Smart Materials : Ferrofluids & Magnetic Levitation

A ferrofluid is a material that belongs to the category of nanostructured materials: it is actually a colloidal solution containing ferromagnetic nanoparticles. For this characteristic it behaves like a magnet, but a liquid magnet!

From Wikipedia : a ferrofluid is a liquid that becomes strongly magnetized in the presence of a magnetic field. Ferrofluids are colloidal liquids made of nanoscale ferromagnetic particles suspended in a carrier fluid (usually an organic solvent or water). Each tiny particle is thoroughly coated with a surfactant to inhibit clumping.  The magnetic attraction of nanoparticles is weak enough that the surfactant’s Van der Waals force is sufficient to prevent magnetic clumping or agglomeration.

General Properties

Ferrofluids are composed of nanoscale particles (diameter usually 10 nm or less) of magnetite, hematite or some other compound containing iron, Fe²⁺ o Fe³⁺, and a liquid. The particles are small enough for thermal agitation to disperse them evenly within a carrier fluid, and for them to contribute to the overall magnetic response of the fluid. The composition of a typical ferrofluid is about 5% magnetic solids, 10% surfactant and 85% carrier, by volume.

Ferrofluids are stable. This means that the solid particles do not agglomerate or phase separate even in extremely strong magnetic fields. However, the surfactant tends to break down over time (in a few years), and eventually the nano-particles will agglomerate, and they will separate out and no longer contribute to the fluid’s magnetic response.

When a ferrofluid is subjected to a strong vertical magnetic field, the surface forms a regular pattern of peaks and valleys, as shown in the image below.

This effect is known as the Rosensweig or normal-field instability. The instability is driven by the magnetic field; it can be explained by considering which shape of the fluid minimizes the total energy of the system. In summary, the formation of the corrugations increases the surface free energy and the gravitational energy of the liquid, but reduces the magnetic energy. The corrugations will only form above a critical magnetic field strength, when the reduction in magnetic energy outweighs the increase in surface and gravitation energy terms.

Ferrofluids have an exceptionally high magnetic susceptibility and the critical magnetic field for the onset of the corrugations can be realised by a small bar magnet.

The images below show this effect.

Ferrofluids have friction-reducing capabilities. If applied to the surface of a strong enough magnet, such as one made of neodymium, it can cause the magnet to glide across smooth surfaces with minimal resistance, as shown in the video below.

Magnetic Levitation of Pyrolytic Graphite

Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets as a result of imperfections in its production.
Pyrolytic carbon is man-made and is not thought to be found in nature. Generally it is produced by heating a hydrocarbon nearly to its decomposition temperature, and permitting the graphite to crystallize (pyrolysis). One method is to heat synthetic fibers in a vacuum.

Few materials can be made to magnetically levitate stably above the magnetic field from a permanent magnet. Although magnetic repulsion is obviously and easily achieved between any two magnets, the shape of the field causes the upper magnet to push off sideways, rather than remaining supported, rendering stable levitation impossible for magnetic objects. Strongly diamagnetic materials, however, can levitate above powerful magnets.

With the easy availability of rare earth permanent magnets in recent years, the strong diamagnetism of pyrolytic carbon makes it a convenient demonstration material for this effect, as shown in the image and video below.

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