Browsing by Author "Abed Zadeh, Aghil"

In a variety of physical systems, slow driving produces self-similar intermittent dynamics known as crackling noise. Barkhausen noise in ferromagnets, acoustic emission in fracture, seismic activities and failure in sheared granular media are few examples of crackling dynamics with substantial differences at the microscopic scale but similar universal laws. In many of the crackling systems, the origin of this universality and the connection between microscopic and macroscopic scales are subjects of current investigations.

We perform experiments to study the microscopic and macroscopic dynamics of a sheared granular medium. In our experiments, a constant speed stage pulls a slider with a loading spring across a 2D granular medium. We measure the pulling force on the spring, and image the medium to extract the local stress and particle displacement. Using novel signal and image analysis methods, we identify fast energy dissipating events, i.e.\ avalanches, and investigate their statistics and dynamics.

The pulling force exhibits crackling dynamics for low driving rates with intermittent slip avalanches. The energy loss in the spring has a power-law distribution with an exponent that strongly depends on the driving rate and is different from $-1.5$ predicted by several models. In our experiments for low driving rate, we find a slip rate power-spectrum of form $\mathcal{P}_v(\omega) \sim \frac{\omega^2}{1+\omega^{2.4}}$, a power-law distribution of the slip rate $P(v) \sim v^{-2.9}$, and average temporal profile of the slider motion (avalanche shape) of form $\mathcal{P}_D(u)=[u(1-u)]^{1.09}$. These findings are different from several theoretical and numerical studies \citep{dahmen2011simple, colaiori2008exactly, Laurson13_natcom}.

Avalanche temporal correlation is also investigated using certain conditional probabilities. At low driving rates, we observe uncorrelated order of the avalanches in terms of Omori-Utsu and B\r{a}th laws and temporal correlation in terms of the waiting time law. At higher driving rates, where the sequence of slip avalanches shows strong periodicity, we observe scaling laws and asymmetrical avalanche shapes that are clearly distinguishable from those in the crackling regime. We provide a novel dynamic phase diagram of granular matter as a function of driving rate and stiffness and characterize the crackling to periodic transition. We also find intermittent fluctuations in internal stress both in the crackling and the periodic regime.

Finally, we observe a narrow shear band with most of particle displacements, but stress fluctuations all over the medium. We identify the spatio-temporal connected components of local stress drops, which we call local avalanches. We find power-law distributions of the local avalanches with an exponent of $-1.7 \pm 0.1$, different from spring energy avalanche distribution with an exponent of $-0.41 \pm 0.05$ for the same experiments.

Our study constrains theoretical frameworks for granular dynamics and crackling noise in sheared granular media. Moreover, it may be relevant for characterizing the role of granular matter in fault gouges during seismic events.