Structure, Thermodynamics and Applications of Ferrofluids

Dear Colleagues,

The number of new materials with extreme physical properties is continuously increasing with the development of nanotechnology, such as supra- and paramagnetic ferrofluids, liquid crystals, magnetorheological (MR), and electrorheological (ER) liquids. Among them, a ferrofluid is a liquid that becomes highly magnetized in the presence of a magnetic field. A ferrofluid is a colloidal suspension of single-domain particles dispersed in a carrier liquid and stabilized by a suitable organic surfactant. The particles have radii ranging from approximately 2–10 nm. Single-domain particles are particles that are in a state of uniform magnetization. The magnetic properties of the dispersion, subjected to a constant magnetizing field, are adequately described by the Langevin theory of paramagnetism. Ferrofluids are characterized by a magnetization curve that displays no hysteresis. The measurement of the complex susceptibility of a colloidal suspension of magnetic particles involves the measurement of inductance and resistance of the suspension. The frequency range is determined by the size of the magnetic particles and their subsequent mechanism of relaxation. For magnetic fluids, two relaxation mechanisms can occur, one by rotational Brownian diffusion and the other by Neel relaxation. The dominant magnetization process of a particle will be that which has the shortest relaxation time. As ferrofluids contain a distribution of particle sizes, both mechanisms contribute to the magnetization with an effective relaxation. The conventional method of determining the frequency dependence of the complex susceptibility of a ferrofluid is to insert the fluid into the alternating magnetic field of a coil and observe the changes in its inductance and resistance.

Ferrofluids have many applications ranging from small electronic devices to space crafts to cancer treatments to art. Ferrofluids are found in many common household devices, including hard drives where they are used to seal the interior of the device. When magnetized they form a barrier to dust and dirt that could damage the delicate plates. Ferrofluids can be viewed as a particularly interesting class of dipolar fluids, which have a wide range of potential applications in biomedicine and technology. Separation, immunoassay, drug delivery, magnetic resonance imaging (MRI), and hyperthermia are enhanced by the use of magnetic nanoparticles and ferrofluids. Ferrofluids can have very high thermal conductivities and their heat transfer properties are exploited in devices such as loudspeakers.

This Special Issue will focus on the structural, transport, and thermodynamic properties of ferrofluids, practical usability of ferrofluid materials in industrial, environmental, and medical applications, as well as the dependence of the phase equilibrium properties of these complex fluids on external electric and magnetic fields.

Prof. Dr. Istvan Szalai
Guest Editor

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