Magnetorheological (MR) fluids are a type of smart material with rheological properties that may be controlled through mesostructural transformations. MR fluids form solid-like fibril structures along the magnetic field direction upon application of a magnetic field due to magnetopolarization of soft-magnetic particles when suspended in an inert medium. A reverse structural transition occurs upon removal of the applied field. The structural changes are very fast on the order of milliseconds. The rheological properties of MR fluids vary with the application of a magnetic field, resulting in non-Newtonian viscoplastic flow behaviors. Recent applications have increased the demand for MR materials with better performance and good long-term stability. A variety of industrial MR materials have been developed and tested in numerous experimental and theoretical studies. Because modeling and analysis are essential to optimize material design, a new macroscale structural model has been developed to distinguish between static yield stress and dynamic yield stress and describe the flow behavior over a wide range of shear rates. Herein, this recent progress in the search for advanced MR fluid materials with good stability is described, along with new approaches to MR flow behavior analysis. Several ways to improve the stability and efficiency of the MR fluids are also summarized.

The magnetorheological (MR) performance of suspensions based on core-shell-structured foamed polystyrene (PSF)/FeO particles was investigated by using a vibrating sample magnetometer and a rotational rheometer. Core-shell-structured polystyrene (PS)/FeO was synthesized by using the Pickering-emulsion polymerization method in which FeO nanoparticles were added as a solid surfactant. Foaming the PS core in PS/FeO particles was carried out by using a supercritical carbon dioxide (scCO) fluid. The density was measured by a pycnometer. The densities of PS/FeO and PSF/FeO particles were significantly lowered from that of the pure FeO particle after Pickering-emulsion polymerization and foaming treatment. All tested suspensions displayed similar MR behaviors but different yield strengths. The important parameter that determined the MR performance was not the particle density but rather the surface density of FeO on the PS core surface. The morphology was observed by scanning electron microscopy and transmission electron microscopy. Most FeO particles stayed on the surface of PS/FeO particles, making the surface topology bumpy and rough, which decreased the particle sedimentation velocity. Finally, Turbiscan apparatus was used to examine the sedimentation properties of different particle suspensions. The suspensions of PS/FeO and PSF/FeO showed remarkably improved stability against sedimentation, much better than the bare FeO particle suspension because of the reduced density mismatch between the nanoparticles and the carrier medium as well as the surface topology change.

Hollow polydivinylbenzene@Fe3O4 (h-PDVB@Fe3O4) nanoparticles with a relatively narrow size distribution were prepared by depositing Fe3O4 nanoparticles on h-PDVB. Because of the cavity in the hollow structure, the density of h-PDVB@Fe3O4 (ρ = 1.83 g/cm3) was significantly reduced from that of Fe3O4 (4.52 g/cm3). Deposition of Fe3O4 particles of 10–20 nm size (average particle size ≃14.3 ± 2.5 nm) on the h-PDVB made the h-PDVB@Fe3O4 particle surface quite rough while preserving the spherical shape. The MR suspensions were prepared by dispersing h-PDVB@Fe3O4 in silicone oil medium, and their magnetorheological properties were investigated. The dynamic modulus and the yield stress under magnetic field decreased compared to those of pure Fe3O4 suspension, but the MR behavior of h-PDVB @ Fe3O4 suspension was well preserved. Interestingly, contrasting MR performance of two suspensions (h-PDVB@Fe3O4 (Fe3O4 nanoparticle size ≃14.3 ± 2.5 nm) and foamed PS/Fe3O4 (Fe3O4 nanoparticle size ≃50–100 nm)) with similar densities was observed at high and low magnetic field strength regions due to the particle size difference. The long-term sedimentation stability of the suspensions was investigated with a Turbiscan apparatus. Because of reduced density mismatch between particles and silicon oil medium, the h-PDVB@Fe3O4 suspension exhibited a significantly improved stability compared to that of the pure Fe3O4 suspension, with only 13% of light transmission after 24 h. The MR performance and enhanced long-term sedimentation stability represent a viable application of h-PDVB@Fe3O4 suspensions to microfluidic devices.

The magnetic Fe3O4 nanoparticles were produced by chemical precipitation from ferric chloride (FeCl3) and ferrous chloride (FeCl2) in an alkaline medium (ammonia hydroxide). The magnetic particle suspension was stabilized with citric acid. Hierarchically structured Fe3O4 (HS-Fe3O4) particles of submicrometer size were successfully produced from citric acid-capped Fe3O4 solution by using a simple electrospray (ES) process and applied for magnetorheolgical (MR) fluids. Submicrometer-sized spherical HS-Fe3O4 particles were formed by evaporation of the solvent and agglomeration of capped Fe3O4 particles during the ES process. With formation of hierarchically structured particles, the density of HS-Fe3O4 particles (ρ = 3.32 g/cm3) was noticeably decreased from that of the capped Fe3O4 (ρ = 4.29 g/cm3) without use of light materials, while the saturation magnetization was comparable to primary capped Fe3O4 (M s = 73 emu/g). The MR fluid based on the submicrometer HS-Fe3O4 particles showed better MR performance with a large static yield stress (337 Pa), which was 300% greater than that of the hollow polymer/Fe3O4 suspension. Reduced density mismatch between the particles and the silicon oil medium significantly improved the stability of the HS-Fe3O4 suspension compared to the pure Fe3O4 suspension. The high MR performance and the improved sedimentation stability showed a possibility of the practical application of the submicrometer HS-Fe3O4 particle suspensions to microfluidic devices.

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