Ferrofluid
Ferrofluids are liquids that react to magnetic fields without solidifying. They consist of magnetic particles just a few nanometers in size that are colloidally suspended in a carrier liquid . The particles are usually stabilized with a polymeric surface coating.
Real ferrofluids are stable dispersions , which means that the solid particles do not settle over time and even in extremely strong magnetic fields do not attach to one another or separate from the liquid as a different phase .
Ferrofluids are superparamagnetic and have a very low hysteresis .
Components
The particles are usually made of iron , magnetite, or cobalt and are smaller than a magnetic domain , typically 5-10 nm (nanometers) in diameter. The surrounding liquid is usually oil or water , less often wax . Surfactants are added to make the suspension more stable in that the particles bound in micelles repel one another due to steric interactions .
Behavior in a magnetic field
In a magnetic field, the magnetic moments of the particles of the magnetic fluid tend to be deflected in its direction and thereby acquire a macroscopic magnetization . However, the random movement of the particles still outweighs the force pulling them together; they do not form chains, their viscosity almost does not change, but they tend to stay in highly magnetic fields.
Ferrofluids sometimes form very interesting three-dimensional shapes in magnetic fields and, when limited to a thin layer (e.g. between two glass plates), stripe patterns. This is caused by the aligning and repelling magnetic fields of the individual particles, whereby the force of the surface tension of the liquid holds them together (Stachel or Rosensweig instability, discovered in 1966 by Ronald E. Rosensweig ).
In a magnetic field, ferrofluids can show birefringence ( Cotton-Mouton effect ).
Comparison with magnetorheological fluids
In contrast to ferrofluids , magnetorheological fluids (MRF) solidify in a magnetic field. They consist of a suspension of magnetic particles that measure 10 nanometers to a few micrometers and are therefore one to three orders of magnitude larger than those of ferrofluids. The relatively large particles of the MRF form chains in the magnetic field that increase the viscosity ("toughness") of the MRF. This solidifies the MRF as long as no compressive force is applied that is great enough to break the chains.
Applications
Industry
Ferrofluids are used in loudspeakers to dissipate the heat between the voice coil and the magnet assembly and to passively dampen the movements of the diaphragm . They sit where the air gap around the voice coil would otherwise be and are held there by the field of the permanent magnet.
Similarly, they are used to form fluid (and therefore wear- free , low-friction ) seals around rotating shafts through walls. These seals are used for shaft feedthroughs in vacuum chambers or clean rooms, for example in a hard disk .
For density separation ferrofluid used water-based. A magnetic field gradient is used to generate an additional pressure with which even high-density bodies can swim.
Matsushita Electric Industrial Co. produced an inkjet printer that uses ferrofluid ink (printing performance: 5 pages per minute).
measuring technology
Ferrofluids have numerous optical applications because of their refractive properties ; H. each nanoparticle , a “mini magnet ”, reflects light. These applications include measuring the specific viscosity of a liquid using a polarizing microscope .
Medical technology
In medicine, attempts are being made to use ferrofluids for cancer detection ( diagnostics ) or with specially modified surfaces with deposited active ingredients for cancer therapy . For example, B. Tests with surface-modified ferrofluid nanoparticles that were introduced into cancer cells and then heated up when a high-frequency magnetic field was applied. This heat is transferred to the inside of the cell and the cell can thus be put into an artificial fever. In this way, tumor growth can be stopped and the tumor can also be completely removed under favorable conditions. After successfully completing clinical studies in 2009, the German company MagForce applied for approval of the procedure for the treatment of glioblastoma (brain tumor).
See also
literature
- Elmars Blums, Andrej Cebers, Michail M. Majorov: Magnetic fluids . De Gruyter, Berlin et al. 1996, ISBN 3-11-014390-9
- Christian Lang: Nanorod Ferrofluids . Dissertation, Saarland University, Saarbrücken 2006 ( full text )
- Sebastian Lissek: The general theory of magnetic fluids . Dissertation, University of Hannover 2001 ( full text as PDF )
- Arnim Nethe, Thomas Scholz, and Hanns-Dietrich Stahlmann: Ferrofluid-assisted electric motors and actuators . Köster, Berlin 2006, ISBN 978-3-89574-618-5
- Stefan Odenbach (Ed.): Ferrofluids. Magnetically controllable fluids and their applications . (= Lecture notes in physics; Vol. 594). Springer, Berlin et al. 2002, ISBN 3-540-43978-1
- Ronald E. Rosensweig: Ferrohydrodynamics . Dover Publications, Mineola NY 1997, ISBN 0-486-67834-2
Web links
- Saarland University: Materials determined by interfaces: Transport, response and instability behavior of ferrofluids
- Arnim Nethe: Research Ferrofluids