'Impossible' material would stretch when compressed

Imagine cushions that lift up instead of sinking when you sit on them. Impossible? Not according to a blueprint for new materials with "negative compressibility": the materials compress when they are pulled and expand when they are pushed.

Metamaterials that do this have been built before. For example, vibrating aluminium bars with tiny cavities inside them create waves that oppose the push or pull applied (Nature Materials, DOI: 10.1038/nmat1644). But the designs must be vibrated at just the right frequency to see the effect.

Zachary Nicolaou and Adilson Motter of Northwestern University in Evanston, Illinois, have now designed a metamaterial that stretches when compressed, and vice versa, under any circumstances.

"What is interesting is that they study systems that are not responding to a vibration but to a steady applied force," says John Pendry of Imperial College London.

That should be impossible, as any material that behaves this way (stretching when compressed, and vice versa) would be inherently unstable and instantly collapse into a stable state without displaying such behaviour. Nicolaou and Motter got around this by designing a material with an internal structure that does transition to a stable state, but a state that is more compressed or expanded than the original state.

Their theoretical design involves a row of four "particles" ? each made of groups of molecules ? that attract each other to varying degrees. The force attracting the two inner particles is weak, so that pulling on the material breaks that bond. "As soon as that happens, the outer particles attract each other more," says Motter, so overall the material compresses. If this material is squeezed, though, the two inner particles are brought close enough together to reform the weak bond ? and the material can expand.

Miniaturised versions that work on similar principles could one day be used as protective coatings for military vehicles, says Christopher Smith at the University of Exeter, UK. "If a blast hit the side of your vehicle, it would push back and try to cancel out some of the effect," he says.

Motter says much of the previous work on metamaterials has focused on creating novel electromagnetic properties, such as bending light to create invisibility cloaks. The new study is part of a budding branch of research into "mechanical" metamaterials with unusual responses to stresses and strains.

"We've gone almost as far as we can with high-strength materials," Smith says. "The next phase has to be materials that do completely different things."

Journal reference: Nature Materials, DOI: 10.1038/nmat3331

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