Summary: A new study shows that non-Brownian suspensions can form, retain, and overwrite multiple microscopic memories at once. These physical memories follow dynamics that parallel how cognitive memories interact in neuroscience.
Researchers examined mixtures of relatively large particles suspended in a viscous fluid. They found that two distinct types of memory—directional memory imprinted by steady stirring and amplitude memory imprinted by back-and-forth rocking—can coexist within the same material.
Increasing the intensity of rocking produces more frequent particle encounters that systematically erase earlier directional memory, mimicking how short-term and long-term biological memories can interact, compete, and reshape each other.
Key Facts
- Non-Brownian suspension mechanics: The experiments use particles large enough that Brownian motion (random thermal movement) is negligible. That ensures microscopic rearrangements arise only from applied mechanical forces such as stirring or rocking.
- Coexistence of dual memories: A single sample can simultaneously store a directional memory from steady rotation and an amplitude memory from oscillatory rocking.
- Intensity-dependent memory erasure: At low rocking amplitudes, the directional memory remains intact. When rocking intensity surpasses a threshold, increased particle collisions disrupt the microstructure and erase the prior directional memory.
- Writing new structural directionality: Beyond the erasure threshold, intense rocking can imprint a new directional memory aligned with the recent mechanical forcing.
- Cross-disciplinary implications: These findings provide a physical framework that may inform understanding of memory consolidation in neuroscience and reveal how stress histories in granular or rock packings could affect geomechanical outcomes such as earthquakes or sinkholes.
Source: Penn State
Materials, like biological systems, can retain records of past deformations at the microscopic level. A familiar example is the visible crease left after folding and unfolding a sheet of paper. Such structural memories determine how materials respond to later forces, and studying them can guide the design of materials that react predictably to changing conditions.
Researchers at Penn State have demonstrated that two distinct material memories can coexist and compete in a simple non-Brownian suspension. These interactions resemble how new and old memories in the brain influence one another, and the results were recently published and highlighted in the journal Physical Review Letters.
The team focused on non-Brownian suspensions—thick fluids such as chocolate syrup or fresh concrete where suspended particles are sufficiently large that thermal fluctuations do not drive their motion. This makes the system an ideal model: particle rearrangements occur only because the experimenters apply mechanical forcing.
In earlier work, steady shearing was shown to write a directional memory into the particle arrangement, while oscillatory shear encoded an amplitude memory. For the new experiments, the researchers first stirred the suspension to imprint a directional bias, then applied rocking at various amplitudes to study how the two memory types interact.
At modest rocking amplitudes, the sample preserved both the initial directional memory and the new amplitude memory—that is, both memories occupied the material simultaneously. As rocking amplitude increased, however, repeated particle encounters gradually weakened the original directional memory until it disappeared entirely. If rocking grew even stronger, the oscillatory forcing began to imprint its own directional bias, effectively overwriting the earlier history.
“When you save a file on a computer, new files do not typically alter existing ones,” said Surendra Padamata, the paper’s first author. “But biological memories change over time as new experiences reshape older ones. We were inspired by that neuroscience idea and sought a material analogue where memories interact and compete.”
The researchers propose that the competition arises when particle encounters become sufficiently frequent. At small oscillation amplitudes these encounters are rare, allowing the original directional structure to persist; at larger amplitudes collisions proliferate and the structure becomes symmetric before possibly acquiring a new bias. While this detail depends on particle concentration and fluid properties, similar coexistence and competition of memories have also been observed in solid disordered materials, suggesting a broader principle for how nonequilibrium matter stores limited histories of deformation.
“We see analogous behavior in soft glasses and granular packings despite very different microscopic mechanics,” said Nathan Keim, associate professor of physics at Penn State and leader of the study. “That points to a simple motif for limited memory capacity in disordered materials under basic driving protocols like stirring and rocking.”
Funding: The work was supported by the Human Frontier Science Program.
Key Questions Answered:
A: A non-Brownian suspension is a mixture in which particles are large enough that thermal Brownian motion is negligible. Because thermal noise does not change the particle arrangement, any structural evolution is driven purely by mechanical forcing, making the system an excellent model for structural memory studies.
A: Stirring first stores a directional memory in the particle arrangement. Subsequent rocking can imprint an amplitude memory while the directional memory persists at low amplitudes. If rocking intensity increases beyond a threshold, frequent particle collisions erase the original directional memory and may allow a new direction to be written by the rocking itself.
A: Unlike isolated computer files, biological memories interact, influence, and modify one another over time. The Penn State experiments show that a similar form of interference and restructuring can occur in physical materials: new mechanical experiences can alter or erase previously encoded structural states.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full by editorial staff.
- Additional context was added by the staff.
About this neurotech and memory research news
Author: Sam Sholtis
Source: Penn State
Contact: Sam Sholtis – Penn State
Image credit: Neuroscience News
Original research: “Memories of Amplitude and Direction Coexist and Compete in Non-Brownian Suspensions” by Surendra Padamata and Nathan C. Keim. Physical Review Letters. Open access.
Abstract
Memories of Amplitude and Direction Coexist and Compete in Non-Brownian Suspensions
Steady shearing of a non-Brownian suspension encodes a directional memory, while oscillatory shearing encodes an amplitude memory. Each memory reveals itself through the system’s response to subsequent shear and demonstrates strong history dependence. By combining steady and oscillatory protocols, the authors show these memories are distinct yet intersecting aspects of the same nonequilibrium physics: they can coexist, but a specific oscillation amplitude can suppress directional memory and restore symmetry. Together with earlier results from disordered solids, this work suggests a simple motif for the limited memory capacity of nonequilibrium materials.