particles of this size varies with the curvature of the body, the researchers made the particles spherical with a uniform size distribution. This is called a monodisperse sphere.
“During the experiment, we unexpectedly found that the particle size of the microspheres has an impact on the structural color. We can produce a variety of colors depending on the particle size,” said Dr. Hey.
“We also found that Spherical CLC coatings. Particles with polydimethylsiloxane polymer improved stain and light resistance.”
An application that can do this research is to create a more secure QR code that cannot be copied. They can be created using a characteristic of CLC called chirality.
Chirality refers to the property of an element or molecule that cannot be aligned with its mirror due to asymmetry.
CLC is chiral and has optical activity, so anti-counterfeiting QR codes can be created by combining the colors of CLC molecules around with commercially available non-chiral pigments. The code can only be read when a circular polarizer is used to pass the non-chiral light but not the chiral light of the QR code.
The development of spherical CLC particles resulting from this research will provide new opportunities for low-cost color processing projects in a variety of colorants,” said Dr. Takeoka said. “In addition to being used as a special anti-counterfeiting pigment, it can also be used for other applications that use the advantage of circularly polarized structural colors and small background angles.”
The study, “Particle Size Controlled Chiral Structural Color of Monodisperse Cholesteric Liquid Crystals Particles,” was published in the journal Advanced Optical Materials.
Source: Nagoya University
