**3. Magnetic fluid-based applications and devices**

The possibility of controlling the physical properties of MFs by external magnetic fields makes them versatile smart materials that can be employed in numerous technological applications, ranging from fields as different as civil and mechanical engineering, aerospace, biomedicine or optics (Carlson et al., 1996; Durán et al., 2007; Jeong et al., 2007; Jolly et al., 1999; Klingenberg, 2001; Park et al., 2010; Popplewell, 1984; Raj et al., 1990; Tran et al., 2010).

The best known application for MRFs is their use as lubricants with field-dependent viscoelasticity in shock absorbers or dampers. It was in the early 1990s when *Lord Corporation* started commercializing the first MR dampers. Since then, further progress has followed, and commercial MR dampers have become available for large trucks or race and high-quality cars (Klingenberg, 2001). A typical monotube design of a MR damper used in vehicles was reported by Jolly et al. (Jolly et al., 1999). As shown in this previous work, this kind of MR damper has one reservoir for the MR fluid and an accumulator receptacle full with a compressed gas, which is used to accommodate the volume changes due to the entrance of the piston rod into the housing. When needed, an electric current in the coil generates a magnetic field in the required direction that makes the MRF change its rheological behaviour to a *solid-like* one; as a result, the vibrations provoked by external forces during driving are absorbed. Therefore, this kind of damping function can be labeled as semi-active (Jolly et al., 1999).

The technological applications of MR dampers are not only restricted to vehicles. For example they are also used as shock/vibration absorbers in structures (i.e. seismic control of buildings or bridges). In this particular case, the stability against sedimentation plays a very important role, since the damper is expected to remain inactive most of its lifetime (Jolly et al., 1999; Klingenberg, 2001; Park et al., 2010). In addition, MR shock absorbers can also be used in semi-active human prosthetic legs.

In the case of FFs, many biomedical applications have been described, which take advantage not only of their superparamagnetic behaviour, but also of their high surface-volume ratio, high reactivity, etc. Medical applications include cell labeling and targeting, separation and purification of cell populations, tissue repair, targeted drug delivery, magnetic resonance imaging or hyperthermia for cancer treatment (Durán et al., 2007; Tran et al., 2010). In all these applications biocompatibility and non-toxicity are of crucial importance. For this reason, iron oxides are preferred as the material for the dispersed phase and, in addition, they are often made biocompatible by means of surface coating by polymers (PEG, dextran, polyvinyl alcohol) or functional groups (thiols, amines, and carboxyls). All these additives prevent from particle aggregation too, which should be almost completely avoided so that particle size does not increase (Tran et al., 2010).

From a more engineering point of view, the control of both the position and the physicochemical properties of FFs by using magnets or solenoids makes them very interesting too. In fact, FFs have been used as lubricants, heat transfer agents or integrated in devices such as dynamic seals, dampers, magnetic inkjets or optic devices. As an example, companies like *Ferrotec* have successfully commercialised FF-based dynamic seals for 40 years. Dynamic seals are used when two different environments have to be isolated one from each other but energy has to be carried between them, by a shaft for example (see figure 6). Here, FFs can be used as filling materials, since they hermetically isolate both environments due to their fixed position if the hole for the shaft is made into a magnet. An important advantage of using a seal like this, apart from being almost completely hermetic, is a prolongation of the seal lifetime, since friction between its rotating and stationary components is almost negligible (i.e. the FF acts as a lubricant agent too) (Raj et al., 1990).

**Figure 6.** Sketch of a FF-based seal. Reprinted from Journal of Magnetism and Magnetic Materials, 85/1- 3, K. Raj and R. Moskowitz, Commercial applications of ferrofluids, 233-245, Copyright (1990), with permission from Elsevier.

Finally, as it has been pointed out above, FFs can also be used as heat transfer materials, provided that their thermal conductivity increases when applying a magnetic field in a direction parallel to the temperature gradient. In this way, magnetically-induced particle chains, able to transfer heat by conduction, are formed in the field direction. When the field is removed, thermal conductivity reaches its original value (Shima et al., 2011). This working principle is used to dissipate heat from devices such as high power loudspeakers. In this application, the use of FFs has additional advantages like the absorption of undesirable vibrations and avoiding resonances without disturbing the quality of the sound (due to their liquid condition), or the possibility of fixing their position by magnets (as it happens in dynamic seals) (Popplewell, 1984; Raj et al., 1990).

All the applications mentioned in this section are quite well known, and some of them have already been developed at the industrial scale. However, it is quite clear that the range of technological applications of MFs will expand as progress in their dynamics and preparation is made.
