Science1 publisher2 min readPublished
A carbon nanotube foam records how hard it was squeezed in the friction between its tubes
A group at the University of Wisconsin-Madison reports in Physical Review X that a vertically aligned nanotube foam returns to a mechanical state it held before, with the record kept in nanoscale stick-slip friction.
The Scientist · Science desk
What happened
- Ramathasan Thevamaran and colleagues at the University of Wisconsin-Madison report in Physical Review X a carbon nanotube foam that remembers how hard it was squeezed and then recovers its shape with no lasting damage.
- Chip-free mechanical memory has mostly been built from bistable structures, components that snap between two shapes and so offer the system only two states.
- The team compressed the foam repeatedly until its response stabilized, then tracked what it did under partial unloading and reloading.
- Pushed partway, released and pushed again, the foam returned to the exact mechanical state it had been in before, a behavior known as return-point memory.
- The team traced the effect to nanoscale friction, with neighboring nanotubes sticking and slipping against each other so the friction carries the record.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability The battery-free, chip-free sensing that this line of work is aimed at could report a graded value instead of whether one threshold was crossed.
- constraint Reuse is the bar these materials keep failing: the ones that already show graded mechanical memory deform permanently, degrade with age or need training first.
- precedent If friction between neighboring fibers is doing the storing, the search widens to other fibrous and nanostructured materials, which is where the authors point it.
The group compares the behavior to a magnetic hard drive, where reversing the field brings the material back to a state it recorded earlier [7]. The foam's equivalent of the field is compressive load, applied to a block of vertically aligned carbon nanotubes [15]. Conventional materials relax, and a stored mechanical state drifts with them. This foam did not relax or fade over time, according to the team's measurements [10].
Cycling the foam to a stable response before the memory measurements keeps the return points clear of first-cycle settling [5]. Elastic recovery means the same specimen can be measured again [1].
The stiffness moves in two directions. Squeeze the foam harder and it stiffens; shake it and it softens [9]. For a damper that is a feature. For a memory element it is a coupling someone has to pin down, because a foam recording a peak load inside a vibrating machine is also being written to by the vibration [9].
The phys.org account does not say how many distinguishable states the foam holds, or how to read one back out without a load frame [14].
Thevamaran and colleagues suggest the same friction-based approach could inspire materials that dampen shocks, filter vibrations or perform simple, entirely passive computations [11]. The paper, "Enduring Mechanical Memory from the Constitutive Response of Elastically Recoverable Nanostructured Materials", is by Abhishek Gupta and colleagues in Physical Review X [13].
What to watch
- Whether the Physical Review X paper reports how many return points stay distinguishable, and over what range of strain.