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 magnetic polystyrene (PS)/inorganic particles were fabricated by Pickering emulsion polymerization using nanosized Fe2O3 particles as a solid stabilizer. Scanning electron microscopy and transmission electron microscopy confirmed the synthesized PS/Fe2O3 particles to be comprised of a PS surface coated with Fe2O3 nanoparticles. The chemical structure of the composite nanospheres was characterized by Fourier transform infrared spectroscopy and X-ray diffraction. The thermal properties of composite nanospheres and corresponding pure polymer were examined by thermogravimetric analysis. The rheological properties of the core-shell-structured magnetic PS/inorganic particles dispersed in silicone oil were investigated under an external magnetic field strength using a rotational rheometer. The particles with extremely lower density than common magnetic particles exhibited solid-like magnetorheological phase characteristics, and the flow curves were fitted to the Cho-Choi-Jhon model of the rheological equation of state.

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.

Core-shell structured polystyrene (PS)-graphene oxide (GO) microspherical particles were synthesized by adsorbing the GO sheets on the PS surface through a strong p-p 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 via X-ray diffraction and Fouriertransform 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.

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.

Novel polarizable graphene oxide (GO) particles with oxidized groups on their edge and basal planes were prepared by a modified Hummers method, and their electro-responsive electrorheological (ER) characteristics when dispersed in silicone oil were examined with and without an electric field applied. The fibrillation phenomenon of this GO-based electro-responsive fluid was also observed via an optical microscope under an applied electric field. Both flow curves and dielectric spectra of the ER fluid were measured using a rotational rheometer and a LCR meter, respectively. Its viscoelastic properties of both storage and loss moduli were also examined using a vertical oscillation rheometer equipped with a high voltage generator, finding that the GO-based smart ER system behaves as a viscoelastic material under an applied electric field.

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Magnetorheological (MR) elastomer was prepared using silicone rubber and soft magnetic carbonyl iron microspheres, and then examined as dielectric materials for manufacturing electric capacitors. As a specific element, capacity of the capacitors located in a magnetic field was found to be sensitive to both the MR suspension proportion to the silicone rubber and the intensity of the applied magnetic field. Viscoelastic characteristics of the MR elastomer, represented by storage modulus and creep behavior, were also studied.