This study investigates the steady shear rheological behavior of water-based ferrofluids composited with hydrophilic fumed silica under different magnetic field strengths, with particular attention to avoiding gelation that reduces fluidity. Seven composite ferrofluid samples were prepared and characterized. By adjusting silica particle size and volume fraction, their effects on viscosity and yield stress were explored. As a result, pronounced shear-thinning behavior is observed in this dispersion, with their flow curves under different magnetic field strengths effectively scaled by the Mason number. Higher silica concentration or larger particle size increases the critical Mason number, showing that field-induced structures become more stable. In contrast, only high silica concentrations significantly enhance shear-thinning, as reflected by a larger flow index, whereas particle size has little influence. Yield stress analysis further shows that macroscopic models capture normalized Bingham yield stress, while microscopic models better predict normalized static yield stress. Overall, this work demonstrates that hydrophilic fumed silica offers a simple and effective route to tune the magnetorheology of water-based ferrofluids without inducing gelation, ensuring controllable rheology and good fluidity.

Magnetic fluids are a type of novel functional material that has emerged over the past two decades, which have found applications in various aspects of production and daily life. However, the application of conventional magnetic fluids in low-temperature environments is severely limited and often unsatisfactory. To address this issue, we have developed a new magnetic fluid with excellent low-temperature resistance. Initially, bare Fe₃O₄ magnetic nanoparticles (FMNPs) were synthesized via co-precipitation without a protective gas. Subsequently, these particles were modified using polyethylene glycol (PEG)-4000 as a surfactant. The bare and modified FMNPs (MFMNPs) were characterized using X-ray diffraction (XRD), vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis-differential thermal gravimetric (TGA-DTG) analysis. The rheological properties, low-temperature resistance of the magnetic fluid were also evaluated. The characterization results indicate that the MFMNPs are spherical and monodisperse, with a narrow size distribution and a mean particle size of approximately 12 nm. Furthermore, the FTIR spectra and TGA-DTA results suggest that PEG-4000 is linked to the bare Fe₃O₄ particles via hydrogen bonding, indicating successful modification of the Fe₃O₄ magnetic nanoparticles using PEG. VSM measurements demonstrate that surface modification does not alter the crystal morphology or superparamagnetic of Fe₃O₄. However, it does reduce the saturation magnetization from 68.17 to 54.75 emu/g. Additionally, the prepared magnetic fluid exhibits shear thinning and magnetic viscosity effects. It also exhibits excellent low-temperature resistance, maintaining good fluidity without freezing even at –60 °C. In summary, these results collectively indicate that the new low-temperature resistant magnetic fluid developed in this study is stable and has broad application prospects.

Magnetic fluids are the suspensions composed of magnetic nanoparticles, surfactants, and non-magnetic carrier liquids. Magnetic fluids are widely used in various fields, especially in sealing, because of their excellent features, including rapid magnetic response, flexible flow ability, tunable magneto-viscous effect, and reliable self-repairing capability. Here, we provide an in-depth, comprehensive insight into the theoretical analyses and diverse applications of magnetic fluids in sealing from three categories: static sealing, rotary sealing, and reciprocating sealing. We summarize the magnetic fluid sealing mechanisms and the development of magnetic fluid seals from 1960s to the present, particularly focusing on the recent progress of magnetic fluid seals. Although magnetic fluid sealing technology has been commercialized and industrialized, many difficulties still exist in its applications. At the end of the review, the present challenges and future prospects in the progress of magnetic fluid seals are also outlined.

Ferrofluids are a type of nanometer-scale functional material with fluidity and superparamagnetism. They are composed of ferromagnetic particles, surfactants, and base liquids. The main characteristics of ferrofluids include magnetization, the magnetoviscous effect, and levitation characteristics. There are many mature commercial ferrofluid damping applications based on these characteristics that are widely used in numerous fields. Furthermore, some ferrofluid damping studies such as those related to vibration energy harvesters and biomedical devices are still in the laboratory stage. This review paper summarizes typical ferrofluid dampers and energy harvesting systems from the 1960s to the present, including ferrofluid viscous dampers, ferrofluid inertia dampers, tuned magnetic fluid dampers (TMFDs), and vibration energy harvesters. In particular, it focuses on TMFDs and vibration energy harvesters because they have been the hottest research topics in the ferrofluid damping field in recent years. This review also proposes a novel magnetic fluid damper that achieves energy conversion and improves the efficiency of vibration attenuation. Finally, we discuss the potential challenges and development of ferrofluid damping in future research.