A ferrofluid is a stable colloidal suspension of sub-domain magnetic
particles in a liquid carrier. The particles, which have an average
size of about 100Å (10 nm), are coated with a stabilizing
dispersing agent (surfactant) which prevents particle agglomeration
even when a strong magnetic field gradient is applied to the ferrofluid.
The surfactant must be matched to the carrier type and must overcome
the attractive van der Waals and magnetic forces between the particles.
The colloid and thermal stabilities, crucial to many applications,
are greatly influenced by the choice of the surfactant. A typical
ferrofluid may contain by volume 5% magnetic solid, 10% surfactant
and 85% carrier.
the absence of a magnetic field, the magnetic moments of the particles
are randomly distributed and the fluid has no net magnetization.
When a magnetic field is applied to a ferrofluid,
the magnetic moments of the particles orient along the field lines
almost instantly. The magnetization of the ferrofluid responds immediately
to the changes in the applied magnetic field and when the applied
field is removed, the moments randomize quickly.
In a gradient field the whole fluid responds as
a homogeneous magnetic liquid which moves to the region of highest
flux. This means that ferrofluids can be precisely positioned and
controlled by an external magnetic field. The forces holding the
magnetic fluid in place are proportional to the gradient of the
external field and the magnetization value of the fluid. This means
that the retention force of a ferrofluid can be adjusted by changing
either the magnetization of the fluid or the magnetic field in the
is designed as a component of a device and therefore it must meet
specific performance objectives of the device. The selection of
ferrofluid depends on many factors such as environments, operating
life, etc. There are many different combinations of saturation magnetization
and viscosity resulting in a ferrofluid suitable for every application.
The operating life of the product depends on the
volatility of the ferrofluid. Products that require long life must
use ferrofluids with low evaporation rate or vapor pressure. Also,
seals operating at high vacuum must incorporate low vapor pressure
fluids. On the other hand, ferrofluids for domain observation must
evaporate quickly so that the process time can be minimized. The
lower the volatility, the higher the viscosity of the ferrofluid.
Thermal conductivity of a ferrofluid depends linearly
on the solid loading. Fluorocarbon based ferrofluids have the lowest
thermal conductivity of all commercial ferrofluids, therefore they
are the least desirable materials for heat transfer applications.
In devices, ferrofluids come in contact with a
wide variety of materials. It is necessary to ensure that ferrofluids
are chemically compatible with these materials. The fluids may be
exposed to hostile gases, such as in the semiconductor and laser
industries; to liquid sprays in machine tool and aircraft industries;
to lubricant vapors in the computer industry; and to various adhesives
in the speaker industry. Furthermore, ferrofluids may be in contact
with various types of plastics and plating materials. The surface
morphology can also affect the behavior of the fluid. The selection
of ferrofluid is carefully engineered to meet application requirements.
Additionally, ferrofluids may be expected to perform
at temperature of 150°C continuously or 200°C intermittently,
in winter conditions (-20°C) and space environments (-55°C).
They may also be required to withstand nuclear radiation without
The thermal stability of a ferrofluid is related
to particle density. The particles appear to behave like a catalyst
and produce free radicals, which lead to cross linking of molecular
chains and eventual congealing of the fluid. Catalytic activity
is higher at elevated temperatures and, therefore, ferrofluids congeal
more rapidly at these temperatures.
Oxidation is another mechanism that contributes
to congealing of ferrofluids, and again the higher the temperature,
the faster the rate of reaction. The ferrofluids in sealed environments
stay in a liquid state significantly longer than those in open air.
High magnetization ferrofluids are of interest
as they produce volumetric efficiencies of magnetic circuit designs
leading to lightweight and lower cost products. They can also be
used to reduce reluctance of magnetic circuits and fringing field
thus increasing useful flux density in the air gap. The domain magnetization
of magnetite ultimately limits the maximum magnetization value that
can be realized in a ferrofluid.
In summary we can say that Ferrofluids are a unique
class of material. Ferrofluid technology is well established and
capable of solving a wide variety of technical problems. There are
many successful applications of this engineering material and there
is immense future potential.
many applications, ferrofluid is an active component that contributes
towards the enhanced performance of the device. These devices are
either mechanical (e.g., seals, bearings and dampers) or electromechanical
(e.g., loudspeakers, stepper motors and sensors) in nature. In other
cases, ferrofluid is employed simply as a material for nondestructive
testing of other components such as magnetic tapes, stainless steels
and turbine blades. When correctly applied, Ferrofluid can produce
dramatic improvements in a products' performance; or achieve a level
of performance unattainable by any other technology or product.
Ferrotec Corporation (formerly Ferrofluidics) has
led the development of Ferrofluidic® technology since
1968 and has worked closely with many companies as their new product
teams incorporate ferrofluids in next-generation products. With
a comprehensive fluid development and applications laboratories
in both the US and Japan, and an experienced staff of scientists
and engineers available to assist you, Ferrotec is well placed to
help you solve your engineering challenges using ferrofluid.
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