Material memories can be erased

Particle locations
Particle locations. (Courtesy: Keim research group / Penn State)

Disordered materials can “remember” deformations they have previously experienced – and they can be made to forget them, too. This is the finding of researchers at Penn State University and Cal Poly San Luis Obispo in the US, whose experiments on erasing material memories could improve the design of foams and emulsions employed in the food and pharmaceutical industries.

Disordered solids are commonplace in food science. Ice cream, for example, is made up of ice crystals, fat droplets and air pockets combined in an erratic way. Emulsions such as mayonnaise also contain particles arranged in a random fashion, and many cosmetics and pharmaceutical products share similar characteristics.

Inscribing a memory of the deformation

In the latest work, researchers led by physicist Nathan Keim studied a two-dimensional disordered material made by pouring oil on top of water in a dish, then spreading a closely-packed layer of 25 000 microscopic plastic particles at the boundary between the liquids. The particles are electrostatically charged and thus repel each other, which allows them to form a soft mayonnaise-like solid. This soft solid can be deformed in a controlled fashion, and the motion of the particles is tracked using a microscope.

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“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 to the side so it becomes a parallelogram,” Keim explains. This type of deformation is known as mechanical annealing, and performing it lowers the overall energy of the structure. By repeating this annealing at the same magnitude many times, Keim says, “you can essentially inscribe a memory of the deformation” that subtly affects how the material responds to deformation of other magnitudes in the future.

After the researchers prepared their material, they performed experiments designed to show that the annealing had indeed formed a memory. “Without knowing its past, we can probe a sample to reveal the strain amplitude γa that was used to anneal it,” they explain. To do this, they applied a series of cycles with increasing amplitude γread, starting with a small value and ending at a value higher than γa.

At the end of each readout cycle, the researchers compared the positions of the particles with those at the end of annealing. For small γread, the average change in the particles’ positions – the mean squared displacement – grows, but it drops near γread = γa when the annealed state of the system is recovered. This observation and others show that the material approximates a generic behaviour known as “return point memory”, which appears to be a property of annealed samples.

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“Ring down” erasing

The researchers also found a new way to erase this memory. To do this, they used a method called “ring down” that involves applying distortions of smaller and smaller magnitudes until the memory has been removed. This is somewhat similar to the method for removing memories in ferromagnets, where a strong magnetic field is applied and its direction alternated while gradually making the field weaker, Keim says.

Keim hopes that some of the advances made in this work and other recent research will find their way into applications. “When a material has been deformed cyclically, it is possible to recover one or more of the past strains it has been subjected to,” he tells Physics World. “There may be a role for this kind of test alongside established techniques like failure analysis. There may also be a use for mechanically erasing the effects of past loading or for estimating a sample’s capacity to form memories.”

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Erasing a memory could provide materials scientists with a way to essentially start from a clean state and then prepare a material in the most advantageous way, he adds.

The researchers, who detail their work in Science Advances, say their technique could be used to probe mechanical annealing and memory formation in a wide variety of disordered solids and other forms of glassy matter. “In the future, we’d like to verify these properties of material memory in three-dimensional disordered solids – the equivalent of mayonnaise or ice cream,” says Keim.

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