Magnetorheology of Carbonyl Iron Dispersions in 1‑Alkyl-3-methylimidazolium Ionic Liquids

Abstract

In this work, a range of ionic liquids (IL), all based on 1-alkyl-3-methylimidazolium, [EMIM]⁺, and [BMIM]⁺ cations, with different anions, [NTf₂]⁻, [CF₃SO₃]⁻, [SCN]⁻, [BF₄]⁻, [PF₆]⁻, and [CH₃COO]⁻, were used as carrier fluids to prepare magnetorheological (MR) fluids with 30 vol % carbonyl iron powder (CIP), grade EW (BASF SE), without additives. IL-MR fluids were characterized in the presence of magnetic fields with conventional steady shear stress ramps, constant shear rate magnetosweeps, and strain amplitude oscillatory shear tests. In the absence of magnetic fields, samples were subjected to temperature sweep (from +80 to −20 °C), settling, and redispersibility tests. We conclude that, under large enough magnetic field strengths, above 10 kA/m, there is not a significant effect of the IL employed, in terms of MR effect or MR performance, both in steady shear as well as oscillatory shear. On the other hand, for long shelf life, without applied magnetic field at rest, [EMIM]⁺[CF₃SO₃]⁻, [BMIM]⁺[SCN]⁻, and [BMIM]⁺[CH₃COO]⁻ are not good candidates as carrier fluids for MR fluids, since they are very difficult to redisperse because of existing specific chemical interactions with the particle surface. In terms of settling stability, [BMIM⁺]­[PF₆]⁻ and [BMIM]⁺[CH₃COO]⁻ presented slower settling rates, because of their higher viscosities; however, the sedimentation curves collapse when normalized by the viscosity of the IL. Overall, the presence of fluorinated anions, such as [NTf₂]⁻, [BF₄]⁻, and [PF₆]⁻ facilitates redispersibility, as demonstrated by penetration tests