Magnetocaloric Fluids - Magnetocaloric Energy Conversion: From Theory to Applications - page 170

Fig. 5.4 The classication of magnetic fl uids according to Lopez et al. [ 16 ]

eld will be the reduced in fl uence of the magnetically induced yield stress, by

changing the solid MR uid into its liquid suspension form.

Note that there is no clear de

fl

nition of the transition from a ferro fl uid to an

magnetorheological uid. Lopez et al. [ 16 ] presented an example for such a tran-

sition, as shown in Fig. 5.4 . They have de

fl

uid as a colloidal
suspension of nanoparticles of ferro- or ferrimagnetic materials dispersed in a liquid
carrier which do not settle, despite the potential long-term exposure to the applied
(gravitational or magnetic)

ned the ideal ferro fl

-
uids, dispersed particles should be smaller than approximately 10 nm and coated
with a molecular layer of surfactant to prevent the van der Waals attractive forces.
Under these conditions, Brownian motion dominates over the forces of interaction.
The real ferro

eld. As denoted by Lopez et al. [ 16 ], in ideal ferro fl

dipole energy of the interaction
between them overcomes Brownian motion and causes clustering under the applied
magnetic

fl uids are polydispersed. The dipole -

eld. The latter increases the viscosity and can also lead to yield stress.
Genc and Derin [ 17 ] also de ned ideal ferro fl uids as colloidal suspensions of

ultra uids,

the size would be in the range from 5 to 15 nm. For a particle size between 15 and
40 nm they considered the magnetic

ne (5 - 10 nm) single-domain magnetic nanoparticles and for real ferro fl

fl uid to be a ferro fl uid. Above this limit, the

fl uid would exhibit the characteristics of an MR fl uid.

5.2.1 Rheology of Ferro fl uids

Since the uids have neglected the interaction

between solid particles, according to Odenbach [ 18 ], these theories can only be
used for quantitative predictions of the behaviour of highly diluted

rst theories on the rheology of ferro fl

uids. This is
especially because experimental results have shown that a strong in

fl

fl uence on

viscosity can be induced by a magnetic eld, not only in magnetorheological fl uids,

but also in ferro fl uids with suf cient particle particle interaction.

-