Rocking matter to make it forget its past

Many materials we use in our daily lives, such as hygiene products and cosmetics, or in modern technologies, such as additive manufacturing and ceramic extrusion, are paste-like materials that behave like solids at rest and like liquids when subjected to mechanical stress. As they solidify from a liquid state, they set into a non-equilibrium state that retains the directional memory of the stresses and strains they underwent during flow. Their properties depend not only on thermodynamic parameters such as temperature and pressure but also on their history. This mechanical memory can have adverse consequences in forming processes and for the durability of the materials.

Researchers [1] from ESPCI Paris–PSL, in collaboration with colleagues from the University of Texas at Austin, have studied the physical origin of these memory effects by combining experiments with molecular dynamics simulations. The materials are highly concentrated suspensions of deformable particles, similar to emulsions, and physical gels. They demonstrated that the materials “store” their mechanical memory through a network of internal stresses that are trapped by the particles during flow and relax only very slowly.

This better understanding of stress storage has allowed researchers to explore methods for erasing the memory and restoring the internal structure of materials to equilibrium. To do this, they devised a protocol that involves “rocking” the materials by applying periodic oscillations with an amplitude close to the yield threshold. This protocol alters the statistical distribution of local internal stresses, frees the material from residual stresses, and renders it “amnesic.”

This breakthrough will lead to advances in the control and optimization of soft materials. By erasing their mechanical memory and controlling their aging, it becomes possible to characterize their intrinsic properties and improve their performance in applications. Initial tests show that the same method can be extended to other classes of disordered materials, thus providing a new tool for controlling and designing complex materials.