Ferrofluid

Ferrofluids are fluids that react to magnetic fields without solidifying. They consist of a few nanometers in size magnetic particles are colloidally suspended in a carrier liquid. The particles are generally stabilized with a polymeric surface coating. True ferrofluids are stable dispersions, which means that the solid particles do not settle with time and even in extremely strong magnetic fields do not adhere to each other or may be deposited from the liquid phase than others.

Ferrofluids are superparamagnetic and have a very low hysteresis.

• 3.1 industry
• 3.2 Measurement Technology
• 3.3 Medical

Components

The particles usually consist of iron, magnetite or cobalt, and are smaller than one magnetic domain, are typically 5-10 nm (millimicrons ) in diameter. The surrounding fluid is usually oil or water, rare wax. Surfactants are added to make the suspension more stable by repel the bound in micelles particles due to steric interactions with each other.

Behavior in a magnetic field

In a magnetic field, the magnetic moments of the particles of the magnetoresistive fluid will tend to be deflected in the direction thereof, and thereby obtain a macroscopic magnetisation. However, the random movement of the particles still dominates the force that pulls them together; they do not form chains, their viscosity almost does not change, but they tend to stay in high magnetic fields.

Ferrofluids in magnetic fields form some very interesting three-dimensional shapes and at a limit of a thin layer (eg between two glass plates ) stripe pattern. This is caused by the aligning themselves and repulsive magnetic fields of the individual particles, the force of surface tension of the liquid but it holds together ( sting 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

Unlike ferrofluids to solidify Magnetorheological fluids (MRF ) in a magnetic field. They consist of a suspension of magnetic particles, which measure 10 nanometers to several micrometers, and thus one to three orders of magnitude greater than that of the ferrofluid. The relatively large particles in the magnetic field of the MRF form chains ( " toughness " ) increase the viscosity of the MRF. Solidifies the MRF as long as no pressure force is applied that is large enough to break the chains.

Applications

Industry

Ferrofluids used in loudspeakers to dissipate heat from the voice coil and the magnet assembly and to the movements of the diaphragm to damp passive. You sit there, where else would the air gap around the voice coil, and are held there by the permanent magnet field. Similarly, they are used for rotating shafts to form liquid (and therefore wear-free, low-friction ) seals by walls. These seals are used for shaft bushings in vacuum chambers or clean rooms, for example, a hard disk. For density separation of ferrofluid used in water-based. Here, a further pressure is generated by a magnetic field, which allows you to leave and high density bodies float. Matsushita Electric Industrial Co. produced an inkjet printer, the ink used ferrofluid ( Capacity: 5 pages per minute).

Measurement

For ferrofluids there are numerous optical applications because of their light scattering properties, i.e., each nanoparticle, a "mini magnet ," reflected light. Such applications include the measurements of the specific viscosity of a liquid with the aid of a polarization microscope.

Medical

In medicine, one tries to use ferrofluids for cancer detection ( diagnostics) or a specially modified surfaces with deposited drugs for cancer therapy. Very successfully ran eg testing with surface-modified ferrofluid nanoparticles were introduced into cancer cells and then heated at applying a high frequency magnetic field. This heat is transferred to the cell interior and the cell can thus be put into an artificial fever. Thus, tumor growth can be stopped and the tumor can also be completely removed under favorable conditions. The German company has Magforce after successful completion of clinical studies in 2009 applied for admission of the method for the treatment of glioblastoma ( brain tumor).