Memory, adaptation, and aging in crumpled sheets and networks of instabilities

A thin sheet that has been crumpled many times exhibits many of the hallmark behaviors of driven and nonequilibrium disordered systems: intermittent global responses, emission of correlated crackling noise, slow relaxations and aging, and a range of mechanical memory effects. Here, through experiments in thin crumpled sheets and simulations of a minimal mechanical model, we reveal a microscopic, real-space, structural mechanism underlying and linking these behaviors.

Using experiments that combine global mechanical measurements, local probing, acoustic measurements, and 3D imaging of crumpled sheets, we build a mesoscopic description of their mechanics. The global measurements reveal a range of memory effects, including hysteresis, memory of largest strain, and return point memory, as well as clear signatures of underlying intermittent dynamics. Intermittent dynamics are also observed during slow, logarithmic aging of crumpled sheets under load. In this case, however, the intermittent events are grouped into highly correlated, scale-free avalanches. The complimentary local measurements reveal that intermittency, memory, and aging behaviors emerge from the collective dynamics of mesoscopic, bistable elements within the sheet: localized geometric instabilities that act as coupled, hysteretic, two-state degrees of freedom.

Based on this picture, we develop a numerical model of a disordered network of bistable elastic elements that corroborates all our findings: hysteresis, intermittencies, memory formation, return point memory, slow relaxations, aging, and avalanches. The model highlights the role of interactions and frustration between instabilities in driving these behaviors. The emerging picture is of a disordered system with a complex energy landscape, reminiscent of a mechanical spin-glass, that self-organizes to a state which lies on the verge of instability.

References:

Shohat, D., Hexner, D. & Lahini, Y. Memory from coupled instabilities in unfolded crumpled sheets. Proc National Acad Sci 119, e2200028119 (2022).

Shohat, D. & Lahini, Y. Dissipation Indicates Memory Formation in Driven Disordered Systems. Phys Rev Lett 130, 048202 (2023).

Lahini, Y., Rubinstein, S. & Amir, A. Crackling noise during slow relaxations in crumpled sheets (in review)

Shohat. D., Friedman, Y. & Lahini, Y. Aging on the edge of stability in disordered mechanical systems (in review)