Directed aging: Training elastic responses in disordered systems*

Disordered materials exist in a rugged energy landscape and become trapped in metastable states that are not a global energy minimum. These far-from-equilibrium systems relax slowly as they search for lower-energy configurations. This aging often leads to material degradation and is thus thought of as undesirable. Here I show how this aging can be directed to produce a final state that has advantageous properties and unique functions not typically found naturally [1]. Aging a system subject to external constraints leaves a memory of how it was aged and creates stresses that direct its evolution [2]. Aging obeys a natural “greedy algorithm”: the material simply follows the path of most rapid and accessible relaxation. During aging, the material modifies stressed regions differently from those under less stress. Our goal is to find out what range of behaviors can be achieved by directed-aging protocols. We do this through experiments and with simulations that use simple models of the aging process in disordered networks. Our experimental networks are laser cut from EVA foam and then aged under external stress. (We can apply heat not only to accelerate the aging process but also to help the material retain a memory of how it was aged after the system has been re-cooled.) Using different applied stresses, we can achieve different functions; a memory of the stress protocol can be read out by measuring the mechanical response of the aged systems. Directed aging provides a pathway for modifying and tuning a material’s elastic properties in the non-linear as well as the linear regime without having to control the material at the microscopic level.

[1] N. Pashine, D. Hexner, A.J. Liu, S. R. Nagel “Directed aging, memory and Nature's greed,” ArXiv:1903.05776.
[2] D. Hexner, N. Pashine, A.J. Liu, S. R. Nagel “Effect of aging on the non-linear elasticity and memory formation in materials” ArXiv:1909.00481.