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26 September 2003

Making Fluids into Solids with Magnets

by Daniel J. Klingenberg
Excerpted from "Scientific American's The Amateur Scientist", first published October, 1993.

Anyone can turn a liquid into a solid using an ice tray and a freezer. But can you solidify a fluid and then liquefy it within a few seconds? Actually, all you need is a strong magnetic field and a simple recipe for a magnetorheological fluid.

Figure 1: IRON FILINGS in corn oil produce a simple magnetorheological fluid. The mixture remains liquid (a) until a magnet is brought close to it. The filings line up between the poles, forming stiff stringy masses that can be pushed to one side of the container (b). The material reverts back to its original liquid state once the magnetic field is removed. Click image to enlarge.

Magnetorheological fluids are not as common as a similar class of materials that solidify in the presence of electric fields. These electrorheological fluids have been incorporated in prototype brake systems, clutches, shock absorbers, engine mounts, actuators, control valves and even artificial joints.

Yet amateur scientists would be wise to experiment with magnetorheological fluids before they attempt anything with the electrical counterparts. Most electrorheological fluids require high voltages. For example, several thousand volts would be needed to solidify a mixture of corn oil and cornstarch. (The actual amount of electric current that would flow through the liquid would be quite low, however.)

A simple magnetorheological fluid is made of iron filings and corn oil. You can find iron filings at a toy store, at your workbench or at a machine shop Specifically, you could empty the filings out of a toy magnetic sketch pad; you could scrape a piece of iron with a good metal file; or you could gather the iron filings produced by a metal lathe.

The size of the filings will determine the magnitude of the magnetorheological effect observed. The filings should be large enough to identify single particles with a magnifying glass but preferably less than half a millimeter long.

The type of oil needed to suspend the filings does not matter much. It just has to be more viscous than water, to slow down the rate at which the filings settle. I have used corn oil, linseed oil, silicone oil and motor oil. Corn oil is perhaps best: it is cheap and cleans up relatively easily with soap and water.

As for the magnetic field, almost any type of strong, permanent magnet will work: horseshoes, bars, even the ones from old loudspeakers. (Refrigerator magnets are typically far too weak.) Magnets are often rated by the number of pounds they can pull. You should buy the most powerful ones available. I purchased two "100 pound" bar magnets from a local hardware store for about $12 apiece.