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 plungers, connected by a narrow tube. The space between the two plungers is filled with a grey-colored liquid. Initially, the plungers move easily.
When one is pushed in, the other moves out. But when he snaps a small magnet into a bracket that holds it next to the connecting tube, suddenly the plungers are frozen: They barely move even when pushed vigorously. The Stanford researchers are the first group to study the microscopic properties of these exotic materials using optical tweezers - an instrument that uses laser light to exert pressure and physically move microscopic objects floating in water.
Because the particles in MR fluids are difficult to manipulate in this fashion, Furst and Gast use polyvinyl microspheres, which are about one-fiftieth the width of a human hair. These microspheres are commercially available in plain and - coated with iron oxide particles - paramagnetic varieties. Unfortunately, the presence of the iron oxide disrupts the action of the optical tweezers. So the researchers attached "tethers" consisting of non-magnetic microspheres. Then, using two optical tweezers, they were able to grip the tether spheres and use them to begin pulling apart individual chains of magnetized spheres, all the while measuring the amount of force that was required.
That resulted in their first surprise. They found that it takes about four times the force to pull the chains apart than the simple models had predicted. The researchers think they know why. The models, which treat the particles as simple point dipoles, do not include the fact that each particle generates its own local field, which acts to stiffen particles nearby.
The researchers found another unexpected effect. They found that, as they were pulling chains apart, extra spheres frequently popped into the chain. Such additions lowered the tension in the chain momentarily, and caused the chains to fail more gradually than predicted.
Another effect not predicted in the models was lateral aggregation. Individual chains join together to form columns. Although the aggregation effect was well known, Furst and Gast were the first to measure the amount of strengthening that aggregation provides at the microscopic level.
Finally, the study found a greater-than-expected range of behavior as the strength of the magnetic field is varied. At very high field strengths, the chains form and cross-link to give the material a solid form. At low field strengths, the chains essentially disintegrate.
At intermediate strengths, however, the material becomes elastic. The researchers were surprised to find that, under certain conditions, the chains can undergo a reorganization into mechanically stronger configurations: an effect strikingly similar to work hardening in plastics. Such detailed knowledge of how MR fluids behave will help the researchers determine how they can be used in microfluidic applications. Considerable research is being conducted in both the public and private sector to extend the techniques used to make computer chips and other microelectronic circuitry to make microscopic systems that can mix, separate and identify a wide range of chemicals. Such devices could have widespread application in the environmental, manufacturing, health care and pharmaceutical industries.
Furst and Gast's research suggests that MR fluids can be used as valves in these "chemistry lab on a chip" systems that appear likely to replace many of the conventional beakers and retorts traditionally used in chemical research. Stanford has applied for a patent to cover such an application. The research was supported by the National Aeronautics and Space Administration.
Other relevant material: Gast Group web pagehttp://chemeng.stanford.edu/~gastgrp/
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