Ferrofluid a Stable Collidal

Technology Overview

Ferrofluid is a stable collidal suspension of sub-domain magnetic particles in a liquid carrier. The particles, which have an average size of about

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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.

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 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.

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