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Multilayered Liquid Crystal Formed Through Phase Separation Exhibits Multiple Colors

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SEOUL, South Korea, Aug. 19, 2020 — A technique for fabricating multilayered liquid crystals could potentially replace the color-shifting ink used for preventing forgery of bank notes, ID cards, and other items. Developed jointly by researchers at the Korea Institute of Science and Technology (KIST) and the Korea Advanced Institute of Science and Technology (KAIST), the liquid crystals can exhibit diverse optical characteristics from having multiple layers. To make such a material, each layer must be built on top of the one before it in a controlled manner. The researchers developed a technology for this complex process.

The research team added a chiral dopant to the liquid crystal material to form cholesteric liquid crystal (CLC). Due to its periodic nanostructure, CLC can exhibit color, without the addition of pigments, by selectively reflecting light of certain wavelengths. This light is circularly polarized, and by changing certain polarization conditions, it is possible to make the colors displayed by the CLC appear and disappear.

The researchers also added a cosolvent that dissolved in both oil and water. The three-part mixture of liquid crystal (a hydrophobic material), moisturizing agent (a hydrophilic material), and cosolvent was microfluidically emulsified to form uniform oil-in-water drops, which underwent liquid-liquid phase separation (LLPS) to form drops composed of alternating CLC-rich and CLC-depleted layers.

Liquid crystal particles with multiple layers formed through phase separation process developed by KIST-KAIST joint research team. Courtesy of Korea Institute of Science and Technology (KIST).

Liquid crystal particles with multiple layers formed through a phase separation process developed by a KIST-KAIST joint research team. Courtesy of Korea Institute of Science and Technology (KIST).

The researchers could control the multiplicity of these layers from levels of one to five by adjusting the initial composition of the mixture, which dictated the number of consecutive steps of LLPS. The concentration of the chiral dopant became reduced from the outermost to the innermost CLC drop due to uneven partitioning during LLPS, which allowed the multilayered liquid crystals to display multiple structural colors.

A schematic diagram of multi-layered liquid crystal particles developed by the KIST-KAIST joint research team. Courtesy of Korea Institute of Science and Technology (KIST).
Multilayered liquid crystal particles developed by the KIST-KAIST joint research team. Courtesy of Korea Institute of Science and Technology (KIST).

Through this process, the research team made a material that can exhibit two or more different characteristics at the same time. This new approach to fabricating multilayered liquid crystals through a process of emulsifying could provide new opportunities for optical applications. The multilayered liquid crystals also provide band-edge lasing at two different wavelengths.

“What we’ve developed is a simple method of creating multilayered liquid crystals and we expect it will serve as the basis for adding unique optical characteristics to materials,” KIST researcher Sang-seok Lee said. “Based on this new technology, we plan on developing diverse functional particles to develop composite materials.”

To the best knowledge of the researchers, this process is a new technology and the results constitute a new class of photonic multishells.

The research was published in Advanced Materials (www.doi.org/10.1002/adma.202002166). 

Photonics.com
Aug 2020
Research & TechnologyeducationAsia-PacificKorea Institute of Science and Technologymaterialslight sourcesopticsSensors & DetectorsKorea Advanced Institute of Science and Technologyphotonic shellsliquid-liquid phase separationlaserslaser resonators

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