Ferrofluid: Magnetic Liquid Technology
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.
Magnetic Behavior of Ferrofluid
In 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 region.
Ferrofluid Properties and Their Application
Ferrofluid 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 performance and operating life of a product that uses ferrofluid can be significantly affected by the characteristics of the ferrofluid. From ferrofluids with low evaporation rate or vapor pressure to ferrofluids with viscosity-optimized products, the characteristics of ferrofluid can dramatically shape the capabilities of the end product.
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 breakdown.
Characteristics of Ferrofluid that Affect Performance
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.
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.
As the worldwide leader in the production of ferrofluid, 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.
In 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.