What is Ferrofluid?

Ferrofluid is a liquid which responds to magnetic fields, if placed near a magnet the liquid will be attracted to it and leap up, forming itself into spikes. If you touch the spikes you will find that they have not skinned over or solidified, the spikes are still liquid.

To do this, the fluid piles up into the shape which minimises the amount of pole and puts it as far from itself as possible. If you do this experiment with an electromagnet you will find that as the magnetic field increases the spikes will sharpen and increase in numbers.To understand why the ferrofluid does this, think about the nature of magnets. For a blob of ferrofluid on a magnet, the whole surface is one pole - so it tries to repelitself.

Originally developed for NASA in the '60s as a way of sealing spacecraft parts in vacuum and controlling liquids in weightlessness, ferrofluids are now used for a variety of applications.

In top quality speakers, ferrofluids are used to lubricate and cool the coil, upgrading sound reproduction at high volume. Ferrofluids allow a speaker to carry more sound volume before heat build-up causes sound distortion, especially in large speakers like PA systems.

Ferrofluids are used to make rotary and feed-through seals in industry, particularly for applications where there will be a pressure gradient across the seal, e.g. in vacuum pulling of silicon crystals for the chip industry. A rotary seal is where a shaft passing from one environment to another must rotate, e.g. a submarine propeller. A feed through seal is where the shaft must pull back and forth, e.g. a shock absorber. If there is pressure or vacuum on one side of the seal, a conventional seal will fail as the pressure forces out the grease. If the seal is magnetised, the magnet can hold a ferrofluid in place. Properly constructed commercial ferrofluid seals are capable of holding pressures of over 1000psi long term without failing. Ferrofluids are finding use in hard disc drives, where they allow a faster operation.

Increasingly, ferrofluids are finding use as an educational or artistic material, especially in conjunction with strong neodymium magnets, or with electromagnets, which can make the fluids move in fantastic forms.

Q & A

Will the ferrofluid keep its magnetic properties even when dried out?
Yes, the ferrofluid will dry and the dried fluid retains its magnetic properties. I have a piece of filter paper, which I used to filter some of the fluid over a year ago and it still responds to a magnet. I would just mention that ferrofluid responds to magnets. It does not retain magnetism after the magnet is removed. This applies to the fluid and to the dried film.

Is there anything I can add to the ferrofluid that WILL NOT mix with it? Basically, I was hoping to put some
ferrofluid in a clear container along with water and the two won't mix. No, water will not mix with the ferrofluid. The ferrofluid is heavier than water so will be the bottom layer. If you need the ferrofluid to be the top layer you will need to thin it a little with kerosene or something similar to it.

What size are the particles in the ferrofluid?
The particles within the ferrofluid are approximately ten nanometres. A nanometre is approximately one billionth (10-9) of a meter, so is pretty small.

Whenever the ferrofluid touches the sides of a container it leaves a fine film behind. Is there any way I can
stop this from happening? This is a problem in working with ferrofluid. It does coat most things. If you are going to find something it will not stick to, I would suggest something very hydrophilic (having an affinity for water) to repel it. Wax or similar will only make it worse. We have always worked on the basis of setting up the experiment then leaving it to settle before starting. If your experiment is not amendable to this regime, the only other suggestion I can make is to use water based ferrofluid, which is much cleaner than the solvent based.

It would be interesting to give the ferrofluid a brighter more pleasing colour, Is there anything (such as a dye), which can be added to the ferrofluid to change the colour?
The problem is the fluid is so intensely coloured to begin with, so it would be difficult for a dye to show through. Unfortunately, a pigment would not work because as soon as you place the fluid near a magnet, the pigment would be rejected and the mixture of the two would separate. The best option would be a very intense fluorescent dye dissolved in the solvent phase of the fluid. Alternatively, you could try a pigment or fluorescent dye in the water based ferrofluid then maybe try illuminating the fluid with a UV light. Alternatively, for solvent based fluid, you would need oil soluble fluorescein.

Could you give me some ideas of what I can use the ferrofluid for and are there any interesting experiments that I can play with?
In industry, ferrofluid is used to make pressure seals and as a coolant in high quality speaker systems. One experiment you might like to try is to make a sink/float separator. Take two strong magnets and fix them to be about a centimetre apart, attracting each other. You will need to fix them strongly so they do not meet as there is a danger they could shatter. Put some ferrofluid into the space between the magnets and it will form a plug of fluid. Because the fluid is attracted to the magnets it will try to occupy all of the space and if you introduce anything non-magnetic into the space it will be repelled. If the substance is light, like aluminium, it will float on top, even though the metal is denser than the fluid and would normally sink. If you use a heavier metal like lead, it will overcome the repulsion and sink through. In this way you can make a simple sink/float separator.

Here's an interesting one to think about. Take a piece of glass tube and place this in the gab between the magnets. Introduce the fluid into the tube to form a plug. This plug will seal the tube and you can float a head of water on top of it. Thread a line through the plug. Attach a float to the line and pull it up through the seal. Once the float is in the water, it will float up and in theory it could pull up another float. Join them in a circle and you may find you have a perpetual motion machine.

Is ferrofluid stable under water indefinitely as I am conducting an experiment where I am attempting to keep the water in a constant motion (sort of like a lava lamp) without using a visible agitator. I am toying with the idea of using ferrofluid instead of a magnetic stirring bar, would this work?
Ferrofluid is stable long term under water although, you may notice a slight oil film. Ferrofluid will follow a moving magnet in the same way that a stirring bar does. It has a much weaker saturation magnetisation than steel, so the effect may not be as strong. If you want to use ferrofluid as the actual lava lamp medium, you would need to add some salt to the water phase to increase its density until the ferrofluid floats on top. Then you could use a pulsing electromagnet to draw the fluid down.

Could you tell me more about the magnetorheological properties of your ferrofluid (i.e. how much the viscosity increases in the presence of a saturating magnetic field). I am aware that normally fluids with bigger magnetic particles are used for this general application but for my specific task the small particle size is attractive.
Ferrofluid is strange stuff. One of the strange things about it is that it continues stiffening beyond its saturation magnetisation. The oil based fluid we supply has a saturation magnetisation of 160 gauss but continues stiffening up to about 1 tesla and for all I know, beyond. In the end it stiffens enough to support lead or gold. I know this is a function of pseudo density rather than viscosity but I hope it will be helpful. In the absence of a magnetic field the fluid is water thin.

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Revised: 19th September 2012