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Paradoxical Materials Can Expand When Compressed

Paradoxical materials could grow when compressed



Thomas Fuchs

Call it the reverse psychology of stuff. Imagine a cushion that swells up instead of compressing when you sit on it. Or a rubber band that shrinks instead of elongating when you stretch it. If two physicists at Northwestern University are right, scientists may soon be able to make materials with such mind-boggling behavior.

The two researchers, Adilson Motter and Zachary Nicolaou, describe their proposal in work that appeared online in May in Nature Materials. (Scientific American is part of Nature Publishing Group.) They show how the unusual response, called negative compressibility, could theoretically emerge from putting together the right building blocks into a “metamaterial”—a material whose behavior is dictated not by its chemical or molecular composition but by its patterning at larger scales.

The molecules of such a material would act like springs in a jack-in-the-box: when slightly compressed, they transition into an expanded state. And just as it takes effort to put jack-in-the-box springs back into the box, the materials would require energy to be restored to their original state. A negatively compressible material could be built by stacking up many such springlike molecules (or something equivalent to them) like Lego bricks. “Everything that's needed to build this material exists,” Motter says, although no one has done the actual engineering to build it.

And what would the material be good for? The most promising applications might be in sensors and actuators, where the materials could amplify a force by expanding or contracting, or in safety gear such as seat belts, Motter notes. For now, though, he says, the idea is just a curiosity.

This article was originally published with the title "Fabrics That Push Back."

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