Recent Announcement
New Article in ACS Nano
Jaeho Choi, Wonhee Cho, Seung Yeol Lee, Yeon Sik Jung, Shin-Hyun Kim, and Hee-Tak Kim, "Flexible and Robust Superomniphobic Surfaces created by Localized Photofluidization of Azopolymer Pillars", ACS Nano, Accepted for publication (2017). (Co-corresponding author) Springtails, insects which breathe through their skins, possess mushroom-shaped nanostructures. As doubly re-entrant geometry in the mushroom head enhances the resistance against liquid invasion, the springtails have robust, liquid-free omniphobic skins. Although omniphobic surfaces are promising for various applications, it remains an important challenge to mimic the structural feature of springtails. This paper presents a pragmatic method to create doubly re-entrant nanostructures and robust super-omniphobic surfaces by exploiting localized photofluidization of azopolymers. Irradiation of circularly-polarized light reconfigures azopolymer micropillars to have a mushroom-like head with a doubly re-entrant nano-geometry through protrusion and inward bending of polymer film from the top edge. The light-driven reconfigured micropillars facilitate the pining of triple line as the springtails do. In particular, the unique geometry exhibits super-omniphobicity even for liquids whose equilibrium contact angles are almost zero in the presence of a practical level of external pressure. In addition, the simple fabrication process is highly reproducible, scalable, and compatible with various substrate materials including flexible polymeric film. Our results suggest that our photofluidization technology will provide a practical route to develop robust super-omniphobic surfaces.  |
New Article in Journal of Materials Chemistry C
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.  |
Frontispiece in Advanced Materials
Sang Seok Lee, Hyeon Jin Seo,Yun Ho Kim, and Shin-Hyun Kim, "Structural Color Palettes of Core-Shell Photonic Ink Capsules containing Cholesteric Liquid Crystals", Advanced Materials, 29, 1606894 (2017).(Co-corresponding author)  |
Front Cover Paper in Advanced Materials
Soojeong Cho, Tae Soup Shim, Ju Hyeon Kim, Dong-Hyun Kim, and Shin-Hyun Kim, "Selective Coloration of Melanin Nanospheres through Resonant Mie Scattering", Advanced Materials, 29, 1700256 (2017). [pdf]  |
New paper in Small
Jing Fan, Shin-Hyun Kim, Zi Chen, Shaobing Zhou, Esther Amstad, Tina Lin, and David A. Weitz, "Creation of faceted polyhedral microgels from compressed emulsions", Small, Accepted for publication (2017) (Fan & Kim contributed equally, Co-corresponding author) Compressed monodisperse emulsions in confined space exhibit highly ordered structures. We investigate the influence of the volume fraction and the confinement geometry on the organized structures and study the mechanism by which structural transition occurs. Based on the understanding of ordering behavior of compressed emulsions, we develop a simple and high-throughput method to fabricate monodisperse polyhedral microgels using the emulsions as the template. By controlling the geometry of the confined spaces, we implement a variety of shapes such as hexagonal prism, Fejes Toth honeycomb prism, truncated octahedron, pyritohedron, and truncated hexagonal trapezohedron. Moreover, the edge sharpness of each shape is controllable by adjusting the drop volume fraction. This design principle can be readily extended to other shapes and materials, and therefore provides a useful means to create polyhedral microparticles for both fundamental study and practical applications.  |
New article in ACS AM&I
Sang Seok Lee, Juwoon Park, Yutaek Seo, and Shin-Hyun Kim, "Thermo-Responsive Microcarriers for Smart Release of Hydrate Inhibitors under Shear Flow", ACS Applied Materials & Interfaces, Accepted for publication (2017)  |
New paper in Small
Tae Yong Lee, Minhee Ku, Bomi Kim, Sangmin Lee, Jaemoon Yang and Shin-Hyun Kim, "Microfluidic Production of Biodegradable Microcapsules for Sustained Release of Hydrophilic Actives", Small, Accepted for publication (2017) (Corresponding author)  |
New Paper in Small
Dong Jae Kim, Tae Yoon Jeon, Sung-Gyu Park, Hye Ji Han, Sang Hyuk Im, Dong-Ho Kim and Shin-Hyun Kim, "Uniform Microgels containing Agglomerates of Silver Nanocubes for Molecular Size-Selectivity and High SERS Activity", Small, accepted for publication (2017). Microgels containing agglomerates of silver nanocubes are microfluidically created to provide SERS-active substrates. The microgels permit selective infusion of small molecules and nanogaps among the nanocubes serve as hot spots for SERS. Therefore, the microgels enable us to selectively analyze Raman spectra of small molecules dissolved in complex mixtures with high sensitivity in the absence of interruption from large adhesives.  |
New Communication in Adv. Mater.
Sang Seok Lee, Hyeon Jin Seo,Yun Ho Kim, and Shin-Hyun Kim, "Structural Color Palettes of Core-Shell Photonic Ink Capsules containing Cholesteric Liquid Crystals", Advanced Materials, accepted for publication (2017). Photonic microcapsules with onion-like topology are microfluidically designed to have cholesteric liquid crystals with opposite handedness in their core and shell. The microcapsules exhibit structural colours caused by dual photonic bandgaps, resulting in a rich variety of colour on the optical palette. Moreover, the microcapsules can switch the colours from either core or shell depending on the selection of light-handedness.  |
New Communication in Adv. Mater.
Soojeong Cho, Tae Soup Shim, Ju Hyeon Kim, Dong-Hyun Kim, and Shin-Hyun Kim, "Selective Coloration of Melanin Nanospheres through Resonant Mie Scattering", Advanced Materials, accepted for publication (2017). Black melanin inks are prepared to selectively exhibit colors under strong light, inspired by human hair. High absorbance of melanin suppresses multiple scattering, causing resonant Mie scattering predominant. Various colors can be developed as the resonant wavelength dictated by nanosphere diameter. Therefore, the melanin inks can be used to encrypt and selectively disclose multicolor patterns for anti-counterfeiting applications.  |
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