**5.1. Ionic-liquid based magnetorheological fluids**

Guerrero-Sánchez et al. (Guerrero-Sánchez et al., 2007) were pioneers in the preparation of IL-based MRFs. More specifically, they prepared several suspensions consisting of either micron- or nanosized magnetite particles dispersed in eight different ILs. The most stable MRF was obtained for a composition of 10 vol % of magnetite microparticles suspended in [BuMeIm][PF6]. As a matter of fact, this MRF only exhibited a sedimentation ratio of 0.95 after two months of preparation. Having prepared a highly stable IL-based MRF was important, but even more important was the fact that colloidal stability was achieved without the addition of any stabilizing agents. The enhancement of stability was attributed to the physical adsorption of the IL ions on magnetite surface (for which they had strong affinity) that gave rise to steric repulsion between the dispersed particles. In the same work, the rheological behaviour of the samples upon magnetic field application was also studied concluding that the MR effect of the IL-based MRFs was comparable to those obtained for conventional MRFs, and high enough for practical purposes.

In a different work, Guerrero-Sánchez et al. (Guerrero-Sánchez et al., 2009) also used ILbased MRFs to study the influence of temperature on their MR behaviour. Here, ILs allowed extending such study to a broader range of temperature than in the case of conventional MRFs, due to their thermal stability and low vapour pressure. Something similar was pointed out by Dodbiba et al. (Dodbiba et al., 2007) who also took advantage of the good performance of ILs at high temperature to study the rheological behaviour upon magnetic field application of a mixture of two types of carbonyl iron powders dispersed in an IL. Therefore, these works make evident that the preparation of IL-based MFs is important from the fundamental point of view too.

Our group has also studied IL-based MRFs, especially for what concerns their stability. With this aim, Gómez-Ramírez et al. (Gómez-Ramírez et al., 2011) prepared IL-based MRFs differing both in the disperse phase material (pure iron and silica-coated iron particles) and in the liquid carrier ([EtMeIm][EtOSO3] and ([EtMeIm][Et2OPO3]). Additionally, conventional MRFs with mineral oil as liquid carrier were also synthesized. The particle volume fraction of all the samples was kept as 10 vol %. The most stable MRF was obtained by the combination of silicacoated iron particles and [EtMeIm][Et2OPO3]; for example, the viscosity of such MRF for shear rates over 100 s-1 almost coincided with the theoretical prediction of Batchelor equation, what means that only two-body hydrodynamic interactions existed between the particles, and, therefore, almost no magnetic aggregates were present. In fact, microscopic observations of the suspensions upon magnetic field application showed that field-induced particle chains had a more regular structure in the case of the most stable MRF (see figure 7). The worst results were obtained for mineral oil-based MRFs in which the particles were strongly aggregated and the resulting structures became more irregular. An intermediate behaviour was found for the suspensions with [EtMeIm][EtOSO3] as liquid carrier.

**Figure 7.** Field-induced particle structures in MRFs consisting of silica-coated iron microparticles dispersed in [EtMeIm][Et2OPO3] (a) and mineral oil (b). Bar length: 200 μm.

The improvement of MRF stability when using ILs as carriers, with respect to the ones prepared in mineral oil, was also explained by the adsorption of the IL ions on the surface of the dispersed particles. In a first approximation, such stability appears to be related to the electric conductivity of ILs, which, on its part, is also connected to the anion volume, being the first lower when the latter increases. Accordingly, if the adsorbed ions are larger, the steric repulsion between the particles is stronger too, leading to a decrease in particle aggregation. This is the reason for which a low conductivity-IL like [EtMeIm][Et2OPO3] gave rise to better results than the high conductivity-one [EtMeIm][EtOSO3]. The explanation for the higher stability in the case of MRFs consisting of silica-coated iron particles was the interaction between silica and the constituent IL ions. In fact, the wettability of both ILs was better in the case of silica-coated iron than for bare iron (Gómez-Ramírez et al. 2011).

The direct consequence of the improved stability of IL-based MRFs is a simpler MRF formulation, since no stabilizing additives are needed. This makes easier their preparation at larger scale given that no additional mixing steps are needed, for example. Therefore, stable MRFs with adequate MR response that can be subjected to ultra-high vacuum or very high/low temperatures are obtained, just by using ILs as carrier fluids.
