Particulate dispersions frequently take advantage of surfactant stabilizers to overcome the density mismatch between dense particles and low-density oily carrier fluids. For magnetorheological fluids (MRFs) surfactant stabilizers may also have the additional benefit of improve oxidative stability, thus improving the long-term stability of the fluid in more than one way. The addition of surfactants to MRFs, however, generally decreases the performance of the fluid by reducing the magnetization of the material per unit mass. Despite the tradeoff between stability and performance, little has been done to study the fundamental mechanisms of MRF stabilization, specifically with iron particles, in terms of surface free energy or particle surface coverage. This study focuses on alkanethiols, a class of molecules that readily self-assemble at metal surfaces including that of iron. While alkanethiols have been studied extensively for gold surfaces as well as some other metals there is little fundamental knowledge about their adsorption to and self-assembly on iron surfaces. Using a combination of goniometry and pendant drop analysis, alkanethiols with varying hydrocarbon lengths, linearity, and branched behavior were studied to evaluate surface packing and surface free energy. It was found that not only did the structure of the alkanethiol make a significant difference to the iron/thiol self-assembly, but the solvent that the adsorption took place in also had a significant impact. The packing of the thiols on the iron surface and the reduction in surface free energy was correlated to colloidal stability in silicone oil and mineral oil, two common MRF carrier fluids, at iron loadings as high as 25 vol%. While viscosity of the dispersion played a large role in MRF stability, dispersion bulk viscosity ranging from 10 to 1,000 Pa s, there was a noticeable change in the movement of the MRF turbid region solvent front as more alkanethiol was added to the surface. Surprisingly, for some MRFs additional alkanethiol increased the speed with which the solvent front dropped, and the iron-rich phase settled. Finally, the MRF performance was evaluated as a function of thiol surface loading to probe the effect of the surface adsorbed material on yield stress and magnetized viscosity. This was further correlated with DSM-VMS saturation magnetization values.

Fundamental characterization of alkanethiol surface adsorption and self-assembly for MRF stabilization

ACS Fall 2021

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