Magnetorheological fluids (MRFs) are functional materials, prepared by dispersing magnetic particles in a nonmagnetic carrier fluid, that exhibit a change in mechanical properties (e.g., increase in viscosity and yield stress) when subjected to a magnetic field. Change in mechanical properties is demonstrated by a fast (i.e., fraction of milliseconds) and reversible transition from a liquid‐ to a solid‐like state. This transition, due to a structural reorganization of magnetic particles in the carrier fluid, makes MRFs a valuable material for damping, breaking systems, medical and prosthetics, and robotics. Since the discovery of MRFs in 1948, developing MRF preparation methods that result in improved performance and the storage without oxidation and settling is paramount. This article presents a review on recent developments in the preparation process of MRFs with a special emphasis on the state of the art in additives and coatings used in enhancing MRF chemical, colloidal, and thermal stability. Recent advances in MRF materials and formulations that have increased yield stress and magnetic properties are discussed. Finally, this review analyses the present‐day challenges in MRF research and makes suggestions for the field to improve MRF stability and performance drawing on previous MRF work and work outside of the fields.

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Self-assembled monolayers (SAMs) of organic molecules on metal surfaces are a type of inexpensive surface coating often used to improve metal substrate properties for sensors, electrochemistry, and nanofabrication applications. Iron, specifically, is one of the most commonly used metals, both as a pure metal and as an alloy due to its high conductivity, strong ferromagnetism, and low cost. However, magnetorheological fluids, which have shown impressive energy dampening in fields from civil infrastructure to biomedical devices utilizing iron dispersions, have suffered from low reliability and efficiency due to iron particle oxidation, corrosion, and settling. To understand the effect of self-assembled monolayers on iron and both the adsorbed particle's resistance against aggregation and performance impact, this work performs an in-depth study on alkanethiol-based self-assembled monolayers on iron particles. Adsorption of alkanethiols and the generation of SAMs on micronsized iron particles were evaluated as a function of adsorption solvent polarity and alkanethiol chain length. Maximum alkanethiol loading, determined from appropriate isotherms, was found to strongly be a function of both parameters. Alkanethiol adsorption increased with increasing alkyl chain length and increasing solvent log P values in polar solvents. With respect to magnetorheologically relevant parameters, alkanethiol adsorption did not show any significant effect on both the magnetic properties of iron (as particles) and fluid on-state yield stress. The colloidal stability of n-alkanethiol adsorbed iron-based magnetorheological fluids (MRFs) was a function of both n-alkanethiol chain length and the iron particle adsorption solvent. MRFs composed of hexadecanethiol adsorbed iron prepared in polar solvents like methanol and ethanol showed excellent sedimentation stability compared to all other MRFs prepared in this study.

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