When squashed down, most materials expand outwards, but one class of 'metamaterials' gets narrower instead.
Katia Bertoldi at Harvard University in Cambridge, Massachusetts, and her colleagues crafted several structures using arrangements of spherical shells with regularly spaced holes. Whereas most structures are engineered to avoid mechanical instabilities, this metamaterial exploits them by buckling and folding to become narrower under compression before returning to its original shape after the pressure is removed. X-ray images and numerical simulations showed how the holes in the structures (one example pictured) distort under increasing strain.
These reversible architectures could be useful in applications such as energy-absorbing materials or acoustic dampeners, the authors suggest.
Adv. Mater. http://dx.doi.org/10.1002/adma.201301986 (2013)
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Odd performance under pressure. Nature 500, 8 (2013). https://doi.org/10.1038/500008c