Global Memory From Local Hysteresis and Disorder in a Jammed Solid

Cyclically shearing a soft 2D jammed material can train it with a memory of strain amplitude that can be retrieved later. Using experiments with particles at an oil-water interface, we show that this memory arises from the hysteresis of particles' rearrangements within the material. A stable population of rearrangements is created by training, and disorder ensures that they collectively discriminate among different inputs. These results point to a generic way of encoding memories in hysteresis and disorder, and explain why jammed and dilute suspensions must have different kinds of memory. The behavior in jammed systems is reminiscent of the return-point memory found in ferromagnets and many other systems, but in light of jammed systems' unquenched disorder, it is unexpected.