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.
Scientists have now designed a metamaterial that stretches when compressed, and vice versa, under any circumstances.
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. The scientists 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. Learn more here.