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The theory of elasticity is essential in engineering. The most basic law is that any piece becomes smaller if one compresses it. My question is:

How can a piece of material expand (some minutes, hours, or days) after being compressed?

It is against our intuition. A related topic is the development of (meta)material with negative Poisson's ratio, which is also counterintuitive.

I suppose such material is possible, for example, a linear spring (as harmonic oscillator) with a very long period (you press it today, it expands tomorrow). I am wondering whether there is subfield developing such unusual material.

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    $\begingroup$ Welcome to our site! $\endgroup$ – Camps Nov 20 at 15:21
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I think you need to know the potential energy surface (PES) of the system. Assuming the system is in equilibrium before applying the deformation, any modification will take it to a non-equilibrium state. Depending on in which points of the PES the system is/go and the amount of strain/stress accumulated during the compression, the tendency is to free the extra energy and came back to equilibrium.

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  • $\begingroup$ Thank you very much! Are there physical implementation of such materials (could be informed by external force, as I understand)? $\endgroup$ – whitegreen Nov 21 at 3:05
  • $\begingroup$ It's worth adding that the relaxation time to return to equilibrium (the original shape) can vary dramatically if the underlying dynamics are slow. $\endgroup$ – taciteloquence Nov 21 at 13:39

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