Penn State Physicists Show a Chocolate-Syrup-Like Fluid Can Store Two Competing Mechanical Memories

Overview

Physicists at Pennsylvania State University have shown that a viscous suspension of particles — the same basic mixture found in chocolate syrup or wet concrete — can hold two distinct types of mechanical memory at once, and that the two memories interact and compete rather than sitting side by side untouched, according to Phys.org. The finding, described in a paper published in Physical Review Letters and highlighted there as an editors’ suggestion, adds a new twist to the study of how disordered materials retain traces of their own history, Phys.org reports.

What We Know

• The research team, led by Penn State physicist Nathan Keim, studies memory effects in what are known as non-Brownian suspensions — mixtures in which relatively large particles sit in a viscous liquid, similar to chocolate syrup or fresh concrete, according to Phys.org. The particles are large enough that their motion is not driven by Brownian motion, the random jostling of thermally energetic atoms, so any movement in the material comes from how it is deformed, Phys.org explains.
• The underlying paper’s abstract, co-authored by first author Surendra Padamata, a graduate student in physics at Penn State’s Eberly College of Science, and Keim, an associate professor of physics, states: “Steadily shearing a non-Brownian suspension forms a memory of direction, while shearing back and forth forms a memory of amplitude,” according to the arXiv preprint.
• The team first confirmed each memory type on its own — the suspension remembers the direction it was stirred, and separately remembers how vigorously it was rocked back and forth — before testing what happens when both are imprinted together, Phys.org reports. “Each of these memories had been studied on its own,” Padamata said, according to Phys.org. “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.”
• At lower rocking intensities, the suspension retained both the directional memory from stirring and the amplitude memory from rocking simultaneously, Phys.org reports. As the rocking grew more intense, the directional memory weakened and was eventually wiped out. “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, according to Phys.org.
• The paper itself frames the result in more general terms: combining steady and oscillatory shear experiments shows the two memories are “distinct but intersecting aspects of the same non-equilibrium physics,” with “a specific amplitude” able to suppress directional memory and make the system symmetric, according to the arXiv preprint. The authors write that, taken together with prior results from disordered solids, the work “presents a simple motif for limited memory capacity in non-equilibrium matter,” the preprint states.
• Keim said the pattern is not unique to liquid suspensions. “A similar combination of directional memory and amplitude memory appears in soft glasses and granular packings with very different microscopic physics,” he said, according to Phys.org. He added: “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 paper, titled “Memories of Amplitude and Direction Coexist and Compete in Non-Brownian Suspensions,” appears in Physical Review Letters, according to Phys.org, and carries the journal citation Phys. Rev. Lett. 136, 258201 (2026), according to the arXiv preprint page. The preprint was first posted in October 2025 and revised in May 2026, the arXiv submission history shows.

What We Don’t Know

• Neither source discloses the specific rocking speeds, frequencies, or amplitudes used to trigger the erasure threshold, so the exact experimental parameters that separate “memory coexistence” from “memory erasure” remain unpublished in the coverage reviewed here.
• Padamata raised the possibility that similar memory effects could exist in rock formations, potentially influencing risks such as earthquakes or sinkholes through changes in temperature and vibration, but described this only as a research interest rather than a tested result. “We are interested in how this model can inform biological memory and, potentially, geophysical processes as well,” he said, according to Phys.org. “Changes in temperature and vibrations might impart memories in rock that influence the risk of earthquakes and sinkholes, for example. It could be possible to find some way to erase these memories and reduce the risk or make better predictive models.”
• It is not yet established whether the competing-memory effect measured in a lab suspension translates directly to denser real-world materials like poured concrete, where particle-to-particle contact is more constant.

Analysis

The result is a small but conceptually tidy addition to the physics of disordered matter: rather than simply asking whether a jostled material “remembers” what was done to it, the Penn State team asked what happens when a material is asked to remember two different things at once. The answer — that one memory can overwrite the other once a threshold is crossed — mirrors, at least descriptively, the kind of interference familiar from human short- and long-term memory, an analogy Padamata drew directly in describing the researchers’ motivation, according to Phys.org. Keim’s comment that the same pattern shows up in soft glasses and granular packings suggests the researchers see this less as a quirk of chocolate-syrup-style suspensions specifically and more as a candidate general rule for how disordered systems handle limited memory capacity, according to Phys.org.