# What are the uses of ferromagnetic fluids

## Manufacture and applications of ferrofluids S. Reuss and T. Wilhelm

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1 Production and applications of ferrofluids S. Reuss and T. Wilhelm 1. Ferrofluids The development of liquids with homogeneous ferromagnetic properties, so-called ferrofluids, can be seen as an important achievement in nanotechnological research. The particular advantage of the materials is that they combine two physical properties in a form that has never been seen before. On the one hand, ferrofluids are liquid at room temperature and, on the other hand, they have magnetic properties at the same time, which was previously due to the loss of ferromagnetic properties of solids above the so-called Curie temperature, which is usually the case. is well below the melting temperature of the substance, was unthinkable. This liquid is realized through the homogeneous distribution of the smallest particles of ferromagnetic materials, such as magnetite, in a liquid. Here it is extremely important for long-term use of the fluid that these magnetite particles neither agglomerate nor sediment. Ferrofluids can be produced in very different ways. The aim of this article is to describe a bottom-up process in which larger magnetite particles are produced by chemical reactions from individual iron ions. 2. Physical requirements for a ferrofluid So that the particles in a carrier liquid do not sediment, the kinetic (thermal) energy of the particles must be greater than their potential energy. This approach can be used to estimate the maximum particle size: It is known that an iron particle with a volume V T has the weight force: F G = V T ρ T g where ρ T represents the density of the particle material and g the acceleration due to gravity. Since the particles are dissolved in a liquid, they are acted on by a buoyancy force that is opposite to the weight of the particles: FA = VT ρ Fl g The resulting force on a particle, which is now ideally assumed to be spherical with diameter d, now follows: F = FG + FA = VT (ρ T ρ Fl) g = d3 6 π (ρ T ρ Fl) g In a vessel with the level h, a particle dissolved in a liquid can gain the following energy through sedimentation: E pot, max = d3 6 π (ρ T ρ Fl) gh So that no sedimentation takes place, the thermal energy of the particle E th = k BT at a certain temperature T must be greater than its energy gain through sedimentation. If one now equates both energies, the maximum diameter of a ferrofluid particle can be determined:

2 3 6k BT d max = (ρ T ρ Fl) gh π If one uses realistic densities for the magnetite particles ρ T 4.7 g cm3 and the carrier liquid (e.g. decane) ρ Fl 0.7 g cm3, the maximum is calculated Particle size has a diameter of about 13 nm. A similar order of magnitude of the particles is calculated if one considers the magnetic separation of the particles. For this, reference is made to [1, p.104]. Another important criterion for preventing sedimentation of the particles consists in avoiding their agglomeration. A wide variety of attractive interactions, such as Van der Waals forces or dipole-dipole interactions, act on the individual particles in the solution. These can be prevented by repulsive interactions. This can be achieved, for example, by chemically enclosing the magnetite particles in a carrier liquid with other so-called surfactant molecules. A distinction is made between two types of molecules. Some are water-soluble in the part that does not dock on the particles, while others dissolve in carbon. Depending on the situation, you get a water-based or petroleum-based ferrofluid. Alternatively, the repulsive potential can also be achieved by charging all iron particles of the same name with the help of chemical reactions. However, this electrostatic potential can be overcome more easily at appropriate temperatures, so that agglomeration of the fluid can occur more easily. 3. Production of your own ferrofluid In order to use ferrofluids in physics lessons, it is advisable to produce them yourself with the help of a so-called bottom-up process. The larger magnetite particles are produced by chemical reactions from individual iron ions. A wide variety of manufacturing variants were tried out and modified for this purpose. These differ mainly in that the magnetite particles are either dissolved in an aqueous solution or in an oily solution. The most promising and one of the cheapest production variants, which is based on [2], will now be presented. In numerous experiments, which were shown in [1, p.113 ff.], It has been shown that the production of a water-based fluid is the simplest. A disadvantage of this liquid, however, is that it has a limited shelf life. The following chemicals are required for the production of the ferrofluid: 7 ml hydrochloric acid (37%) 3.976 g iron (II) chloride tetrahydrate 6.758 g iron (III) chloride hexahydrate 30 ml ammonium hydroxide (25%)

3 5 ml Tetramethylammonium Hydroxide 800 ml distilled water Further required utensils are: spatula 1 beaker à 50 ml 2 beakers à 500 ml spray water bottle Magnetic stirrer with magnetic fish or glass rod for stirring Crucible tongs Disposable plastic bowl, plate or beaker Neodymium magnet Pipette resp. Burette, protective goggles, coat or old clothes, disposable protective gloves, scales (max. Error ± 0.01 g) citric acid or any other acid to neutralize the decanted solutions ph indicator paper. Under a fume cupboard, the hydrochloric acid is 35 ml and the ammonium hydroxide 170 ml Diluted water to prevent working with highly concentrated substances. Now the iron (II) chloride is dissolved in 10 ml and the iron (III) chloride in 25 ml of the dilute hydrochloric acid in separate beakers. This is done with the help of the magnetic stirrer. These two dissolved salts are then mixed together and the dilute ammonium hydroxide solution is slowly added drop by drop while stirring (see Fig. 1). It is important that the addition is slow, otherwise magnetite particles that are too large will form. The solution will gradually get darker and darker until it is pitch black. Fig. 1: The dropwise addition of dilute ammonium hydroxide leads to the precipitation of

4 magnetite. If the solution has not yet reached this color after adding the estimated amount of ammonium hydroxide, slowly diluted ammonium hydroxide must be added successively. The suspension is now left to stand so that the magnetite particles settle. This process can be accelerated with the help of a magnet on the lower edge of the beaker (see Fig. 2). Fig. 2: After the complete addition of ammonium hydroxide, the precipitate is allowed to settle. The clear liquid is decanted several times. The clear solution is now decanted into another beaker, i.e. the liquid above the solid is poured off over the edge of the vessel. The magnet fixes the magnetite at the bottom of the glass to be poured (see Fig. 2). The remaining black sludge is then floated up two more times with about 200 ml of water each time and after a while it is decanted off again. The precipitate, which has now been washed and decanted again, is then poured into a plastic dish or the like. The part that remains in the beaker is floated up with a little water and also poured into the bowl, whereby it is important that all of the magnetite is removed from the beaker. Then the black mass from the plastic bowl is decanted again, but this time also part of the black precipitate, until only a tough paste remains. Under certain circumstances this can be a very small amount. A few drops of tetramethylammonium hydroxide are then added to this paste and the mixture is stirred for two minutes with a glass rod. The amount of base added is increased until the fluid has a viscosity of viscous oil. If you hold a magnet to the bottom of the cup and vary its distance, you can discover the Rosensweig instabilities (see Fig. 3), an indication of a successful production. The ferrofluid can now be preserved in a test tube using n-decane, for example. For disposal

5 of the poured off liquids must be neutralized with any acid (e.g. citric acid). Fig. 3: After adding the 25% tetramethylammonium hydroxide solution, the ferrofluid is obtained, which shows the Rosensweig instabilities in a magnetic field. Ferrotex Europe and NanoBionet e.v. also sell finished fluids. The advantages here are the high quality of the fluids and their long shelf life. 4. Experiments with ferrofluids Ferrofluids can be used to illustrate the course of magnetic field lines. The so-called Rosensweig instabilities arise due to the surface tension of the liquid. In addition to showing the effects of magnetic forces on small particles, ferrofluids can also be used to illustrate the behavior of various substances in a magnetic field. The permeability of matter to magnetic fields depends on the magnetic permeability. In the case of materials with low permeability, the magnetic field lines are hardly influenced (see Fig. 4) and the ferrofluid collects at the point of the strongest magnetic field and makes its direction visible. In the case of materials with high permeability, such as iron or mu-metals, on the other hand, there is no longer a strong magnetic field above the material and thus hardly any effect on the ferrofluid (see Fig. 5). Fig. 4: Course of the B magnetic field lines in non-magnetic materials (low permeability) and its visualization with a ferrofluid

6 Fig. 5: Course of the B magnetic field lines in magnetic materials (high permeability) and its visualization The following two experiments are intended to clarify the force effect of an external magnetic field on the fluid particles. They show that the magnetic force on the individual fluid particles is greater than their gravitational force. In the experiment in Figure 6, the magnet is held over the ferrofluid so that it is pulled upwards. Fig. 6: Abolition of the force of gravity That this force effect is even more powerful can be shown with the help of a non-magnetic material, the density of which is greater than that of the fluid. For example, a 10 cent coin is lost in a ferrofluid without an external magnetic field (see Fig. 8). If you create a field, however, the coin appears on the surface (see Fig. 9). It can be said that the virtual density of the ferrofluid increases when a magnetic field is applied. Fig. 7: The 10 cent coin was lost in the ferrofluid.

7 Fig. 8: When an external magnetic field is applied, the coin reappears. 5. Conclusion Nanotechnology is still not mentioned at all in many curricula in the various federal states. However, it is possible to enrich the magnetism lesson sequence through the use of such fluids, especially since they show that there are also liquid magnetic substances. In combination with the production of a ferrofluid, an interdisciplinary project with the subjects chemistry and biology is a good option, since ferrofluids have proven to be an effective means of treating cancer, not only for sealing rotating axes, as a heat transfer agent for loudspeakers, but also in everyday life. Literature: [1] S. Reuss & T. Wilhelm, Nanotechnology in School Lessons, Staatsexamensarbeit, 2011, [2] D. Chun, S. Karlen, JB Kolodziej-Chris, V. Shabnam, M. Weinberger Michelle: Syntheses of an Aqueous Ferrofluid - Version 3.0., Instructions for the production of a water-based ferrofluid, ual% pdf, as of 01/2015 Address of the authors StR Sebastian Reuß, Staatliches Gymnasium Holzkirchen, Jörg-Hube-Straße 4, Holzkirchen, Prof. Dr. Thomas Wilhelm, Institute for Physics Education, Goethe University Frankfurt am Main, Max-von-Laue-Str. 1, Frankfurt am Main,

8 Five key words: nanotechnology, ferrofluid, magnetism, magnetic fluid, magnets Abstract A special branch of research in nanotechnology deals with the development of magnetic fluids - ferrofluids. In this article, some physical aspects of this liquid are highlighted and instructions are given on how to make such a fluid yourself. In addition, some experiments with the liquids are presented, how one can integrate ferrofluids into the subject of magnetism in the classroom.