Magnetocaloric Fluids - Magnetocaloric Energy Conversion: From Theory to Applications - page 169

5.2 Rheology of Magnetic Fluids

Ferro fl uids are magnetically controllable nano fl uids and have been synthesized

since the 1960s [ 7 - 9 ]. They can be used in a variety of applications [ 10 , 11 ].

A typical ferro fl uid consists of magnetic particles (about 5 - 10 % in terms of volume

fraction) having size of about 3 15 nm and monodispersed in a base liquid. The

-

basic type of ferro nely

dispersed particles in a base medium, including suspensions in which particles tend
to settle. Despite the fact that a small concentration gradient can be established after
longer exposure to a magnetic or gravitational

fl uid is the colloidal ferro fl uid. This is a suspension of

eld, the particles will stay sus-
pended [ 7 ]. Particles will keep suspended because of Brownian motion. However,
they can agglomerate. To prevent agglomeration due to van der Waals forces the
particles can be coated with an adsorbed layer of surfactant with a thickness of
approximately 2 nm [ 7 ]. Macroscopically looking, these

fl uids manifest themselves

as magnetizable single-phase liquid media.
The applied magnetic eld may cause the formation of particle chains in the

direction of the magnetic eld will increase interaction

between the particles. Also, particle chains will become longer. The longer the
chain of particles, the larger the resistance of the

eld. An increased magnetic

ow and, consequently, the
larger will be the viscosity. Therefore, with the induced magnetic

fl uid to fl

eld, this will

certainly affect the viscosity of the ferro fl uid, but will in most cases induce very

small yield stress. In most cases these fl uids do not exhibit any magnetorheological

behaviour.
Magnetorheological (MR) fl uids also represent magnetically controllable fl u-

ids. However, in contrast to ferro uids are suspensions of micrometre

range ferro- or ferrimagnetic multi-domain particles in a liquid matrix [ 12 , 13 ]. MR
fl

fl uids the MR fl

uids were introduced in the early 1940s; however, the majority of research

activities relate to the past two decades. Today, these uids represent an important

part of many applications. One of them is in damping devices [ 14 ]. A typical
magnetorheological (MR) fluid consists of solid particles, typically in the range
from 1 to 20

fl

μ m, with volume fraction of about ϕ V = 0.4 − 0.5 [ 15 ]. Because of the

micron-sized particles the Brownian motion in MR fl uids is negligible. With respect

to MR uids the coupling constant represents an important parameter, which relates

the level of tendency towards the formation of chains and agglomerates. In MR
fl

fl

uid tends to create
particle chains or forms agglomerates. The result is a yield stress that depends not
only on the common rheological parameters, but also on the applied magnetic

uids, the coupling constant is high, which means that the fl

eld.
As a consequence, the chaining of solid particles, which is established along the
suspension (depends on the magnetic eld direction), transforms the liquid form of

MR uid into a solid (the apparent viscosity can increase up to about 1,000 times).

If the shear stress (induced by the pressure difference on the MR

fl

fl uid) overcomes

the yield stress, the MR fl uid will start to fl ow. Note that the particle aggregation

processes in MR fl uids are reversible. Therefore, the result of the removed magnetic