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”: these materials compress when pulled and expand when pushed.
Metamaterials that do this have been built before, but the designs must be vibrated at just the right frequency to see the effect.
Zachary Nicolaou and of Northwestern University in Evanston, Illinois, have now designed a metamaterial that stretches when compressed, and vice versa, under any circumstances.
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That should be impossible, as any material that behaves this way would be inherently unstable and instantly collapse into a stable state without the behaviour. Nicolaou and Motter got around this problem by designing a material with an internal structure that transitions to a stable state, but one which is more compressed or expanded than the initial 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 (Nature Materials, ).
Because it might be a struggle to imagine the material working in practice, Nicolaou and Motter came up with a model to help envision what happens when the material is pulled (see diagram).
Miniaturised versions that work on similar principles could one day be used as protective coatings for military vehicles, says 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.
“We’ve gone almost as far as we can with high-strength materials,” Smith adds. “The next phase has to be materials that do completely different things.”