New Article in Journal of Materials Chemistry C

posted Jul 2, 2017, 4:39 PM by Shin-Hyun Kim
Hyeon Jin Seo, Sang Seok Lee, Jieun Noh, Jae Won Ka, Cheolmin Park, Shin-Hyun Kim, and Yun Ho Kim, "Robust Photonic Microparticles comprised of Cholesteric Liquid Crystals for Anti-forgery Materials", Journal of Materials Chemistry C Accepted for publication (2017)  (Seo & Lee contributed equally, Co-corresponding author)

Cholesteric liquid crystals (CLCs) possess photonic bandgap owing to the helical arrangement of molecules. The CLCs reflect circularly-polarized light of a specific handedness and wavelength, exhibiting colors. The wavelength of the selective reflection, or the structural color, can be easily controlled by varying the concentration of a chiral dopant. Although such a unique optical property renders CLCs promising for various applications, their fluidity severely limits the ease of processing and structural stability. To overcome the limitation, we design CLC microparticles (CLC-MPs) by photopolymerization of reactive mesogens (RMs) in CLC droplets. With capillary microfluidic devices, highly uniform emulsion drops of CLC-RM mixtures are prepared in an aqueous phase drops, which are then exposed to ultraviolet (UV) to yield solid microparticles. The diameter of the CLC-MPs is precisely controlled by either manipulating the flow rates of dispersed and continuous phases or varying the diameter of the capillary orifice in the microfluidic devices. The wavelength of reflection and handedness of helical structure are selected by the composition of the dispersed phase. The photo-polymerization of RMs leads to the formation of a three-dimensional rigid network, thereby yielding the CLC-MPs with high mechanical stability. The CLC-MPs could be further assembled to form two-dimensional hexagonal arrays on flat surfaces or deposited in pre-defined trenches or holes by a mechanical rubbing. Two distinct CLC-MPs with opposite handedness can be patterned to show different color graphics depending on the selection of handedness of circularly-polarized light, which are appealing for anti-forgery patches.