Hyoung Jin Choi

Magnetorheology: materials and application

As one of the most important field-responsive intelligent and smart soft matter materials, magnetorheological (MR) fluids, consisting of magneto-responsive magnetizable particles suspended in non-magnetic fluids, have drawn a lot of attentions in both academia and industry as their physical and rheological characteristics can be controlled with external magnetic field strength. In this highlight, preparation methods and MR properties of various magnetic composites with soft magnetic particles and polymers are reviewed. In addition, some industrial applications, such as a MR dampers and a MR polishing, are briefly summarized.

Electrorheology of polymers and nanocomposites

This highlight aims to report electrorheological (ER) materials in state-of-the art polymeric particles and their various nanocomposites with clay, mesoporous inorganics and carbon nanotubes along with their potential application. ER fluids, suspensions of these particles having higher dielectric constant or electrical conductivity than the low-viscosity fluids in which they are suspended, are currently regarded as a smart/intelligent material, because their structural and rheological properties can be systematically tuned by controlling electric field strengths. In this highlight, various conducting polymers, including polyaniline, polypyrrole, poly(p-phenylene), poly(naphthalene quinone) and copolyaniline, are introduced and different types of polymer nanocomposites are emphasized. Flow curves for shear stress of the ER fluids are also examined.

A yield stress scaling function for electrorheological fluids

The yield stress dependence on electric field strength for electrorheological (ER) fluids is examined. A proposed scaling function incorporates both the polarization and conductivity models. Proper scaling allows yield stress data for ER fluids to collapse onto a single curve for a broad range of electric field strengths.

Synthesis and electrorheological properties of polyaniline-Na+-montmorillonite suspensions

We synthesized polyaniline-Na+-montmorillonite nanocomposite particles using an emulsion intercalation method and prepared electrorheological (ER) fluids by dispersing the synthesized nanocomposite particles in an electrically insulating silicone oil. The conducting polymer (polyaniline) was inserted into the layers of clay, and this insertion of polyaniline was confirmed by X-ray diffraction. For the first time, ER properties were determined via a rotational rheometer equipped with a high voltage generator.

Graphene oxide coated core–shell structured polystyrene microspheres and their electrorheological characteristics under applied electric field

Core–shell structured polystyrene (PS)–graphene oxide (GO) microspherical particles were synthesized by adsorbing the GO sheets on the PS surface through a strong π–π stacking interaction. As core materials, monodispersed PS microspheres were prepared using a dispersion polymerization, while the shell part of GO was synthesized by a modified Hummers method. Morphology of the composite particles was studied by both scanning electron microscopy and transmission electron microscopy, while their structure and chemical components were examined viaX-ray diffraction and Fourier-transform infrared spectroscopy, respectively. All the data confirmed the coexistence of PS and GO with the expected core–shell structure of the composite. In addition, for the study on the electroresponsive behavior, the composite was dispersed in silicone oil and its electrorheological (ER) characteristics were examined via both an optical microscope and a rotational rheometer which was equipped with a high voltage source. Without an electric field, it behaved like a fluid, however, when an external electric field is present, the particles became polarized and demonstrated typical chain-like ER structures.

Electrorheological fluids: smart soft matter and characteristics

An electrorheological fluid, a special type of suspension with controllable fluidity by an electric field, generally contains semiconducting or polarizable materials as electro-responsive parts. These materials align in the direction of the applied electric field to generate a solid-like phase in the suspension. These electro-responsive smart materials, including dielectric inorganics, semiconducting polymers and their hybrids, and polymer/inorganic composites, are reviewed in terms of their mechanism, rheological analysis and dielectric characteristics.

Core-shell structured semiconducting PMMA/polyaniline snowman-like anisotropic microparticles and their electrorheology.

Core-shell structured semiconducting snowman-like particles were synthesized, and their electrorheological (ER) characteristics under an applied electric field were examined. Monodispersed snowman-like poly(methyl methacrylate) (PMMA) particles were fabricated previously using a seed emulsion polymerization procedure. These anisotropic particle-based ER fluids, which were tested using a rotational rheometer, exhibited unusual ER properties in the flow curves at various electric field strengths when analyzed using the Cho-Choi-Jhon model. The dielectric spectra, as supporting data for the ER effect, were measured using a LCR meter. The relaxation time of the ER fluid was relatively shorter than typical ER fluids.

Encapsulation of spherical iron-particle with PMMA and its magnetorheological particles

Composite magnetic particles (CMP) with carbonyl iron (CI) core and poly(methyl methacrylate) shell were prepared by an in-situ dispersion polymerization method via surface treated CI with acrylic acid. These CMP were adopted as dispersed phase of magnetorheological (MR) fluids, and has better MR fluids characteristics than fluid with CI alone as they have severe sedimentation and poor dispersion quality. Flow properties of the MR fluids were analyzed via a rotational rheometer equipped with a magnetic field supplier in parallel plate geometry.

Silica-Graphene Oxide Hybrid Composite Particles and Their Electroresponsive Characteristics

Silica-graphene oxide (Si-GO) hybrid composite particles were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of hydrophilic GO obtained from a modified Hummers method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images provided visible evidence of the silica nanoparticles grafted on the surface of GO, resulting in Si-GO hybrid composite particles. Energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) spectra indicated the coexistence of silica and GO in the composite particles. The Si-GO hybrid composite particles showed better thermal stability than that of GO according to thermogravimetric analysis (TGA). The electrorheological (ER) characteristics of the Si-GO hybrid composite based ER fluid were examined further by optical microscopy and a rotational rheometer in controlled shear rate mode under various electric field strengths. Shear stress curves were fitted using both conventional Bingham model and a constitutive Cho-Choi-Jhon model. The polarizability and relaxation time of the ER fluid from dielectric spectra measured using an LCR meter showed a good correlation with its ER characteristics.

Core-shell structured carbonyl iron microspheres prepared via dual-step functionality coatings and their magnetorheological response.

The dispersion stability of soft magnetic carbonyl iron (CI)-based magnetorheological (MR) fluids was improved by applying a unique functional coating composed of a conducting polyaniline layer and a multiwalled carbon nanotube nest to the surfaces of the CI particles via conventional dispersion polymerization, followed by facile solvent casting. The coating morphology and thickness were analyzed by SEM and TEM imaging. Chemical composition of the polyaniline layer was detected by Raman spectroscope, which also confirmed the coating performance successfully. The influence of the functional coating on the magnetic properties was investigated by measuring the MR performance and sedimentation properties using a vibrating sample magnetometer, rotational rheometer, and Turbiscan apparatus. Improved dispersion characteristics of the MR fluid were observed.