6.3
Ferrofluids
6.3.1
General Considerations
One fascinating application of superparamagnetic particles – and one that already has
been widely applied in technical products – is that of ferrofluids, which are a special type
of nanofluids. A ferrofluid is a stable suspension of superparamagnetic particles in a
liquid. In order to avoid magnetic coagulation of the particles, they must be coated with
a second, distance-holder phase. Ferrofluids contain between 3 and 8 vol% of magnetic
nanoparticles, and usually more than 10 vol% of the surfactant. Normally, oil or water is
used as the carrier liquid. Brownian molecular movements thwart the sedimentation of
the particles in the absence of any external magnetic field, while in the presence of a
magnetic field such movements prevent demixing of the suspension. Magnetic
sedimentation can be avoided if the magnetic moment of the particles is not too
large. Characteristically, in the absence of an external magnetic field the net magnetic
moment of a ferrofluid is nil. However, the particles adjust within a few milliseconds in
the direction of an external magnetic field, resulting in to a net magnetic moment. As is
typical for superparamagnetic systems, following removal of the magnetic field the
magnetic moments of the particles randomize almost immediately, leading again to a
net magnetic moment of nil. In a magnetic field gradient, the whole fluid moves to the
region of highest flux, and consequently an external magnetic field can be used for the
precise positioning and control of ferrofluids. This also allows the design of actuators
that are based on ferrofluids. It is essential, however, that the ferrofluids are stable
against the sedimentation of magnetic nanoparticles, even in strong magnetic fields.
Rosenzweig [5] has provided a complete theoretical basis for ferrofluids and also
explained the series of instabilities that bears his name. The most famous of these
shows the surface of a ferrofluid in an inhomogeneous magnetic field forming
spikes, rather than a flat or convex surface (see Figure 6.6). The spikes, which follow
the gradient of the magnetic field, are the consequence of an interaction of surface
energy, gravitational, and magnetic energies. These are formed above a critical
magnetic field, where the reduction in the magnetic field energy is greater than the
increase in surface and gravitational energies. The forces that act at magnetic fluids
are proportional to the gradient of the magnetic field and the magnetization of the
fluid. Therefore, the retention force of a ferrofluid may be adjusted by changing
either the magnetization of the fluid or the external magnetic field.
6.3.2
Properties of Ferrofluids
Ferrofluids, as magnetic materials, may be used to transfer magnetic fields or to close
magnetic circuits in a simple way, without the need for any complicated and shaped
parts. In addition to these possibilities, the variation of viscosity as a function of an
external magnetic field represents one of the most striking properties of a ferrofluid.
6.3 Ferrofluids j127
Ferrofluids
6.3.1
General Considerations
One fascinating application of superparamagnetic particles – and one that already has
been widely applied in technical products – is that of ferrofluids, which are a special type
of nanofluids. A ferrofluid is a stable suspension of superparamagnetic particles in a
liquid. In order to avoid magnetic coagulation of the particles, they must be coated with
a second, distance-holder phase. Ferrofluids contain between 3 and 8 vol% of magnetic
nanoparticles, and usually more than 10 vol% of the surfactant. Normally, oil or water is
used as the carrier liquid. Brownian molecular movements thwart the sedimentation of
the particles in the absence of any external magnetic field, while in the presence of a
magnetic field such movements prevent demixing of the suspension. Magnetic
sedimentation can be avoided if the magnetic moment of the particles is not too
large. Characteristically, in the absence of an external magnetic field the net magnetic
moment of a ferrofluid is nil. However, the particles adjust within a few milliseconds in
the direction of an external magnetic field, resulting in to a net magnetic moment. As is
typical for superparamagnetic systems, following removal of the magnetic field the
magnetic moments of the particles randomize almost immediately, leading again to a
net magnetic moment of nil. In a magnetic field gradient, the whole fluid moves to the
region of highest flux, and consequently an external magnetic field can be used for the
precise positioning and control of ferrofluids. This also allows the design of actuators
that are based on ferrofluids. It is essential, however, that the ferrofluids are stable
against the sedimentation of magnetic nanoparticles, even in strong magnetic fields.
Rosenzweig [5] has provided a complete theoretical basis for ferrofluids and also
explained the series of instabilities that bears his name. The most famous of these
shows the surface of a ferrofluid in an inhomogeneous magnetic field forming
spikes, rather than a flat or convex surface (see Figure 6.6). The spikes, which follow
the gradient of the magnetic field, are the consequence of an interaction of surface
energy, gravitational, and magnetic energies. These are formed above a critical
magnetic field, where the reduction in the magnetic field energy is greater than the
increase in surface and gravitational energies. The forces that act at magnetic fluids
are proportional to the gradient of the magnetic field and the magnetization of the
fluid. Therefore, the retention force of a ferrofluid may be adjusted by changing
either the magnetization of the fluid or the external magnetic field.
6.3.2
Properties of Ferrofluids
Ferrofluids, as magnetic materials, may be used to transfer magnetic fields or to close
magnetic circuits in a simple way, without the need for any complicated and shaped
parts. In addition to these possibilities, the variation of viscosity as a function of an
external magnetic field represents one of the most striking properties of a ferrofluid.
6.3 Ferrofluids j127