Ferrofluid

Figure 1: Example of ferrofluid interacting with a strong magnetic field. Image from, Popular Science.

Demonstrate the mystifying properties of magnetic fields with ferrofluid.

Materials:

  1. Bottle of ferrofluid or vegetable oil and iron filings
  2. Shallow dish
  3. Assortment of magnets

Demo 1:

  1. Pour pre-made ferrofluid into shallow dish.
  2. Put a magnet underneath the dish, and see how the ferrofluid behaves.

Demo 2 (Making your own ferrofluid):

  1. Pour some vegetable oil into the shallow dish so as to make a thin film across the bottom.
  2. Add iron filings until you get a thick, sludge-like material.
  3. Put a magnet on the outside of the container to solidify a portion of the fluid, and use a paper towel to dab up any excess oil.
  4. Continue to adjust the mixture until it behaves as you’d like.

Recipe and instructions from, National High Magnetic Field Laboratory.

Explanation:

Ferrofluids are made of many nano-sized iron particles (approximately 10 nanometers across) suspended in an organic carrier fluid. The iron particles are paramagnetic, so they are free to be manipulated by an applied magnetic field. Our pre-made ferrofluid includes a surfactant, which prevents conglomeration of iron particles from van der Waals forces and magnetic forces.

Figure 2: Ferrofluids can form spikes that align with the magnetic field lines of a permanent magnet. Image from, BackerClub.

1. What is paramagnetism?

Although ferrofluid has ferro in the name, it is important to note that it is not ferromagnetic, as the iron particles do not retain their magnetism when the external magnetic field is removed (the ferro portion of the name comes form the Latin ferrum, meaning iron). This behavior classifies the iron as paramagnetic. Paramagnetism is one of five major ways for materials to produce magnetic fields, with the other four being ferromagnetism, diamagnetism, ferrimagnetism, and anti-ferromagnetism. For more on magnetism, see the Curie Point demo.

2. Why does ferrofluid form spikes instead of a raised dome?

Spikes form as a result of normal-field instability (also known as Rosensweig instability), a phenomenon dependent on the interplay of a uniformly applied and vertically oriented magnetic field, gravity, and surface tension. As an external magnetic field grows in strength, it reaches a critical value where it overpowers the forces of surface tension and gravity. As this imbalance grows, hexagonally arranged spikes form along the magnetic field lines.

Figure 4: When seen from above, the hexagonal arrangement of normal-field instability spikes are clearly shown. Image from, Non Linear Physics Group – Eric Falcon.

Notes:

  • Demo 2 does not include a surfactant to keep the iron filings from sticking together, so results are less visually spectacular (one may find it more emotionally satisfying when considering the joy of being the ferrofluids creator, though).

Written by Aleksander Beck