Two scientists at Pennsylvania State University have shown how a range of everyday solid materials retain a memory of how they were previously stretched out, and this has an impact on how they respond to such deformations in the future. The researchers, led by Nathan Keim, demonstrated how to read, write and also erase the memory of previous deformation in the foams and emulsions used in many products, which could help guide how they are prepared for future use.
Creases in paper can act as a memory of being folded or crumpled, while many materials form memories when deformed, heated up or cooled down. Here it is key to ask the right questions, as improving our understanding of how to write, read and erase such memories could bring opportunities for diagnostics and programming of materials.
The preparation of materials can involve manipulating them, often altering the arrangement of their molecules, moving them from a higher energy state to a lower, more stable state. For some materials, heating has unwanted side effects, and so where heating is not an option, mechanical annealing is used to physically deform the material and bring it to a lower energy state.
As reported in Science Advances [Keim N. C., Medina D., Sci. Adv. (2022) DOI: 10.1126/sciadv.abo1614], here memory was assessed in disordered solids, which have particles that can often be erratically arranged. Disordered solids are regularly found in consumer products such as foams, including ice cream and emulsions like mayonnaise, and pharmaceuticals.
A disordered solid was simulated using 25,000 tiny plastic particles positioned at the interface of water and oil in a dish. The particles are electrostatically charged, repelling each other, and can be deformed with a needle moving along the interface in a controlled way. Repeating this deformation at the same magnitude many times essentially inscribes a memory of the deformation and can affect how it responds to deformation of other magnitudes in the future.
The issue of “avoiding” memories could therefore prove useful and lead to identifying some memory-less states among the huge number of ways to prepare a material. Most of the methods used here could also find applications where it is difficult to see individual atoms or particles.
The team will keep investigate other examples of memory behaviors, and are keen to understand how this picture breaks down. As Keim told Materials Today, “At very large strains these complex memory behaviors are destroyed, but at slightly larger strains they might actually be enriched, so that for instance the material could encode how many cycles of deformation were applied, not just their magnitude”.
“At very large strains these complex memory behaviors are destroyed, but at slightly larger strains they might actually be enriched, so that for instance the material could encode how many cycles of deformation were applied, not just their magnitude.”Nathan Keim
Structure of rearrangements in a single cycle