**6. Future challenges**

458 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

Oleic acid-double layer formed when adding an excess of oleic acid (b).

sample before and after centrifugation respectively (see text).

the yield stress obtained for all of them.

able to stabilize maghemite nanoparticles in [EtN][NO3] by coating them by a layer of short acrylic-acid-*b*-acrylamide copolymer; however, they were not in the case of [BuMeIm][BF4] and [EtMeIm][SCN] just because the polyacrylamide block of the acrylic-acid-*b*-acrylamide copolymer is not soluble in these ILs, and therefore, no compatibility exists. Therefore, as it has been said above, the best way to ensure long-term stability is the use of surfactants with long enough, carrier liquid-compatible tails adsorbed on the surface of magnetite nanoparticles.

**Figure 9.** Monolayer of oleic acid molecules adsorbed on the surface of magnetite nanoparticles (a).

Finally, some results about the rheological behaviour upon magnetic field of the samples mentioned above are shown (Rodríguez-Arco et al., 2011b). In particular, figure 10 shows

**Figure 10.** Yield stress for IL-based suspensions consisting of bare (triangles), citric acid-coated (circles) and oleic acid-double layer-coated (squares) magnetite. Full and open squares correspond to this latter

With regards to the results presented in figure 10 it is important to remind that, theoretically, a true FF should not display considerable yield stresses, since its response to magnetic field is too weak for this to happen. However, in the case of the suspension of bare magnetite, the yield stress is quite high, likely due to the strong particle aggregation, and therefore, to the formation of field-induced structures by the aggregates. When the particles Many of the future challenges in the field of IL-based MFs are related to ILs in general. One of the most important disadvantages of ILs, for instance, is their cost, that can be much higher than that of conventional organic solvents. However, in some specific applications, it is probably worth using them (i.e. space applications). Nevertheless, their price will decrease if they begin to be produced at a larger scale.

Another disadvantage of ILs when compared with traditional carriers is the lack of enough data about their physicochemical properties and toxicity (Keskin et al., 2007). In the same direction, a thorough analysis of the relationship (e.g. adsorption, wettability) between the particle surface and the constituent ions in MFs would be of the utmost importance, in order to gain a better understanding of the stability mechanisms.

Since ILs could be used to prepare tailor-made MFs for each specific application, it would be interesting to broaden their preparation by changing the IL carrier, the nature of the dispersed phase or its concentration. But before this is accomplished, it is necessary to determine which solid materials (or surfactants in the case of FFs) fit best each particular IL. As a result, deeper studies on the compatibility among all the MF constituents may be needed in the future. Additionally, further research has to be done, not only with regards to the formulation of IL-based MFs, but also to their performance in each particular application. As a consequence, detailed magnetization and magnetorheological studies should be faced too.

The final step for IL-based MFs would need a much more applied study which could result in a number of patents susceptible of being exploited. For example, they should be included in prototypes before developing any industrial device. This would allow additional industrial implementation and commercialization of this new breed of MFs. Here it is worth mentioning the work by companies like *Ioniqa Technologies* that are starting to commercialize different IL-based magnetic smart materials, including MFs, magnetoresponsive elastomers, and magnetic ILs. Therefore, this new type of MFs will pose new questions not only to scientists and engineers, but also to businesspeople.
