15 Second Summary: Memory and slow relaxations of amorphous media Crumpled sheets, elastic foams and granular materials exhibit a similar repertoire of glassy-like behaviors when driven far from equilibrium. These systems also allow probing of the underlying structural dynamics, which we exploit in search of a deeper understanding of these behaviors.
When driven far from equilibrium, many disordered systems exhibit a similar repertoire of behaviors, such as slow, non-exponential relaxations that can span a huge range of time scales - from a fraction of a second to weeks, aging and memory effects. While recurring motifs have been observed in very different systems - electronic, molecular, mechanical and more - the underlying mechanisms giving rise to these behaviors are not well understood.
Our research in this direction is focused on soft mechanical disordered systems - elastic foams, randomly crumpled thin sheets and granular materials. While exhibiting the universal behaviors of disordered systems, these systems also offer ways of measuring the underlying structural dynamics. In crumpled Mylar sheets for example, we observed logarithmic relaxation and memory effects, but we can also look at the evolution of the underlying network of crumples and folds and listen to the crackling sounds the system emits as it evolves. To detect the hidden internal dynamics of elastic foams during relaxation and memory dynamics we use speckle interferometry - when illuminated with a coherent laser beam these foams show beautiful multiple scattering effects resulting in a dynamic speckle pattern. Similar aging and memory effect are displayed by a shaken granular pile as it compacts - here we develop methods to image and track all the grains, in an attempt to identify the underlying dynamics.
Using these and other systems, we aim to uncover the underlying mechanisms that lead to the macroscopic, glassy-like behavior, in an attempt to identify universal principles that govern the out of equilibrium dynamics of disordered systems.
Figure: observation of a memory effect in crumpled thin sheets. A 200gr weight is places on a crumpled ball for approximately 25 seconds, during which the volume of the ball relaxes non-exponentially. Then the weight is replaced by a slightly lighter one. Rather then exhibiting slightly slower relaxation as one might expect, the system first exhibits a slow increase in volume, pushing the Wight upwards. Only after many seconds the systems finally ‘forgets’ the previous load it was under, and renews its non- exponential relaxation.
Disordered Granular Compaction
A sheet of bidisperse granular material, when repeatedly tapped, displays a consistent, logarithmic decrease in volume over many orders of magnitude. Logarithmic processes are often observed in disordered systems, (e.g. paper) and this system avoids long-range order via frustration of the two sizes of grains. After taking images of our system after many numbers discrete taps we use image analysis, particle identification, and tracking software to investigate the cause(s) of this logarithmic decay, and to explore possible memory and non-monotonic compaction effects.Right: Particle tracking results for ~2000 bidisperse beads over 30,000 discrete taps.
Particle tracking results for ~2000 bidisperse beads over 30,000 discrete taps.