When a spring is compressed, it returns to its original shape. This property of materials is called positive stiffness. On the other hand a material with 'negative stiffness' does not push back on compression. Such materials are very unstable. A small compression and they fly apart or collapse into a material with positive stiffness. But now, R S Lakes, researcher at the University of Wisconsin, US, has shown that the positive stiffness of a material can be increased by adding small bubbles of negative stiffness into it. This technique could be used to make more rigid airplane wings as well as improve medical implants ( Physical Review Letters , Vol 86, p2897).
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