Soft matter, or complex fluids, is all deformable materials under thermal or mechanical stress around room temperature, including colloids, emulsions, foams, granules, gels, liquid crystals, and various biomaterials. We study the fundamental physics of such materials and engineer them to have well-defined structures to provide new types of functional materials for human beings. Three main themes we have aimed are listed below. But, our research cover a broad range of soft matter physics and applications beyond the list.
■ Microencapsulation and controlled release
Encapsulation technologies have been intensively developed. Recently, these technologies have attracted considerable attention due to their increasing importance in various applications, ranging from drug delivery, cosmetics, and foods to emerging areas of display devices and medicine. It is not surprising anymore that all commercialized E-papers (for example, “Amazon Kindle”) are based on microcapsules containing oppositely-charged black and white microparticles (called “E-ink”). Double-emulsion droplet is one of the most attractive templates for microcapsule fabrication due to their core-shell geometry. Recent advances in microfluidics have enabled to make such double-emulsion droplets in precisely controlled manner. In particular, multiple emulsions with controlled numbers of phases and cores have been prepared, which are useful to make microcapsules for controlled release of core materials.
■ Functional microparticles
Anisotropic or functional microparticles have great potential as a new class of colloidal materials with advanced applications. For example, Janus particles can be used as active pigments in new types of display devices, while chemically-patterned microparticles can be used as building blocks to construct photonic structures through directional interactions. In addition, amphiphilic microparticles are useful for stabilization of the interface between two immiscible fluids. Therefore, there remains intense demand for new classes of microparticles. However, surface energy drives microparticles spherical and isotropic, which makes it difficult to design new types of microparticles. We address this problem using various drop-based approaches in microfluidics.
■ Colloidal photonic crystals
Photonic crystals which have periodic modulation of dielectric constant in half-wavelength scale show unique optical properties related with photonic bandgap. Colloidal self-organization has enabled the construction of such 3D photonic crystals in facile and economic fashion by comparison with direct-writing or e-beam lithographic methods. Most previous studies have been devoted to wide area production with low defect density, controlled embedment of active defects, and patterning of the colloidal crystals with discrete colors. Unfortunately, however, practical uses of colloidal crystals were rarely reported due to their complex production conditions and low chemical resistance or physical rigidity, in spite of their high potential in various photonic applications. We have developed practical platform of colloidal photonic crystals to apply them into real application including display, miniaturized spectrometer, and photonic crystal fibers.
Shin-Hyun Kim (김신현)