Ferrofluids and their applications
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amental properties of ferrofluids Ferrofluids are suspensions of magnetic nanoparticles in appropriate carrier liquids. The magnetic particles are usually magnetite (Fe3O4) or cobalt with a mean diameter on the order of 10 nm. The particles are coated with surfactant molecules to prevent agglomeration due to van der Waals interactions for times up to decades. A liquid dispersion of magnetic nanoparticles is considered to be a ferrofluid if it behaves as a uniform magnetic liquid and the particles are not separated from the solvent by an external magnetic field. Since the first preparation of a ferrofluid by Steve Papell,1 numerous methods to prepare ferrofluids have been reported, including the co-precipitation methods of Khalafalla and Reimers.2 The most common particle used in ferrofluids is magnetite, since oxidation does not cause a degradation of magnetic properties, unlike the reduction in magnetization when Fe or Co oxidizes. Recently, Bönnemann et al.3 have shown a way to produce iron as well as cobalt nanoparticles with an aluminium oxide shell to overcome this. The most important property of ferrofluids is high initial susceptibility (i.e., χ = M/H, at low fields, where M is the magnetization and H is the magnetic field) about four orders of magnitude larger than in common molecular paramagnets. This allows for the possibility to produce strong magnetic forces with weak fields, making numerous applications possible.4–6 In the following, we discuss possibilities to control the flow
of ferrofluids by magnetic fields, changes in their basic fluid properties in the presence of magnetic fields, as well as applications of ferrofluids.
Flow control by magnetic forces Readers interested in a deep theoretical approach to fluid mechanics of ferrofluids are referred to the book by Rosensweig4 as well as to recent developments in ferrofluid dynamics summarized in a review article by Müller et al.7 To calculate magnetic field effects, the Bernoulli equation has to be extended by a magnetic term, including the energy of the ferrofluid in a magnetic field:4 ρ 2 ν + ρgh − μ 0 MH = const . 2
(1)
Here, v denotes the velocity of the fluid, ρ is its density, h is the height used to calculate potential energy, μ0 is the permeability of free space, and g is the gravitational acceleration. Consider a free jet of a ferrofluid, as shown in Figure 1—with a diameter a1 and a velocity v1—which enters a homogenous magnetic field aligned parallel to the flow direction of the jet. The magnetic energy term in Equation 1 changes from zero at point 1 to a finite negative value at point 2. Assuming that pressure as well as potential energy do not change between points (1) and (2), it becomes obvious that the change in magnetic energy has to be compensated by a
Stefan Odenbach, Technische Universität Dresden, Germany; [email protected] DOI: 10.1557/mrs.2013.232
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • NOVEMBER 2013 • www.mrs.org/bulletin
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FERROFLUIDS AND THEIR APPLICATIONS
Figure 1. The magnetic nozzle. The ferrofluid enters
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