Summary: A new study demonstrates that non-Brownian suspensions can form, retain, and overwrite multiple microscopic memories simultaneously, offering physical parallels to cognitive memory dynamics in neuroscience.
Researchers investigated suspensions of large particles suspended in viscous fluid. The team discovered that directional memory (imprinted by stirring) and amplitude memory (imprinted by rocking) can coexist within the same material structure.
However, increasing the intensity of rocking creates particle encounters that systematically erase previous directional memory, mirroring the way short-term and long-term biological memories interact, compete, and reshape one another.
Key Facts
- Non-Brownian Suspension Mechanics: The experimental fluid contains large suspended particles untouched by thermal Brownian motion, ensuring that microscopic structural rearrangements occur solely due to applied mechanical shear (stirring or rocking).
- Coexistence of Dual Memories: The research team confirmed that a single material sample can simultaneously hold a directional memory (from initial rotational stirring) and an amplitude memory (from periodic rocking).
- Intensity-Dependent Memory Erasure: Under low rocking amplitudes, directional memory remains intact. As rocking intensity crosses a specific threshold, frequent particle collisions disrupt the microscopic structure, erasing directional memory and restoring structural symmetry.
- Writing New Structural Directionality: Beyond the erasure threshold, high-intensity rocking overwrites past configurations by writing a new directional memory aligned with the new mechanical force.
- Cross-Disciplinary Applications: The findings offer physical frameworks for understanding cognitive memory consolidation in neuroscience, as well as geomechanical stress histories in rock packings that influence earthquake dynamics and sinkhole formation.
Source: Penn State
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 suggest 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.”
Funding: The Human Frontier Science Program funded the research.
Key Questions Answered:
A: A non-Brownian suspension is a mixture where particles in a viscous liquid are too large to be moved by thermal energy (Brownian motion). Because thermal noise does not disrupt the system, any change in particle arrangement is caused entirely by mechanical forces, making it an ideal model for studying structural memory.
A: When the suspension is first stirred, it stores a directional memory in its microscopic particle arrangement. When subsequently rocked, it can retain both direction and rocking amplitude. However, if the rocking becomes too intense, frequent collisions between particles erase the original directional memory and replace it with a new one.
A: Unlike computer memory where files exist independently, biological memories continuously interact, compete, and modify each other over time. Penn State researchers used this neuroscience concept to show that physical materials experience similar memory interference, where new mechanical experiences alter or erase established structural states.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this neurotech and memory research news
Author: Sam Sholtis
Source: Penn State
Contact: Sam Sholtis – Penn State
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Memories of Amplitude and Direction Coexist and Compete in Non-Brownian Suspensions” by Surendra Padamata and Nathan C. Keim. Physical Review Letters
DOI:10.1103/ckl2-lcpl
Abstract
Memories of Amplitude and Direction Coexist and Compete in Non-Brownian Suspensions
Steadily shearing a non-Brownian suspension forms a memory of direction, while shearing back and forth forms a memory of amplitude. Each memory is evident in the system’s response to further shear, exemplifying its strong history dependence.
By combining the steady and oscillatory experiments, we show these memories are distinct but intersecting aspects of the same nonequilibrium physics: they can coexist, yet a specific amplitude suppresses directional memory and makes the system symmetric.
Combined with prior results from disordered solids, our Letter presents a simple motif for limited memory capacity in nonequilibrium matter.
