A foam made from forests of carbon nanotubes can remember how it was squeezed even after recovering its original shape, showing a form of persistent mechanical memory that researchers compare to the magnetic memory used in hard drives.
The breakthrough, described in a paper published in Physical Review X, sets the experimental foam apart from conventional cushioning materials. Instead of gradually forgetting its previous state, accumulating damage or responding differently at different compression speeds, the material retained a stable mechanical memory.
Researchers then used that memory to control how quickly impact waves traveled through the foam. This ability could eventually lead to smarter helmets, shock-absorbing liners, and mechanical computers that process signals without electronics.
“We report constitutive return point memory in elastically recoverable, vertically aligned carbon nanotube foams, analogous to magnetic hysteresis-based RPM utilized in hard drives,” researchers write. “Unlike viscoelastic fading memory, VACNTs exhibit nonvolatile memory arising from rate-independent nanoscale friction.”
The foam consists of multiwalled carbon nanotubes arranged into dense, vertically aligned forests. Individual test samples were only about 0.2 inches wide. However, their internal structure produced a combination rarely found in soft materials: They dissipated energy while still returning to a repeatable mechanical state.
A Foam That Remembers Where It Turned Around
Ordinary polymer foams are viscoelastic, meaning they possess both fluid-like and solid-like properties. Their behavior depends partly on time, so holding them under strain allows internal stresses to relax. With multiple compressions, their mechanical response can drift through a process known as fading memory.
However, this new carbon nanotube foam behaves much differently.
Researchers compressed samples, partially released them, and then held them at a fixed strain. While a conventional PVC foam gradually relaxed, the stress inside the nanotube foam remained constant. When compression resumed, the nanotube material returned to the exact point where the loading direction had previously reversed.
The behavior is called return point memory. It resembles what occurs when the magnetic field applied to a ferromagnetic material changes direction: The material retains information about previous reversal points and can return to them when the field changes again.
“People refer to this as fading memory,” Dr. Ramathasan Thevamaran, a University of Wisconsin–Madison mechanical engineer and the study’s senior author, told the American Physical Society’s Physics Magazine when describing ordinary foams. His team’s nanotube material, however, did not appear to forget.
Researchers initially compressed each sample through several cycles to secure its internal structure. Once conditioned, the foam produced reproducible stress-strain loops as long as subsequent compression remained below that initial maximum.
They also tested deformation rates spanning three orders of magnitude and oscillations ranging from 1 to 1,000 hertz. The response remained essentially unchanged.
“Whether we did very slow compression or very fast compression, it didn’t matter,” Dr. Thevamaran said. “The foam behaved the same way.”
Nanoscale Friction Stores the Memory
The memory did not come from electronic components or from mechanical pieces snapping between two shapes.
Instead, the researchers traced it to friction between neighboring nanotubes. As the foam is compressed, nanotubes bend, buckle, and slide against one another. Their contact points undergo rapid stick-slip interactions, including repeated “zipping” and “unzipping” as the tubes attach and separate.
Those interactions dissipate energy, but they are not governed by the slow, time-dependent viscosity that causes memory to fade in polymer foams. The researchers developed a deformation-dependent stick-slip friction model that reproduced the observed recovery, hysteresis, and return point memory.
The distinction could make the material more versatile than mechanical memories built from bistable metamaterial structures. Those systems typically encode one of two geometrical states, equivalent to 0 or 1. The nanotube foam’s memory can instead occupy a continuous range of mechanical states determined by its loading history.
Completing a compression cycle also erases the corresponding history through what physicists call the “wiping-out effect.” That makes the material’s memory reusable rather than a permanent record of every force it has experienced.
Harder Impacts Produced Slower Waves
To show that the effect could perform useful work, the team stacked nanotube foam disks between rigid aluminum layers, creating a narrow waveguide resembling a roll of coins. A gas-powered projectile struck one end while strain gauges measured the resulting stress pulse farther along the stack.
The researchers found two independent ways to change the pulse’s speed.
Precompressing the nanotube layers made them stiffer and caused waves to travel faster. Increasing the strength of an incoming impact produced the opposite response: Greater dynamic strain softened the foam, causing the pulse to travel more slowly.
That combination suggests a material could be programmed to handle impacts differently depending on their intensity. An array of differently precompressed waveguides might spread a dangerous pulse over time instead of letting its energy arrive simultaneously at one location.
The researchers modeled one such phased array, showing how low-amplitude waves could converge into a focused peak while stronger shock waves lost synchronization and emerged attenuated. A helmet liner based on the concept might therefore redistribute high-energy impacts without relying on powered sensors or active controls.
The same behavior could support mechanical analog computing. Properly arranged waveguides could potentially filter, gate, or recognize incoming waveforms while the signal remains mechanical, avoiding the energy cost of converting it into digital information.
Ultimately, that could mean protective equipment that responds differently to routine vibrations and dangerous impacts, vehicle or spacecraft components that pass harmless mechanical signals while dispersing damaging shock waves, or ultrasound systems that mechanically isolate and amplify specific wave patterns before they reach electronic processors.
Because the response is built into the material, such devices could be especially useful in isolated or extreme locations where power, sensors, and standard electronics are limited.
For now, those devices remain concepts rather than finished protective systems. The researchers experimentally demonstrated the memory effect and tunable wave speed, but they tested the phased-array shock limiter through analytical modeling.
Nevertheless, the breakthrough shows that a material does not need electronics, or even a visibly different shape, to retain and act on information.
“One of the striking aspects of these VACNT foams is that they appear to be protected from degradation: They don’t age or accumulate damage,” said Dr. Joseph Paulsen, a mechanical physicist at St. Olaf College who was not involved in the research. “The researchers show how this bulk response could be leveraged to engineer useful acoustic materials.”
The recent study, “Enduring Mechanical Memory from the Constitutive Response of Elastically Recoverable Nanostructured Materials,” by Gupta, A., Maheswaran, B., Jaegersberg, N., et al, appeared in Physical Review X.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com