Sang Seok Lee

Robust Microfluidic Encapsulation of Cholesteric Liquid Crystals Toward Photonic Ink Capsules

Robust photonic microcapsules are created by microfluidic encapsulation of cholesteric liquid crystals with a hydrogel membrane. The membrane encloses the cholesteric core without leakage in water and the core exhibits pronounced structural colors. The photonic ink capsules, which have a precisely controlled bandgap position and size, provide new opportunities in colorimetric micro-thermometers and optoelectric applications.

Reconfigurable Photonic Capsules Containing Cholesteric Liquid Crystals with Planar Alignment

Cholesteric liquid crystals (CLCs) reflect selected wavelengths of light owing to their periodic helical structures. The encapsulation of CLCs leads to photonic devices that can be easily processed and might be used as stand-alone microsensors. However, when CLCs are enclosed by polymeric membranes, they usually lose their planar alignment, leading to a deterioration of the optical performance. A microfluidics approach was employed to integrate an ultrathin alignment layer into microcapsules to separate the CLC core and the elastomeric solid membrane using triple-emulsion drops as the templates. The thinness of the alignment layer provides high lubrication resistance, preserving the layer integrity during elastic deformation of the membrane. The CLCs in the microcapsules can thus maintain their planar alignment, rendering the shape and optical properties highly reconfigurable.

Nonspherical Double Emulsions with Multiple Distinct Cores Enveloped by Ultrathin Shells

Microfluidics has provided means to control emulsification, enabling the production of highly monodisperse double-emulsion drops; they have served as useful templates for production of microcapsules. To provide new opportunities for double-emulsion templates, here, we report a new design of capillary microfluidic devices that create nonspherical double-emulsion drops with multiple distinct cores covered by ultrathin middle layer. To accomplish this, we parallelize capillary channels, each of which has a biphasic flow in a form of core-sheath stream; this is achieved by preferential wetting of oil to the hydrophobic wall. These core-sheath streams from the parallelized channels are concurrently emulsified into continuous phase, making paired double-emulsion drops composed of multiple cores and very thin middle shell. This microfluidic approach provides high degree of controllability and flexibility on size, shape, number, and composition of double-emulsion drops. Such double-emulsion drops are useful as templates to produce microcapsules with multicompartments which can encapsulate and deliver multiple distinct components, while avoiding their cross-contamination. In addition, nonspherical envelope exerts strong capillary force, leading to preferential coalescence between innermost drops; this is potentially useful for nanoliter-scale reactions and encapsulations of the reaction products.

Structural Color Palettes of Core–Shell Photonic Ink Capsules Containing Cholesteric Liquid Crystals

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 colors caused by dual photonic bandgaps, resulting in a rich variety of color on the optical palette. Moreover, the microcapsules can switch the colors from either core or shell depending on the selection of light-handedness.

Osmocapsules for Direct Measurement of Osmotic Strength

Monodisperse microcapsules with ultra-thin membranes are microfluidically designed to be highly sensitive to osmotic pressure, thereby providing a tool for the direct measurement of the osmotic strength. To make such osmocapsules, water-in-oil-in-water double-emulsion drops with ultra-thin shells are prepared as templates through emulsification of core-sheath biphasic flow in a capillary microfluidic device. When photocurable monomers are used as the oil phase, the osmocapsules are prepared by in-situ photopolymerization of the monomers, resulting in semipermeable membranes with a relatively large ratio of membrane thickness to capsule radius, approximately 0.02. These osmocapsules are buckled by the outward flux of water when they are subjected to a positive osmotic pressure difference above 125 kPa. By contrast, evaporation-induced consolidation of middle-phase containing polymers enables the production of osmocapsules with a small ratio of membrane thickness to capsule radius of approximately 0.002. Such an ultra-thin membrane with semi-permeability makes the osmocapsules highly sensitive to osmotic pressure; a positive pressure as small as 12.5 kPa induces buckling of the capsules. By employing a set of distinct osmocapsules confining aqueous solutions with different osmotic strengths, the osmotic strength of unknown solutions can be estimated through observation of the capsules that are selectively buckled. This approach provides the efficient measurement of the osmotic strength using only a very small volume of liquid, thereby providing a useful alternative to other measurement methods which use complex setups. In addition, in-vivo measurement of the osmotic strength can be potentially accomplished by implanting these biocompatible osmocapsules into tissue, which is difficult to achieve using conventional methods.

Robust photonic microparticles comprising cholesteric liquid crystals for anti-forgery materials

Cholesteric liquid crystals (CLCs) possess a 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 this unique optical property renders CLCs promising for various applications, their fluidity severely limits the ease of processing and their structural stability. To overcome this limitation, we designed CLC microparticles (CLC-MPs) via photopolymerization of reactive mesogens (RMs) in CLC droplets. Using capillary microfluidic devices, highly uniform emulsion drops of CLC-RM mixtures were prepared in aqueous phase drops, which were then exposed to ultraviolet (UV) irradiation to obtain solid microparticles. The diameter of the CLC-MPs can be precisely controlled by either manipulating the flow rates of the dispersed and continuous phases or varying the diameter of the capillary orifice in the microfluidic devices. The wavelength of reflection and the handedness of the helical structure are selected via the composition of the dispersed phase. The photo-polymerization of RMs leads to the formation of a three-dimensional rigid network, thereby yielding 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 via mechanical rubbing. Moreover, 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.

Ultrathin Double-Shell Capsules for High Performance Photon Upconversion

Triplet-fusion-based photon upconversion capsules with ultrathin double shells are developed through a single dripping instability in a microfluidic flow-focusing device. An inner separation layer allows use of a brominated hydrocarbon oil-based fluidic core, demonstrating significantly enhanced upconversion quantum yield. Furthermore, a perfluorinated photocurable monomer serves as a transparent shell phase with remote motion control through magnetic nanoparticle incorporation.

Thermoresponsive Microcarriers for Smart Release of Hydrate Inhibitors under Shear Flow

The hydrate formation in subsea pipelines can cause oil and gas well blowout. To avoid disasters, various chemical inhibitors have been developed to prevent or delay the hydrate formation and growth. Nevertheless, direct injection of the inhibitors results in environmental contamination and cross-suppression of inhibition performance in the presence of other inhibitors against corrosion and/or formation of scale, paraffin, and asphaltene. Here, we suggest a new class of microcarriers that encapsulate hydrate inhibitors at high concentration and release them on demand without active external triggering. The key to the success in microcarrier design lies in the temperature dependence of polymer brittleness. The microcarriers are microfluidically created to have an inhibitor-laden water core and polymer shell by employing water-in-oil-in-water (W/O/W) double-emulsion drops as a template. As the polymeric shell becomes more brittle at a lower temperature, there is an optimum range of shell thickness that renders the shell unstable at temperature responsible for hydrate formation under a constant shear flow. We precisely control the shell thickness relative to the radius by microfluidics and figure out the optimum range. The microcarriers with the optimum shell thickness are selectively ruptured by shear flow only at hydrate formation temperature and release the hydrate inhibitors. We prove that the released inhibitors effectively retard the hydrate formation without reduction of their performance. The microcarriers that do not experience the hydration formation temperature retain the inhibitors, which can be easily separated from ruptured ones for recycling by exploiting the density difference. Therefore, the use of microcarriers potentially minimizes the environmental damages.

Wavelength-tunable and shape-reconfigurable photonic capsule resonators containing cholesteric liquid crystals

Shape-reconfigurable photonic capsules are microfluidically designed to make wavelength- and intensity-tunable microlasers. Cholesteric liquid crystals (CLCs) have a photonic bandgap due to the periodic change of refractive index along their helical axes. The CLCs containing optical gain have served as band-edge lasing resonators. In particular, CLCs in a granular format provide omnidirectional lasing, which are promising as a point light source. However, there is no platform that simultaneously achieves high stability in air and wavelength tunability. We encapsulate CLCs with double shells to design a capsule-type laser resonator. The fluidic CLCs are fully enclosed by an aqueous inner shell that promotes the planar alignment of LC molecules along the interface. The outer shell made of silicone elastomer protects the CLC core and the inner shell from the surroundings. Therefore, the helical axes of the CLCs are radially oriented within the capsules, which provide a stable omnidirectional lasing in the air. At the same time, the fluidic CLCs enable the fine-tuning of lasing wavelength with temperature. The capsules retain their double-shell structure during the dynamic deformation. Therefore, the CLCs in the core maintain the planar alignment along the deformed interface, and a lasing direction can be varied from omnidirectional to bi- or multidirectional, depending on the shape of deformed capsules.

High-performance solution-processable flexible and transparent conducting electrodes with embedded Cu mesh

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.