Ferrofluid
A magnet placed just below a dish containing ferrofluid generates an array of spikes in the fluid. The magnetic attraction is strong enough to hold a magnet in place and to prevent the "fluid" from escaping when the dish is tipped. When the magnet is removed the spikes disappear and the fluid may be poured.
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Keywords
ferromagnetism, practical applications, solid state, bonding, electronic structure, periodic table, real life/environment, solids and liquids, d block, transition elements, magnetic properties
Multimedia
Entire demonstration
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Part 1 of 3
_Play movie (11 seconds, 0.7 MB)
Brown ferrofluid is contained in a petri dish. When a strong magnet
is brought underneath, ferrofluid "spikes" are observed along
magnetic field lines.
Part 2 of 3
_Play movie (33 seconds, 2 MB)
Ferrofluid is composed of small polar magnetite particles surrounded
by a surfactant and suspended in a nonpolar medium. The spikes form a
close-packed arrangement along magnetic field lines. A strong magnet
will be held to the bottom of the dish by its attraction for the
ferrofluid. Ferrofluids were originally developed by NASA to confine
liquids in space. Commercial applications include ultracentrifuge
rotor seals.
Part 3 of 3
_Play movie (18 seconds, 1.1 MB)
A magnet can confine the fluid, but in the absence of a magnet, the
fluid pours.
Inverse micelles 
Discussion
Magnetite, Fe3O4 is a ferrimagnetic substance containing both Fe2+ and Fe3+ ions. As in ferromagnetic substances, the unpaired electrons in ferrimagnetic substances are aligned in large domains. However, in ferrimagnetic substances some of the spin sets are opposed to others (this depends on the position of the Fe2+ and Fe3+ ions in the crystal lattice). Since more of the spins are aligned in one direction than in another, the spins do not cancel. A magnet will attract a ferrimagnetic substance.
In the ferrofluid, tiny particles of magnetite are suspended throughout the liquid medium. In order to prevent the particles from aggregating, they are surrounded by the polar end of long chain fatty acid molecules, to which they are attracted by ion-dipole forces. (See the diagram inverse micelles.) The long, nonpolar tails of the molecules are attracted by London forces to the molecules of the oil that serves as the liquid medium, but cannot compete with the polar ends in their attraction for the Fe3O4 particles. These structures bear a certain resemblance to the micelles formed by soaps and detergents. However, in the case of soaps, the nonpolar tails are directed toward the interior, where they surround water-insoluble oils, while the polar ends (-COO- or -SO3- groups) are strongly attracted to water molecules by ion-dipole forces.
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Demonstration Notes: Warnings, Safety Information, etc.
Exam and Quiz Questions
1. Describe what happens when a magnet is brought near a ferrofluid.
2. What happens to the viscosity of the ferrofluid when a strong magnet is present?