Chocolate syrup-like fluid stores multiple interacting memories

UNIVERSITY PARK, Pa. — Animals and electronic devices aren’t the only things with memory. Materials can retain memories of past deformations in their microscopic structure. A common example is a crease in a sheet of paper that has been folded then unfolded. Understanding this type of memory could benefit the design of materials that respond to changes in their environment in predictable ways. It can also be a source of ideas about the various types of memory studied by neuroscientists, including how short-term and long-term memories interact and influence each other. Now, researchers at Penn State have shown that two different types of material memory can coexist in a simple mixture of small particles suspended in a viscous liquid. Like long- and short-term memories, these material memories interact and compete.

A paper describing the research was recently published and highlighted as an editors’ suggestion in the journal Physical Review Letters.

“When you save a file on your computer, that new memory does not influence any of the other files that are already there,” said Surendra Padamata, a graduate student in physics in the Penn State Eberly College of Science and first author of the paper. “But in neuroscience, we know that, for example, a long-term memory might change over time, influenced by new short-term memories gained in the intervening years. For example, a novel read in adolescence may seem to be a simple story. Recalling it later in life, after personal experiences that echo its themes, can reveal layers of meaning that went unnoticed the first time. We were inspired by thinking about how memories interact in this way to see if we could find an analogous situation in a material.”

The research team studies memory in non-Brownian suspensions. Like chocolate syrup or fresh concrete, non-Brownian suspensions are composed of relatively large particles in a viscous liquid. The particles are large enough that their movement is not influenced by Brownian motion — the random movement of thermally energetic atoms — so any movement of the particles would be due to their experimental design. They first showed that their suspension could remember the direction it was stirred, and if it was rocked back and forth, the suspension of particles remembered how vigorously it was rocked — the amplitude.

“Each of these memories had been studied on its own,” Padamata said. “So, in our new experiments, we first stirred the mixture, imprinting a memory of direction, then rocked it back and forth at varying intensities to see how the memories interact.”

The team found that at lower rocking intensities, the mixture retained memory of the direction of stirring in addition to the memory of amplitude — the two memories can, in fact, occupy the same material at the same time. But as the rocking intensified, the memory of direction weakened and was eventually erased.

“At a certain threshold of intensity, the rocking completely erased any memory of direction in the suspension, returning it to a perfectly symmetric state, but beyond that threshold, the rocking itself begins to write a new directional memory” Padamata said. “We are interested in how this model can inform biological memory and, potentially, geophysical processes as well. Changes in temperature and vibrations might impart memories in rock that influence risk for earthquakes and sink holes, for example. It could be possible to find some way to erase these memories and reduce the risk or make better predictive models.”

The researchers suggested that the competition between the memories could happen when encounters between particles become too numerous — whereas for smaller rocking motions, these encounters can be rare. However, that detail might be specific to particles suspended in liquid and it depends on the ratio of particles to liquid. Similar memory phenomena have been observed in solid materials, where neighboring particles are always in contact.

“A similar combination of directional memory and amplitude memory appears in soft glasses and granular packings with very different microscopic physics,” said Nathan Keim, associate professor of physics at Penn State and the leader of the research team. “This suggests that there may be a general principle for how disordered matter behaves under simple conditions like stirring or rocking and why they have a limited memory capacity.”

The Human Frontier Science Program funded the research.