Magnetorheological fluids (MRFs) are smart materials that are dispersions of magnetic particles in a non-magnetic carrier liquid. MRFs transit reversibly from a liquid state in the absence of a magnetic field to a solid-like state on application of a magnetic field in a fraction of milliseconds due to the formation of chain-like structures by the magnetic particles. The transition from a liquid to a solid-like state is frequently studied as a function of MRF yield stress. One of the common challenges of MRFs is that with time and storage, magnetic particles exhibit oxidation, corrosion, and settling. Attraction between MRF magnetic particles results in aggregation and settling of high-density aggregates irreparably degrading MRF performance. To improve the colloidal, chemical, and thermal stability of MRFs, this study focuses on introducing self-assembled monolayers on the commonly used magnetic particle, iron. Self-assembled monolayers (SAMs) on metal surfaces are an easy and inexpensive approach to improve particle chemical resistance and colloidal stability. Despite significant work on self-assembled monolayers of alkanethiols on gold, nickel, and copper substrates, limited work has been performed on iron. To improve iron resistance against degradation, in particular for MRFs, this work focuses on self-assembled monolayers of alkanethiols on iron. For the first time, adsorption of three alkanethiols of different chain lengths on iron particle substrates was studied in solvents of varying polarity. Maximum alkanethiol adsorption was strongly a function of alkyl chain length as well as the type of solvent. In the presence of polar solvents, the maximum amount of alkanethiol adsorbed on iron increased with increasing log P values. With respect to application-specific properties, it was found that SAMs had a negligible effect on the magnetic characteristics of the iron and rheological properties of the MRFs suggesting that, unlike the other coatings, MRF performance does not decrease with alkanethiol adsorption. The effect of alkanethiol SAMs on MRF colloidal stability was also studied as a function of SAM composition and processing. This work has provided a new path towards enhancing the durability of iron, allowing a further exploration in the development of alkanethiol-based SAMs on new but significant metal, iron, and a reliable approach towards enhancing MRF performance.

Effect of n-alkanethiol chain length and solvent of alkanethiol self-assembled monolayers on magnetic properties of micron-sized iron substrates

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ACS Fall 2022

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