**2. Magnetic fluids**

As it has been pointed out above, the term "magnetic fluids" (MFs) is used to describe a group of smart materials whose properties can be controlled by means of external magnetic fields. They all are suspensions of magnetisable particles dispersed in a liquid carrier.

Basically, two kinds of MFs can be defined according to the size of the dispersed phase: magnetorheological fluids (MRFs) and ferrofluids (FFs).

MRFs consist of micron-sized ferro- or ferrimagnetic particles dispersed in a liquid carrier. Traditionally, organic solvents such as kerosene or mineral oil are used as carriers. With regards to the dispersed particles, they can be considered as multi-domain from the magnetic viewpoint, since their size is much higher than the one of a single magnetic domain for the constituent material in question (Bossis et al., 2002). For example, typical MRFs can be composed of iron particles with diameters around 1 μm, while the size of one magnetic domain in this material is approximately 30 nm (Herpin, 1968). As a result, MRFs strongly respond to external magnetic fields, giving rise to considerable changes in their flow (rheological) behaviour. More specifically, they behave as Newtonian fluids in the absence of magnetic field application, while their behaviour becomes that of a non-Newtonian plastic fluid when a magnetic field is applied. In this latter case, the MRF only flows when the shear stress applied to the suspension overcomes a finite value, the so-called yield stress. Therefore, it could be said that their rheological behaviour changes from a *liquid-like* to a *solid-like* one due to the application of external fields. This change, known as magnetorheological (MR) effect is a consequence of the formation of magnetically-induced structures by the dispersed particles, that have to be broken by the hydrodynamic forces to make the suspension flow (Bossis et al., 2002). Figure 1 shows typical MRF rheograms (shear stress vs. shear rate curves) both in the absence of and upon magnetic field application; the appearance of the yield stress (σ*y*) can be clearly observed. The field-induced structures and their evolution as the shear rate is increased are schematized too.

**Figure 1.** Rheograms for a MRF consisting of 30 vol % of iron microparticles in mineral oil at magnetic field strengths (H) of 0 and 250 kA/m (circles and squares respectively). The evolution of the fieldinduced structures is showed.

The strong magnetic interaction between the dispersed particles in MRFs, responsible for the MR effect, has its disadvantages too. Actually, it makes them agglomerate and settle down if no stabilizing additives are used. Since complete avoidance of sedimentation in MRFs is almost impossible, the efforts in formulation have focused on trying to reduce it and making particle redispersion easy. Different additives such as surfactants (oleic acid, lecithin, aluminum stearate) or thixotropic agents (organoclay particles) have been used for this purpose (de Vicente et al., 2003; López-López et al., 2005a, 2008).

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

their evolution as the shear rate is increased are schematized too.

induced structures is showed.

magnetorheological fluids (MRFs) and ferrofluids (FFs).

Basically, two kinds of MFs can be defined according to the size of the dispersed phase:

MRFs consist of micron-sized ferro- or ferrimagnetic particles dispersed in a liquid carrier. Traditionally, organic solvents such as kerosene or mineral oil are used as carriers. With regards to the dispersed particles, they can be considered as multi-domain from the magnetic viewpoint, since their size is much higher than the one of a single magnetic domain for the constituent material in question (Bossis et al., 2002). For example, typical MRFs can be composed of iron particles with diameters around 1 μm, while the size of one magnetic domain in this material is approximately 30 nm (Herpin, 1968). As a result, MRFs strongly respond to external magnetic fields, giving rise to considerable changes in their flow (rheological) behaviour. More specifically, they behave as Newtonian fluids in the absence of magnetic field application, while their behaviour becomes that of a non-Newtonian plastic fluid when a magnetic field is applied. In this latter case, the MRF only flows when the shear stress applied to the suspension overcomes a finite value, the so-called yield stress. Therefore, it could be said that their rheological behaviour changes from a *liquid-like* to a *solid-like* one due to the application of external fields. This change, known as magnetorheological (MR) effect is a consequence of the formation of magnetically-induced structures by the dispersed particles, that have to be broken by the hydrodynamic forces to make the suspension flow (Bossis et al., 2002). Figure 1 shows typical MRF rheograms (shear stress vs. shear rate curves) both in the absence of and upon magnetic field application; the appearance of the yield stress (σ*y*) can be clearly observed. The field-induced structures and

**Figure 1.** Rheograms for a MRF consisting of 30 vol % of iron microparticles in mineral oil at magnetic field strengths (H) of 0 and 250 kA/m (circles and squares respectively). The evolution of the fieldFFs, on the other hand, are ultrastable suspensions of magnetic nanoparticles (size around 10 nm) dispersed in a liquid carrier. From the magnetic point of view, these particles are single-domain and therefore, FFs behave as superparamagnetic systems (Charles, 2002; Rosensweig, 1985). The term superparamagnetism is used to describe the magnetic behaviour of systems that exhibit high saturation magnetization values (i.e. typical of ferromagnetic materials), and no magnetic remanence or hysteresis as it happens in paramagnetic materials. FFs only undergo slight changes of viscosity in the presence of external magnetic fields. This phenomenon, known as magnetoviscous (MV) effect is very interesting from the technological viewpoint (Odenbach et al., 2002).

Nevertheless, the most important feature related to FFs is their so-called ultrastability: ideally, true FFs should not settle either when subjected to strong magnetic field gradients or in the presence of gravitational forces during their lifetime (Rosensweig, 1985). As a result, they really behave as magnetic liquids, since no phase separation appears. Figure 2 shows how a FF climbs the tube walls in order to move towards the zones where the magnetic field provided by the magnet is higher.

**Figure 2.** FF (5 vol % of magnetite) subjected to magnetic field application by a powerful rare-earth magnet. It can be seen that the liquid moves as a whole towards the zone where the field is more intense.

Ultrastability in FFs is usually achieved as follows (Charles, 2002; Rosensweig, 1985): (i) particle size is low enough so that Brownian motion prevents from possible particle aggregation induced by the magnetic interaction between the particles; (ii) particle

aggregation by means of van der Waals attraction (short range forces) must also be avoided by imposing some kind of repulsion between the particles. Such repulsion is usually obtained by adsorbing surfactants or polymers on the surface of the particles, and thus providing with a strong enough barrier (steric repulsion) to overcome van der Waals interactions. In polar media (i.e. water-based FFs), this can be alternatively achieved by electrostatic repulsion between the particles derived from their surface charge (Charles, 2002; Rosensweig, 1985).

The efforts made in order to improve MFs in terms of stability, durability, MR and MV effects have led to the development of a new generation of MFs. This group would include inverse FFs, bimodal suspensions, and MRFs consisting of anisotropic particles. In the first case, non-magnetic microparticles are dispersed in a concentrated enough FF. These particles behave as non-magnetic holes in a practically continuous magnetic medium, and, as a result, are able to orientate and form magnetic structures when a magnetic field is applied (figure 3). This sort of MR effect is stronger than in the case of FFs, but it is still far from that of MRFs (de Gans et al., 1999; Ramos et al., 2011).

**Figure 3.** Microscopic picture upon magnetic field application of an inverse FF consisting of PMMA microparticles dispersed in a FF (5 vol % of magnetite in mineral oil). It can be seen that the nonmagnetic particles (white spots) form chains in the direction of the magnetic field. Bar length: 100 μm.

In the case of bimodal suspensions, micron-sized particles are again dispersed in a FF, but this time, the dispersed particles are magnetic. These MFs have proved to be more stable against sedimentation than conventional MRFs due to the stabilization achieved by the formation of nanoparticle clouds around each micron-sized particle. The formation of such clouds can be observed in figure 4 (López-López et al., 2005b, 2006, 2010).

Finally, the anisotropic nature of fibrillar particles leads to an increase of the MR effect in MRFs consisting of them (Kuzhir et al., 2009; López-López et al., 2007, 2009, 2012). Figure 5 shows magnetic fibrillar particles, with a longitudinal axis of about 50 nm, which have been used in the preparation of stable FFs.

from that of MRFs (de Gans et al., 1999; Ramos et al., 2011).

2002; Rosensweig, 1985).

aggregation by means of van der Waals attraction (short range forces) must also be avoided by imposing some kind of repulsion between the particles. Such repulsion is usually obtained by adsorbing surfactants or polymers on the surface of the particles, and thus providing with a strong enough barrier (steric repulsion) to overcome van der Waals interactions. In polar media (i.e. water-based FFs), this can be alternatively achieved by electrostatic repulsion between the particles derived from their surface charge (Charles,

The efforts made in order to improve MFs in terms of stability, durability, MR and MV effects have led to the development of a new generation of MFs. This group would include inverse FFs, bimodal suspensions, and MRFs consisting of anisotropic particles. In the first case, non-magnetic microparticles are dispersed in a concentrated enough FF. These particles behave as non-magnetic holes in a practically continuous magnetic medium, and, as a result, are able to orientate and form magnetic structures when a magnetic field is applied (figure 3). This sort of MR effect is stronger than in the case of FFs, but it is still far

**Figure 3.** Microscopic picture upon magnetic field application of an inverse FF consisting of PMMA microparticles dispersed in a FF (5 vol % of magnetite in mineral oil). It can be seen that the nonmagnetic particles (white spots) form chains in the direction of the magnetic field. Bar length: 100 μm.

In the case of bimodal suspensions, micron-sized particles are again dispersed in a FF, but this time, the dispersed particles are magnetic. These MFs have proved to be more stable against sedimentation than conventional MRFs due to the stabilization achieved by the formation of nanoparticle clouds around each micron-sized particle. The formation of such

Finally, the anisotropic nature of fibrillar particles leads to an increase of the MR effect in MRFs consisting of them (Kuzhir et al., 2009; López-López et al., 2007, 2009, 2012). Figure 5 shows magnetic fibrillar particles, with a longitudinal axis of about 50 nm, which have been

clouds can be observed in figure 4 (López-López et al., 2005b, 2006, 2010).

used in the preparation of stable FFs.

**Figure 4.** Optical microscope picture of a diluted bimodal suspension consisting of ferromagnetic microparticles dispersed in a FF. Note the halo around each microparticle formed by the magnetite nanoparticles of the FF carrier. Bar length: 20 μm.

**Figure 5.** High resolution TEM image of CoNi fibrillar particles for the preparation of MFs. Bar length: 50 nm.
