As a class of intelligent fluids, both electrorheological (ER) and magnetorheological (MR) fluids have attracted widespread attention in various fields as they can rapidly and reversibly transform from a liquid-like to solid-like phase under the stimuli of electrical and magnetic fields, respectively.
For decades, a variety of ER materials have been studied, including the polarizable inorganic mineral oxides, carbon nanotubes (CNT), graphene oxide (GO), semiconducting polymers, and poly ionic liquid (PIL) particles. Among them, the semiconducting polymers, are considered promising candidates as ER materials due to their excellent environmental stability, simple synthesis route, low density, and controllable conductivity that can be tuned using the doping and dedoping process. However, for many semiconducting polymer materials with high conductivity, in order to adjust the conductivity within this appropriate range, a dedoping process with an alkali solution is usually implemented. Furthermore, in the previous studies, we have reported poly(diphenylamine) (PDPA) as an ER material as it has an appropriate conductivity and can be employed directly as an ER material without a dedoping procedure.
As for the MR materials, a large number of magnetic particles have been adopted, such as carbonyl iron (CI), cobalt ferrite, zinc ferrite, MnFe₂O₄, maghemite, and magnetite (Fe₃O₄). In particular, Fe₃O₄ particles with various morphologies have been widely studied as MR materials because they have soft magnetic properties, sufficient saturation magnetization (Ms), a lower density compared to other magnetic materials, and can be easily synthesized. Furthermore, it is well-known that the morphology of materials critically affects the ER and MR properties. Majority of the ER and MR mechanisms are based on the fact that the constituent particles are spherical. However, for semiconducting polymers, it is difficult to synthesize regular spherical particles. Thus, inserting a regular spherical core to build a core−shell material is an effective method.
Furthermore, in recent years, materials simultaneously responsive to both electric and magnetic fifield stimuli have been introduced. In this study, core−shell type poly(diphenylamine)-coated magnetite (Fe₃O₄−PDPA) microspheres were designed (as shown in Figure 1) and adopted as a novel actively tunable smart material which is responsive under both electric and magnetic fields. Their morphology, chemical structure, crystalline structure, and thermal properties were characterized using scanning electron microscopy, transmission electron microscope, Fourier transform-infrared spectroscopy, X-ray diffffraction, and a thermal gravimetric analyzer. Their magnetic and dielectric properties were determined using vibrating-sample magnetometer and an LCR meter, respectively. They were dispersed in silicone oil and their electrorheological (ER) and magnetorheological (MR) responses under the electric and magnetic fields, respectively, were examined. The formation of chain structure of Fe₃O₄−PDPA based E/MR fluid under the application of electric field or magnetic field was observed by an optical microscopy and the sedimentation stability was observed by a Turbiscan optical analyzer system. It was observed that the yield stress, ER effiffifficiency, and leakage current density increased with an increase in the particle concentration, while the slope of the electric field-dependent yield stress decreased. Several models such as the Bingham model, Herschel−Bulkley model, and Cho−Choi−Jhon equations were used to describe the shear stress curves of the ER fluid (as shown in Figure 2); the curves fitted well. For the dielectric properties, the two types of ER fluids tested displayed the same relaxation time and distribution; however, the one with the higher concentration had a higher dielectric constant and polarizability. The Fe₃O₄−PDPA based MR fluid (10 vol %) exhibited typical MR properties. In addition, the Herschel−Bulkley model matched well with the shear stress curves under a magnetic field.
 

Core−Shell,Dual Response,Electrorheological fluid,magnetorheological fluid