Chang-Hyung Choi

One Step Formation of Controllable Complex Emulsions: From Functional Particles to Simultaneous Encapsulation of Hydrophilic and Hydrophobic Agents into Desired Position

This article presents a one-step method for generating complex emulsions that exploits the phase separation of the emulsion droplet generated in the microchannel. This approach easily produces double, triple, quadruple, and Janus emulsions with monodisperse size. These emulsions can be used as useful templates for the synthesis of new functional materials, such as microcapsules, hemispheres, Janus particles and microcarriers that are capable of simultaneously encapsulating hydrophilic and hydrophobic compounds with selective compartmentalization in a one-step process.

Surface-Tension-Induced Synthesis of Complex Particles Using Confined Polymeric Fluids†

Therefore, a novel methodology for the gen-eration of uniform particles with a large diversity of designmorphologies and physicochemical properties would be apromising platform for many advanced applications.Conventional approaches for the synthesis of polymericparticles with various shapes are self-assembly,

Fabrication of Uniform DNA-Conjugated Hydrogel Microparticles via Replica Molding for Facile Nucleic Acid Hybridization Assays

We identify and investigate several critical parameters in the fabrication of single-stranded DNA conjugated poly(ethylene glycol) (PEG) microparticles based on replica molding (RM) for highly uniform and robust nucleic acid hybridization assays. The effects of PEG-diacrylate, probe DNA, and photoinitiator concentrations on the overall fluorescence and target DNA penetration depth upon hybridization are examined. Fluorescence and confocal microscopy results illustrate high conjugation capacity of the probe and target DNA, femtomole sensitivity, and sequence specificity. Combined, these findings demonstrate a significant step toward simple, robust, and scalable procedures to manufacture highly uniform and high-capacity hybridization assay particles in a well-controlled manner by exploiting many advantages that the batch processing-based RM technique offers. We envision that the results presented here may be readily applied to rapid and high-throughput hybridization assays for a wide variety of applications in bioprocess monitoring, food safety, and biological threat detection.

A Facile Synthesis–Fabrication Strategy for Integration of Catalytically Active Viral-Palladium Nanostructures into Polymeric Hydrogel Microparticles via Replica Molding

The synthesis of small, uniform, well-dispersed and active Pd nanocatalysts under mild conditions in a predictable and controlled manner is an unmet challenge. Viral nanomaterials are attractive biotemplates for the controlled synthesis of nanoparticles due to their well-defined and monodisperse structure along with abundant surface functionalities. Here, we demonstrate spontaneous formation of small (1-2 nm), uniform and highly crystalline palladium (Pd) nanoparticles along genetically modified tobacco mosaic virus (TMV1cys) biotemplates without external reducing agents. The ratio between TMV and Pd precursor plays an important role in the exclusive formation of well-dispersed Pd nanoparticles along TMV biotemplates. The as-prepared Pd-TMV complexes are then integrated into the poly(ethylene glycol) (PEG)-based microparticles via replica molding (RM) technique in a simple, robust and highly reproducible manner. High catalytic activity, recyclability and stability of the hybrid Pd-TMV-PEG microparticles are further demonstrated through dichromate reduction as a model reaction. Taken together, these findings demonstrate a significant step toward simple, robust, and scalable synthesis and fabrication of efficient biotemplate-supported Pd nanocatalysts in readily deployable polymeric scaffolds with high capacity in a controlled manner.

Encapsulation and Enhanced Retention of Fragrance in Polymer Microcapsules

Fragrances are amphiphilic and highly volatile, all of which makes them a challenging cargo to efficiently encapsulate and retain in microcapsules using traditional approaches. We address these limitations by introducing a new strategy that combines bulk and microfluidic emulsification: a stable fragrance-in-water (F/W) emulsion that is primarily prepared from bulk emulsification is incorporated within a polymer microcapsule via microfluidic emulsification. On the basis of the in-depth study of physicochemical properties of the microcapsules on fragrance leakage, we demonstrate that enhanced retention of fragrance can be achieved by using a polar polymeric shell and forming a hydrogel network within the microcapsule. We further extend the utility of these microcapsules by demonstrating the enhanced retention of encapsulated fragrance in powder state.

A Rapid One-Step Fabrication of Patternable Superhydrophobic Surfaces Driven by Marangoni Instability

We present a facile and inexpensive approach without any fluorinated chemistry to create superhydrophobic surface with exceptional liquid repellency, transportation of oil, selective capture of oil, optical bar code, and self-cleaning. Here we show experimentally that the control of evaporation is important and can be used to form superhydrophobic surface driven by Marangoni instability: the method involves in-situ photopolymerization in the presence of a volatile solvent and porous PDMS cover to afford superhydrophobic surfaces with the desired combination of micro- and nanoscale roughness. The porous PDMS cover significantly affects Marangoni convection of coating fluid, inducing composition gradients at the same time. In addition, the change of concentration of ethanol is able to produce versatile surfaces from hydrophilic to superhydrophobic and as a consequence to determine contact angles as well as roughness factors. In conclusion, the control of evaporation under the polymerization provides a convenient parameter to fabricate the superhydrophobic surface, without application of fluorinated chemistry and the elegant nanofabrication technique.

Geometrically and chemically anisotropic particles at an oil–water interface

We present the behaviour of particles with chemical and geometrical anisotropy at a planar oil–water interface. We find that Janus cylinders with a small aspect ratio adopt an upright configuration, whereas the particles with a large aspect ratio exhibit both the upright and tilted configurations, which can be explained by the presence of two minima in the attachment energy profile. Such unique configurations significantly affect their assembly structure and lateral interactions. In particular, we observe strong capillary attractions between two tilted Janus cylinders and show that the scaling behaviour of the interaction depends on the lateral alignments of two cylinders. Consequently, this capillarity leads to a variety of assembled structures, which we attribute to the quasi-quadrupolar interface deformation observed around each particle.

Microfluidic Design of Complex Emulsions

Controllable generation of complex emulsions comprising exceptional features such as several compartments and shape anisotropy is becoming increasingly important. Complex emulsions are attracting great interest due to their significant potential in many applications, including foods, pharmaceuticals, cosmetics, materials, and chemical separations. Microfluidics is emerging as a promising route to the generation of complex emulsions, providing precise control over emulsion shape, size, and compartments. The aim of this Minireview is to mainly describe the progress of microfluidic approaches to design complex emulsions using hydrodynamic control and phase separation. The emulsions formed are classified according to their morphology, anisotropy, and internal structure. Emerging applications of complex emulsions formed using these microfluidic techniques are discussed.

Double Hydrophilic Janus Cylinders at an Air–Water Interface

Colloidal particles spontaneously attach to the interface between two immiscible fluids to minimize the interfacial area between the two phases. The shape and wettability of particles have a strong influence on their configuration and interactions at fluid-fluid interfaces. In this study, we investigate the behavior of asymmetrically hydrophilic Janus cylinders (or double hydrophilic Janus cylinders with two different hydrophilic regions) trapped at an air-water interface. We find that these double hydrophilic Janus cylinders with aspect ratios of 0.9, 1.2, and 2.4 adopt both end-on and tilted configurations with respect to the interface. Our numerical calculations show that the coexistence of these configurations is a result of multiple energy minima present in the attachment energy profile that can be represented as a complex energy landscape. Double hydrophilic Janus cylinders with tilted orientations induce hexapolar interface deformation, which accounts for the pair interactions between the particles as well as the nondeterministic assembly behaviors of these particles at the interface.

Synthesis of Monodispersed Microspheres from Laplace Pressure Induced Droplets in Micromolds

These various synthesizing methods can modify their chemical and physical properties such as size, composition, porosity, density, surface charge, and hydrophilicity or hydrophobicity. One of the widely used approaches for the synthesis of spherical polymeric particles is emulsion polymerization. [ 5 ] The most common type of emulsion polymerization is oil-in-water emulsion polymerization; in this method, droplets of monomer (the oil) are emulsifi ed with surfactants in a continuous phase of water. Upon heating this mixture, particles start to grow from the surfactant micelles until the desired diameter is reached. The reaction is terminated at an appropriate time to obtain particles of a desired size, up to a few micrometers. While emulsion polymerization is advantageous for the mass production of particles, the materials and the maximum size of the particle are limited. Another well-known synthesis method is suspension polymerization. [ 6 ] In this process, an agitating impeller is used to mix monomers in an immiscible fl uid while they are dispersed by continuous agitation. Although suspension polymerization has advantages for mass production, the particle size distribution is broad because of the non-uniform shear gradient near a rotating impeller. Moreover, it is not an appropriate technique to produce functional materials for some applications that require precise control such as controlled release of drugs. [ 7 ] Therefore, a novel method for synthesizing uniform particles with a large diversity of design morphologies and physicochemical properties would be a promising platform technology for many advanced applications. Over the past decades, microfl uidic technology has provided an advantage for the high-throughput synthesis of uniform spherical or two dimensional particles. [ 8 ] Although it is feasible to produce highly monodispersed particles using microfl uidic technology, complicate control using pumps is required and the choice of materials for synthesizing functional materials can be limited. To overcome these shortcomings, a particle replication