Magnetocaloric Fluids - Magnetocaloric Energy Conversion: From Theory to Applications - page 177

(continued)
Title Stability and magnetorheological behaviour of magnetic fl uids based

on ionic liquids (Rodrigez-Arco et al. [ 31 ])
tests showed that a true ferrofluid was only obtained when the
nanoparticles were coated with a layer of surfactant compatible with
the ionic liquid. The mean diameter of the nanoparticles for different
samples was about 10 nm, while the volume fractions were between
5 and 10 vol%, depending on the type of material analysed

Constitutive model The authors applied the Bingham model (Eq. 5.14)

Viscosity The authors used Einstein ' s (Eq. 5.2 ) and Batchelor ' s (Eq. 5.11)

equation

5.2.2 Rheology of Magnetorheological Fluids

In contrast to ferro fl uids, in magnetorheological fl uids, even a small magnetic eld

will have very strong in uence on the interactions between the particles and on the

viscosity. Magnetorheological

fl

uids are suspensions that contain micron-sized
magnetic particles. The carrier liquids are usually oils; however, other applications,
such as for instance liquid metals should not be neglected. The volume fraction of
solid particles in these

fl

uids varies between 10 and 50 %. Note again that the
primary use of magnetorheological

fl

ow, but
magnetically controllable viscosity and yield stress. We will come back to this issue
when we speak about the applications in magnetocaloric energy conversion. The
primary applications for magnetorheologic

fl uids is not the manipulation of their fl

uids regard the so-called direct shear
mode (application in brakes and clutches) and the valve mode (application in
dampers). However, due to the fast response and the precise controllability, mag-
netorheological

fl

uids are becoming attractive for many different applications (see,
e.g. Olabi and Grunwald [ 40 ]).
In order to stabilize a dispersion of particles, small amounts of dispersants are
added to the carrier liquid. However, in contrast to ferro

fl

uids, surfactant techniques,
which avoid the agglomeration of particles, are not usually applied. Other methods
relate to polymer core

fl

shell magnetic particles or composite magnetic particles,
applications of nanoparticles, nanotubes or nanowires (see, e.g. Lim et al. [ 41 ], Park
et al. [ 42 ], Cho et al. [ 43 ], Fang et al. [ 44 ]). Because the particles are large, they can
no longer be considered as single-domain particles, as is the case with ferro

-

uids.
Namely, nanosized particles, which can be considered as magnetic single domains,
are in a permanent state of magnetization. Therefore, they can always possess a
magnetic dipole, even in the absence of an applied

fl

eld. On the other hand, micron-
sized particles represent a magnetic polydomain [ 45 ]. Therefore, the soft ferro-
magnetic micron-sized particle will not retain a remanent magnetization after the
removal of the magnetic

eld. Such particles will have a zero overall magnetic
moment and, consequently, the magnetic interactions between the particles will be
small. When the magnetic eld is applied, however, this will result in large magnetic