114 6 Nanofluids
engineering, medicine, and biotechnology. The stabilization of ferrofluids is thus
more difficult as compared to the other nanofluids, as in-between the ferromagnetic particles, there is a dipole–dipole interaction, with the tendency to form
lumps. Furthermore, besides segregation due to gravity and thermophoresis, one
must take note of segregation of the particles in an external magnetic field. Magnetic unmixing is avoided by limiting the size of the magnetic particles; on the
other hand, magnetic phenomena depend strongly on the particle size. Therefore,
a compromise, optimal for the specific application, must be sought. Ferrofluids
for technical applications contain between 3 and 8 vol% magnetic nanoparticles
and usually more than 10 vol% surfactant. Depending on the application, water
or oil are applied as carrier liquid.
In the absence of an external magnetic field, the magnetic moment of a ferrofluid is nil. In the presence of a magnetic field, the particles adjust within milliseconds in the direction of the magnetic field and after removing the external
field, the orientation of the magnetic dipoles (the particles) randomize again. The
dynamics of this process is known as Brown’s superparamagnetism. In a magneticfield gradient, the whole fluid moves to the region of highest flux. This propensity
is the basis of series of interesting phenomena connected to ferrofluids. In a
famous book, Rosenzweig [5] gives a complete theory of ferrofluids and describes
or predicts a series of phenomena carrying his name.
The most famous one of the Rosenzweig phenomena is depicted in Figure 6.6.
It displays the surface of a ferrofluid in an inhomogeneous magnetic field. The
unbiased thinker expects just a convex surface, showing a tendency of attracting
the magnetic fluid by the magnetic field. However, things are more complicated,
as this simple way of looking at the things did not consider the interaction between
the inhomogeneous magnetic field, gravity, and surface energy. Because of this
complicated interaction, one observes a spiked surface, where the spikes follow
the direction of the gradient of the magnetic field. The forces acting at magnetic
fluids are proportional to the gradient of the magnetic field (Figure 6.4).
Figure 6.4 Rosenzweig phenomenon, interaction of a ferrofluid in an inhomogenous magnetic
field with gravitation and surface energy [6].
engineering, medicine, and biotechnology. The stabilization of ferrofluids is thus
more difficult as compared to the other nanofluids, as in-between the ferromagnetic particles, there is a dipole–dipole interaction, with the tendency to form
lumps. Furthermore, besides segregation due to gravity and thermophoresis, one
must take note of segregation of the particles in an external magnetic field. Magnetic unmixing is avoided by limiting the size of the magnetic particles; on the
other hand, magnetic phenomena depend strongly on the particle size. Therefore,
a compromise, optimal for the specific application, must be sought. Ferrofluids
for technical applications contain between 3 and 8 vol% magnetic nanoparticles
and usually more than 10 vol% surfactant. Depending on the application, water
or oil are applied as carrier liquid.
In the absence of an external magnetic field, the magnetic moment of a ferrofluid is nil. In the presence of a magnetic field, the particles adjust within milliseconds in the direction of the magnetic field and after removing the external
field, the orientation of the magnetic dipoles (the particles) randomize again. The
dynamics of this process is known as Brown’s superparamagnetism. In a magneticfield gradient, the whole fluid moves to the region of highest flux. This propensity
is the basis of series of interesting phenomena connected to ferrofluids. In a
famous book, Rosenzweig [5] gives a complete theory of ferrofluids and describes
or predicts a series of phenomena carrying his name.
The most famous one of the Rosenzweig phenomena is depicted in Figure 6.6.
It displays the surface of a ferrofluid in an inhomogeneous magnetic field. The
unbiased thinker expects just a convex surface, showing a tendency of attracting
the magnetic fluid by the magnetic field. However, things are more complicated,
as this simple way of looking at the things did not consider the interaction between
the inhomogeneous magnetic field, gravity, and surface energy. Because of this
complicated interaction, one observes a spiked surface, where the spikes follow
the direction of the gradient of the magnetic field. The forces acting at magnetic
fluids are proportional to the gradient of the magnetic field (Figure 6.4).
Figure 6.4 Rosenzweig phenomenon, interaction of a ferrofluid in an inhomogenous magnetic
field with gravitation and surface energy [6].