Nonergodicity in silo unclogging: Broken and unbroken arches

Abstract

We report an experiment on the unclogging dynamics in a two-dimensional silo submitted to a sustained gentle vibration. We find that arches present a jerking motion where rearrangements in the positions of their beads are interspersed with quiescent periods. This behavior occurs for both arches that break down and those that withstand the external perturbation: Arches evolve until they either collapse or get trapped in a stable configuration. This evolution is described in terms of a scalar variable characterizing the arch shape that can be modeled as a continuous-time random walk. By studying the diffusivity of this variable, we show that the unclogging is a weakly nonergodic process. Remarkably, arches that do not collapse explore different configurations before settling in one of them and break ergodicity much in the same way than arches that break down.

Intermittent flow and transient congestions of soft spheres passing narrow orifices

“…This is a qualitatively new observation for passive granular materials that are not agitated by external forcing. Intermittent clogs that look very similar in terms of the outflow rates are found with hard particles in vibrated containers, 13,[16][17][18] in silos with oscillating bottom, 59 or in flocks of animals passing a gate. 32 In these systems, however, momentarily existing clogs can be broken by external forces like vibrations, or by the activity of bacteria, animals or pedestrians.…”

“…Below a certain outlet diameter, hard particles form a stable clog at the orifice. 6,10 This structure blocks further outflow, and it can only be destroyed by external forcing like vibration of the silo [11][12][13][14][15][16][17][18] or air flushes through the opening. 19 The destruction of clogged states by container vibrations, also called unjamming, has been extensively studied quantitatively in the past.…”

On the broad tails in breaking time distributions of vibrated clogging arches

“…They included a broad distribution of quiescent times (in fact, a stretched exponential distribution similar to the proposed for the aforementioned trap model). Later, Guerrero and coworkers [5] estimated the quiescent times distribution from experimental data and they found that this CTRW reproduces many of the features of the experiments, although quantitative agreement is sometimes lacking. In particular, absence of ergodicity -as revealed by the difference between ensemble averaged and time averaged mean squared displacements of the variable describing the geometry-was found both in the experiments and the model.…”

“…This explanation was elaborated to better understand the dynamics of arches while they were being vibrated. In a series of experiments [4,5] and numerical simulations [10] the positions of the beads in the arch where registered along time, with a high sampling frequency. In order to define the arch geometry, the angles between two consecutive beads were used, taken as a vector; or, alternatively, the standard deviation σ of that set of angles was used.…”

“…We have carried out several sets of experiments, some of which have been previously reported [3][4][5]. The ones used here consist of a monolayer of grains enclosed between two vertical glass plates.…”