Experimenting with magnetorheological fluids
A magnetorheological fluid is a liquid that hardens near a magnet, and becomes liquid again when you remove the magnet. They are simple to make in your kitchen after a trip to a sandbox.
In the introduction to this section I described how to mine iron ore out of the sand from playgrounds or the beach. You may want to spend a while at the beach, because we will need a good handful of ore.
The ore that sticks to the magnets in the plastic bags has quite a bit of sand entrained in it. We can remove the sand by some additional refining. Make sure the ore is dry. Spread the ore out on a paper plate, and hold the bag with the magnet over the plate until a small amount of ore jumps up to the bag. Put this ore onto another plate, and continue until no more ore rises to the magnet in the bag. Don't get the bag too close to the plate, since there are many sand grains with ore stuck to them. We wish to keep only the ore that does not stick to grains of sand. The ore in the second plate should be visibly darker than what is left in the first plate. If you can see a lot of sand in the second plate, repeat the process, using a third plate.
Put the ore into a small cup. Soft plastic cups work nicely. The cup should be small enough that the ore fills it at least a third of the way up. Add some vegetable oil to the ore, and stir with a plastic spoon, or another nonferrous object, such as a popsickle stick. Keep adding oil until you get a thin black paste. Now gently place a strong magnet on the side of the cup. It should stick to the side as it attracts the ore. The ore should become quite stiff. Tip the cup over another cup to let excess oil and ore pour off. What remains in the first cup is our magnetorheological fluid.
We are now ready for the fun part. Hold the cup upright, and remove the magnet. Stir the liquid with the plastic spoon. It may be a little stiff at first, but will soon stir easily. Tip the cup a bit to the side, and bury the bowl of the spoon in the liquid. Now place the magnet on the side of the cup to stiffen the goop. The spoon will now stand upright when the cup is righted. The cup can even be inverted without losing any fluid, although a little oil may still drip out the first few times. Set the cup upright again, remove the magnet, and the solid mass slumps back into the cup, and the spoon falls over.
Put some fluid into a plastic bag, and let a magnet stick to the side of the bag. Now you can form the fluid into shapes by pressing the bag. The fluid will act like clay, and hold its shape. When you remove the magnet, the shapes slump into puddles.
Why does it do that?
Iron ore in oil reacts pretty much the way it reacts without the oil. Only one thing is really different. The oil allows the powder to slump more easily than it can when dry. This is because of its extra weight, its lubricating ability, its viscosity, and the fact that the ore is more buoyant in oil than in air.
Saying that it behaves like dry ore doesn't really answer the question unless we know why dry ore acts the way it does. If we look very closely at the ore with a magnifying glass or a microscope, we will notice that the pieces are a little longer than they are wide. They look like small footballs. Shapes like this do interesting things in a magnetic field.
Take a small iron nail in one hand, and a magnet in the other. Move the nail around the magnet, holding the nail loosely so it can move under the influence of the magnet. The nail will align itself with a bar magnet if you hold the two of them parallel to each other. As the nail moves toward one pole of the magnet, it will rotate so the point of the nail points toward the pole. Eventually, when the nail is above the pole, it will point straight at the pole.
There are two ways to think about what is happening. Pretend the attraction of the magnet is the attraction of the Earth, and the nail is a domino standing up on its end. A slight push makes the domino lie flat, and it takes a larger push to make it stand up straight again. We say that the domino has more potential energy when it is standing up than it has when it is lying down. There is a tendency for dominos to lose this energy by lying down. They have much less of a tendency to spontaneously gain energy and stand up by themselves. The nail tends to "fall down" so that it aligns with the magnetic lines of force that surround the magnet.
To illustrate the second way of thinking about the nail and the magnet we need a second nail. Hold the first nail parallel to the magnet, about half an inch to the right. Bring the second nail parallel to the first nail, a little to its right. We might expect the magnet to attract the second nail, just like the first nail. Instead we find that the two nails are repelling one another. If we lower the second nail so its top is near the bottom of the first nail, it now attracts the first nail.
The nails seem to have become magnets themselves while in the presence of the bar magnet. Their poles repel when they are parallel, and attract when they align vertically. If the bar magnet has its north pole facing away from you, the nails will have their south poles facing away from you. The nails attract the magnet because unlike poles attract each other. The nails repel each other because like poles repel each other. It is now easy to see why the nail follows the magnetic lines of force as it moves around the magnet. Its north pole points toward the magnet's south pole. Its south pole points to the magnet's north pole. When the nail is beside the magnet, it is parallel because the attractions are equal. When it is closer to the magnet's north pole, the nail's south pole attracts and the north pole repels, and it rotates.
Magnets repel one another when they are side by side. They attract one another when they are end to end. The natural state of a collection of magnets will thus be a string of them stuck end to end. If there are two strings next to each another, they will stagger so the poles of one string will be next to the centers of the other. Like poles are thus as far apart as possible. The strings will still repel from one another slightly. This is exactly the behavior of grains of iron ore sprinkled on paper above a magnet.
When a magnet is placed on the side of a jar of iron ore powder, the powder arranges itself into strings. Each grain of powder becomes a magnet and attracts the opposite pole if its neighbor. The strings thus formed repel each another, and the powder expands. If the powder has been mixed with oil, the oil wicks into the spaces created by the expansion, and sticks there by surface tension. The result is a dry appearing solid that does not leak oil.
