What does shampoo have in common with a crumpled piece of paper?
In this paper, we show that the mechanical response of a crumpled aluminum foil or
plastic sheet is engaged in a pas de deux with its internal structure in three movements. At the beginning, the ball radius obeys a simple power-law behavior with the external force. Just as we squeeze with open arms a bunch of pens that are thrown randomly on the desk, the layers inside the crumpled ball will gradually align among themselves, which triggers the breakdown of power law. After all layers have participated in the domain formation at high enough pressure, a new scaling relation between density and pressure emerges. This intimate interrelation between the mechanical response and structural reorganization is no stranger in soft-matter systems. One example is the shear thickening in colloidal dispersions, such as shampoos, paints and cements. The advantage of our table-top experiment is that one does not need special equipments or expertise or get dirty to appreciate this interesting property of nature. Furthermore, the simplicity of crumpling made it possible for us to build a mean-field model that can consistently explain all prominent properties related to this phenomenon.
