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.