Ferro fluids are particles of iron suspended in a solution. Unfortunately they are often expensive or difficult to make. This is a relatively cheap and simple method which can make a substance that behaves just like a real ferro fluid.
Warning: This activity uses acetone. Only use acetone in an open, well ventilated area and never around any open flames. Wash your hands after use and read the warning label on the bottle. Take care using the magnet – neodymium magnets are extremely powerful and can pinch fingers if used without caution. Use old bowls and spoons and don't use them for cooking afterwards. An adult should assist with this activity.
You will need
5 old cassette tapes (audio or video – so long as it has the brown recording tape)
1 litre bottle of acetone
2 large bowls
Small plastic container
Spoon
Scissors
Baby oil or cooking oil
Balloon
Large rare earth magnet (also called a neodymium magnet – available at many hobby and electronics stores)
What to do
Remove all of the tape from the cassettes and place it in a large bowl. Take care not to cut or rip the tape into small pieces – keeping it as long ribbons will make it easier later.
Pour the acetone over the pile of tape until most of the tape is submerged. This will dissolve its coating and remove the layer of tiny iron filings. Use a spoon to mix the pile of tape until all of it has been washed.
Let it sit for half an hour.
Pour off the acetone into a second bowl. It should be a murky brown colour. Try to remove any of the larger pieces of tape you can see floating in the acetone using a spoon.
Push the magnet into a balloon and swirl it through the acetone. You should see the liquid turn clear as the brown filings are attracted to the magnet and form a brown sludge on the outside of the balloon. Pour the clear acetone back over the tape and repeat the process another six times.
By now your balloon should have a thick layer of red-brown iron particles covering it. Cut off the neck of the balloon completely and remove the magnet. Take care not to get any of the sludge on the magnet – once there, it is very difficult to remove! Put the magnet aside.
Turn the balloon inside out, so the sludge is now cupped on the inside. Add a small amount (five or six large drops) of oil. Remember, less is best – you can always add more later. Carefully massage the balloon to rub the oil and the iron particles together.
Tip the mixture out of the balloon into a small, clean, plastic container. Place the magnet underneath. How does your magnetic slime respond to it?
What's happening?
The sludge is made up of thousands of tiny iron oxide particles. This type of iron is 'ferromagnetic' – its atoms are affected by magnetic fields, causing them to line up in a special way along the field lines of a magnet. Iron can also take forms which are not ferromagnetic, such as the iron in your blood which will not respond to your magnet at all.
With a powerful magnet underneath, the iron particles align with the magnet's field and form short spikes, showing where the field lines are. In proper ferro fluid the particles are smaller and coated with a chemical which helps keep them separated. This makes the mixture appear smoother and more like a liquid.
Applications
Data tapes use this principle to record information such as music or video images. In a recording machine, a magnet rapidly switches on and off as the tape runs past, making the iron particles line up and form a pattern along the tape. This can be read later by a machine to retrieve and then display the information. Sweeping a magnet over the tapes makes the iron filings all line up in one direction, destroying the pattern and losing the information recorded on them.
