Some solid materials have a memory of how they have been stretched before, which impacts how they react to these types of deformations in the future. A new study from Penn State provides insight into memory formation in foams and emulsions common in food and pharmaceutical products and provides a new method for erasing that memory, which could guide the preparation of materials for future use.
“A crease in a piece of paper serves as a memory of having been folded or crumpled,” said Nathan Keim, a research associate professor of physics at Penn State who led the study. “Many other materials form memories when they are deformed, heated or cooled, and you may not know it if you don’t ask the right questions. Improving our understanding of how to write, to read and erase memories provides new possibilities for diagnosing and programming materials. We can discover the history of a material by doing tests or erase the memory of a material and program a new one to prepare it for consumer or industrial use.
The researchers studied memory in a type of material called disordered solids, which contains particles that are often arranged erratically. For example, ice cream is a messy solid made up of a randomly mixed combination of ice crystals, fat droplets, and air pockets. This contrasts sharply with materials with “crystalline structures”, with particles arranged in highly ordered rows and columns. Disordered solids are common in food science, consumer products, and pharmaceuticals and include foams like ice cream and emulsions like mayonnaise.
“The preparation of materials often includes manipulating them in such a way as to change the arrangement of their molecules, bubbles or drops, by changing them from a higher energy state to a lower energy and more stable state”, Keim said. “For some materials like glass, this involves carefully heating the material so that its molecules separate and can come together in a more organized way. But for some materials, like mayonnaise, heating has destructive or unappetizing side effects. So for materials where heating is not an option, we use a process called mechanical annealing to physically deform the material and bring it to a lower energy state.
Keim and his colleagues previously investigated how mechanical annealing of disordered solids can allow a material to form a memory of that deformation, which impacts how it responds to future deformation. In a new paper published Oct. 5 in the journal Science Advances, researchers provide a finer understanding of how memories are formed in disordered solids and how existing memories can be “read” and even erased.
“We deform our material by shearing, which involves moving one side of the material relative to the other, like pulling the corner of a rectangle sideways so that it becomes a parallelogram,” Keim said. “By repeating this deformation at the same magnitude multiple times, you can essentially inscribe a memory of the deformation, which subtly affects how it responds to deformation of other magnitudes in the future. We have specified the circumstances under which this memory is formed in disordered solids and showed how to determine the extent of a previous deformation which had registered there.
The researchers also demonstrate a new method for erasing memories in disordered solids.
“Some of the memory rules in these materials are very similar to memory rules in ferromagnets, something that physicists have been studying intensely for over a hundred years,” Keim said. “A fridge magnet carries a magnetization which is a sort of memory of the magnetic fields that were applied at the factory. To erase these memories, you can apply a strong magnetic field and alternate its direction while gradually weakening the field. With our new method, which we call a ring method, we apply smaller and smaller strain amplitudes until the memory has been removed.
Erasing a memory could offer materials scientists the opportunity to start from scratch and then prepare a material in the most advantageous way.
For this study, the researchers simulated a disordered solid using 25,000 tiny plastic particles located at the interface of water and oil in a dish – a configuration developed by co-author Dani Medina. , an undergraduate student at California Polytechnic State University, San Luis Obispo, at the time of research. The particles are electrostatically charged, repel each other, and can be deformed with a needle moving along the interface in a controlled manner. The team used a microscope to track the arrangement of particles in the material.
“Disordered solids are more alike than different, and the microscopic details of their structure – whether they are oil drops or foam bubbles, grains or particles – do not seem to have much impact on the overall behavior,” Keim said. “This allows our experiments to provide insight into mechanical annealing and memory formation in many other materials. In the future, we would like to verify these material memory properties in three-dimensional disordered solids – the equivalent of mayonnaise or ice cream.
This work was supported by the National Science Foundation.
- Nathan C. Keim, Dani Medina. Mechanical annealing and memories in a disordered solid. Scientific Advances, 2022; 8 (40) DOI: 10.1126/sciadv.abo1614
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