Sang Seok Lee

Cholesteric liquid crystals (CLCs) exhibit structural colors through selective reflection at their photonic stopband. However, their single-band reflection within the visible spectrum restricts accessible color diversity. While multiple helical pitches can broaden the palette, such approaches typically require complex, multilayered fabrication processes. Here, we report a bioinspired strategy that generates two distinct reflection bands from a single-pitched CLC film, enabling juxtaposition-based color mixing. Drawing inspiration from the dual retroreflection mechanism of Papilio palinurus, we integrate CLCs with a dome-down poly(vinyl alcohol) (PVA) microdome array to form a periodic dimple architecture. The curved PVA domes induce planar alignment of LC molecules, orienting the helical axes radially and establishing two distinct optical pathways: single normal reflection at the dimple bases and interstitial regions and oblique double reflection at the dimple rims with an apparent 45° angle. These spatially separated dot, background, and rim features merge into a unified mixed hue in the far field. By adjusting chiral dopant concentration and dimple areal density, the base wavelength of the CLC stopband and the relative intensity balance between the two reflection pathways can be tuned, enabling near-infrared/visible, visible/visible, and ultraviolet/visible combinations. This single-pitch, dual-pathway design provides a scalable and versatile platform for adaptive photonic coatings, angle-robust displays, and high-security anticounterfeiting.

As electronic systems increasingly operate in space environments, the demand for lightweight shielding materials capable of blocking electromagnetic interference, attenuating radiation, and managing heat continues to grow. However, conventional shielding materials either employ separate materials for each function or rely on simple mixing, leading to excessive weight gain, and particularly, random mixing of conductive and insulating components damages the inherent networks of each component, failing to exploit the synergistic effects of composites. This study presents a novel strategy to overcome these limitations by controlling nanofiller dimensionality and film stacking architecture in carbon nanotube (CNT) and boron nitride (BN) hybrid systems. Through comparative studies of two-dimensional hexagonal BN (hBN) and one-dimensional BN nanotubes (BNNT), we revealed that the one-dimensional morphology of BNNT maintains CNT percolation networks while forming effective interfacial interactions, whereas hBN sheets cause weak interfacial bonding. Based on this dimension-matched filler system, we designed a multilayer architecture with alternating conductive CNT and insulating BNNT layers, preserving the intrinsic properties of each layer while inducing synergistic effects. This CNT/BNNT multilayer structure demonstrated superior performance with enhanced mechanical strength, thermal conductivity, and particularly electromagnetic shielding effectiveness that exceeded uniformly mixed films by 11.7 dB in the X-band. Furthermore, the ¹⁰B isotope in BNNT layers can provide efficient neutron attenuation, enabling application as multifunctional materials for simultaneous electromagnetic and neutron shielding. By demonstrating the effects of nanofiller dimensionality and stacking architecture on composite performance enhancement, this work provides new directions for developing multifunctional shielding materials in aerospace applications.

Boron nitride nanotubes (BNNTs) are cylindrical nanostructures characterized by alternating boron and nitrogen atoms. Unlike carbon nanotubes (CNT), BNNTs are electrically insulating and exhibit high thermal stability and neutron shielding capabilities. Their unique tubular structure enables the formation of 1D arrays, which can achieve a nematic liquid crystal phase, ideal for fabricating high‐density structures. However, the widespread application of BNNTs has been hindered by challenges in purifying and dispersing high‐purity BNNTs. This study aims to exploit the liquid crystal properties of BNNTs by generating high‐purity BNNT dispersions through a series of dispersion and purification processes. Furthermore, this study utilizes microfluidics technology to encapsulate the BNNT dispersions. Using this approach, the transition of liquid crystal phases in relation to BNNT concentration is efficiently examined. This process results in the production of high‐density BNNT films characterized by pronounced anisotropy, which in turn provides significant thermal neutron shielding effects.

Cholesteric liquid crystals (CLCs), also known as chiral nematic LCs, show a photonic stopband, which is promising for various optical applications. In particular, CLCs confined in microcompartments are useful for sensing, lasing, and optical barcoding at the microscale. The integration of distinct CLCs into single microstructures can provide advanced functionality. In this work, CLC multishells with multiple stopbands are created by liquid–liquid phase separation (LLPS) in a simple yet highly controlled manner. A homogeneous ternary mixture of LC, hydrophilic liquid, and co‐solvent is microfluidically emulsified to form uniform oil‐in‐water drops, which undergo LLPS to form onion‐like drops composed of alternating CLC‐rich and CLC‐depleted layers. The multiplicity is controlled from one to five by adjusting the initial composition of the ternary mixture, which dictates the number of consecutive steps of LLPS. Interestingly, the concentration of the chiral dopant becomes reduced from the outermost to the innermost CLC drop due to uneven partitioning during LLPS, which results in multiple stopbands. Therefore, the photonic multishells show multiple structural colors. In addition, dye‐doped multishells provide band‐edge lasing at two different wavelengths. This new class of photonic multishells will provide new opportunities for advanced optical applications.

Microgels, microparticles made of hydrogels, show fast diffusion kinetics and high reconfigurability while maintaining the advantages of hydrogels, being useful for various applications. Here, presented is a new microfluidic strategy for producing polymer‐graphene oxide (GO) composite microgels without chemical cues or a temperature swing for gelation. As a main component of microgels, polymers that are able to form hydrogen bonds, such as polyvinyl alcohol (PVA), are used. In the mixture of PVA and GO, GO is tethered by PVA through hydrogen bonding. When the mixture is rapidly concentrated in the core of double‐emulsion drops by osmotic‐pressure‐driven water pumping, PVA‐tethered GO sheets form a nematic phase with a planar alignment. In addition, the GO sheets are linked by additional hydrogen bonds, leading to a sol–gel transition. Therefore, the PVA–GO composite remains undissolved when it is directly exposed to water by oil‐shell rupture. These composite microgels can be also produced using poly(ethylene oxide) or poly(acrylic acid), instead of PVA. In addition, the microgels can be functionalized by incorporating other polymers in the presence of the hydrogel‐forming polymers. It is shown that the multicomponent microgels made from a mixture of polyacrylamide, PVA, and GO show an excellent adsorption capacity for impurities.

Microcapsules with molecule‐selective permeation are appealing as microreactors, capsule‐type sensors, drug and cell carriers, and artificial cells. To accomplish molecular size‐ and charge‐selective permeation, regular size of pores and surface charges have been formed in the membranes. However, it remains an important challenge to provide advanced regulation of transmembrane transport. Here, smart microcapsules are designed that provide molecular polarity‐ and temperature‐dependent permeability. With capillary microfluidic devices, water‐in‐oil‐in‐water (W/O/W) double‐emulsion drops are prepared, which serve as templates to produce microcapsules. The oil shell is composed of two monomers and dodecanol, which turns to a polymeric framework whose continuous voids are filled with dodecanol upon photopolymerization. One of the monomers provides mechanical stability of the framework, whereas the other serves as a compatibilizer between growing polymer and dodecanol, preventing macrophase separation. Above melting point of dodecanol, molecules that are soluble in the molten dodecanol are selectively allowed to diffuse across the shell, where the rate of transmembrane transport is strongly influenced by partition coefficient. The rate is drastically lowered for temperatures below the melting point. This molecular polarity‐ and temperature‐dependent permeability renders the microcapsules potentially useful as drug carriers for triggered release and contamination‐free microreactors and microsensors.

Shape-reconfigurable photonic capsules are microfluidically designed to make wavelength- and intensity-tunable microlasers.

Hydrate formation in subsea flowlines is one of the main concerns to safely transport the hydrocarbon fluids in the field of flow assurance. To prevent blockage of hydrate, thermodynamic hydrate inhibitors (THIs) have been injected into the subsea flowlines; however, the high injection amount of THIs may drive up operational expenditure (OPEX) with increasing water depth of oil and gas reservoirs. Kinetic hydrate inhibitors (KHIs) have been investigated as an alternative for few decades. In this study, microcapsules were designed using microfluidics concept for efficient release of KHIs at off-shore transport line of hydrocarbon and gas. Hydrate formation kinetics among four different systems, pure water, microcapsules containing poly vinyl caprolactam (PVCap), microcapsules without PVCap, and PVCap aqueous solution was investigated. The dispersive Raman was applied to understand hydrate formation on microcapsules with temperature change under static condition, and autoclave study also was employed to confirm the shear rate effect on microcapsule stability. From the both experiments, it was confirmed that release of KHI solution from microcapsules delayed the formation of hydrate at the target temperature and showed a similar efficacy comparing to the bulk KHI solution under turbulent condition. These results indicated the release of PVCap solution can be controlled by adjusting the membrane thickness of the microcarriers, which would provide operational flexibility to manage the risk of hydrate plug formation.

Alternative transparent and conducting electrodes (TCEs) that can overcome the practical limitations of the existing TCEs have been explored. Although network structures of metal nanowires have been investigated for TCEs because of their excellent performance, characteristics such as high junction resistances, poor surface roughness, and randomly entangled NW networks still pose challenges. Here, we report cost-effective and solution-processable metallic mesh TCEs consisting of a Cu-mesh embedded in a flexible PDMS substrate. The unprecedented structures of the Cu-mesh TCEs offer considerable advantages over previous approaches, including high performance, surface smoothness, excellent flexibility, electromechanical stability, and thermal stability. Our Cu-mesh TCEs provide a transmittance of 96% at 550 nm and a sheet resistance of 0.1 Ω sq⁻¹, as well as extremely high figures of merit, reaching up to 1.9 × 10⁴, which are the highest reported values among recent studies. Finally, we demonstrate high-performance transparent heaters based on Cu-mesh TCEs and in situ color tuning of cholesteric liquid crystals (CLCs) using them, confirming the uniform spatial electrical conductivity as well as the reproducibility and reliability of the electrode.