Magnetocaloric Fluids - Magnetocaloric Energy Conversion: From Theory to Applications - page 176

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Title Rheological properties of a ʳ -Fe 2 O 3 paraf n-based ferro fl uid

(Hosseini et al. [ 30 ])
was estimated that the aggregates contain about 27 - 280 magnetic
nanoparticles. All the particles and aggregates were assumed to be
spherical and the surfactant layer diameter was considered to be 2 nm

Constant model Authors propose the Bingham model (Eq. 5.14)

Viscosity The viscosity of the ferro uid decreases with increasing shear rate,

(i.e. shear-thinning behaviour). The authors conclude that when the
applied shear rate increases, the nanoparticles begin to arrange their
orientation in the shearing direction. The increasing shear rate
destroys the initial bonds existing between the nanoparticles, which
results in a decrease in the viscosity. The magneto-viscous effect
increases versus an increasing magnetic eld for every shear rate.
The magnetic eld also caused aggregation and the formation of
drop-like structures, which could lead to an increase in the viscosity
in accordance with the size of the structures

fl

Title Colloids on the Frontier of Ferrofluids (Lopez et al. [ 16 ])

Materials used for
investigation

Two new kinds of ferrofluids: one was composed of CoNi
nanospheres with a diameter of 24 nm, while the other was
composed of CoNi nanobres 56 nm long and 6.6 nm wide. The
spherical and bre-like CoNi particles were considered to be single-
domain particles. The ferro uids were prepared by dispersing proper

amounts of the synthesized powders in a mineral oil. L -

fl

-Phospha-
tidylcholine was used as the surfactant to avoid irreversible particle
aggregation. The particle volume fraction was 5 % in all cases

ʱ

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

static yield stress were different for both ferrofluids. In the case of the
nanosphere ferrofluid, the yield stress resulted in a strong increase
with an increase in the magnetic eld. The static yield stress is the
shear stress required to induce the ow of a material. However, the

magnetic eld dependence of the static yield stress of the nanobre
ferro

fl

uid was rather weak at low and medium eld (H < 100 kAm − 1 )
and became strong at higher eld values. At high shear rates the
relationship between the shear stress and the shear rate was linear.
The dynamic yield stress, which is needed to continuously break the
aggregates that reform in the presence of the magnetostatic forces,
was higher than the static yield stress

fl

Viscosity For viscosity of the nanosphere ferrofluid, Batchelor ' s equation

(Eq. 5.11) was applied. The authors, however, denoted the difference
between the experimental viscosity and the calculations using
Batchelor s equation. They assumed that the reason for this is due to

the existence of magnetostatic interactions between the particles, as
well as due to the deviation from a spherical shape. For viscosity of
the nanobre ferro

'

uid, the predictions for the suspensions of
spheroidal particles were used with the help of expressions from
Hinch and Leal [ 38 ] and Larson [ 39 ]

fl

Title Stability and magnetorheological behaviour of magnetic fl uids based

on ionic liquids (Rodrigez-Arco et al. [ 31 ])

Materials used for
investigation

Magnetic fluids consisting of magnetite nanoparticles dispersed in a
quaternary onium cation-based ionic liquid. Citric, humic and oleic