Imagine a free-flowing liquid that "freezes" the instant a strong magnet is brought near.

Such strange fluids actually exist. Called "magnetorheological" (MR) suspensions, they are used in some commercial products, such as StairMaster exercise equipment and special shock absorbers for truck seats. Their unusual properties also have attracted the interest of earthquake engineers who think MR fluids might make effective seismic shock absorbers for large buildings. In the future, they also may play an important role in the "chemistry laboratory on a chip" systems currently under development.

But before such microfluidic applications can be designed, researchers need more information about how MR fluids behave at the microscopic level. Obtaining such information was the object of a study performed by chemical engineers at Stanford that was published in the May 17 issue of Physical Review Letters. They report that these materials are considerably more complicated than the current models assume and can behave in unexpected ways.

Commercial MR fluids typically consist of finely ground carbonyl - a compound made by linking metal atoms and carbon monoxide molecules - suspended in a non-magnetic liquid such as mineral oil. Ordinarily, carbonyl is non-magnetic, but it becomes magnetic when exposed to a magnetic field, a characteristic that scientists call paramagnetic.

"In the absence of a magnetic field, these particles are randomly dispersed throughout the liquid," says chemical engineering graduate student Eric M. Furst, who co-authored the paper with Professor Alice Gast. "As a result, the suspension still flows relatively easily. When they are exposed to a magnetic field, however, the particles turn into tiny magnets and rapidly form into long chains. A high magnetic field turns the suspension into a solid."

Furst has a gadget - provided by the Lord Corp. of Cary, N.C. - that demonstrates this effect in his laboratory. It consists of two plastic p
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Contact: David F. Salisbury
david.salisbury@stanford.edu
650-725-1944
Stanford University
26-May-1999