Mechanical fluctuations suppress the threshold of soft-glassy solids: The secular drift scenario

Abstract

We propose a dynamical mechanism leading to the fluidization by external mechanical fluctuations of soft-glassy amorphous material driven below the yield-stress. The model is based on the combination of memory effect and non-linearity, leading to an accumulation of tiny effects over a long-term. We test this scenario on a granular packing driven mechanically below the Coulomb threshold. We bring evidences for an effective viscous response directly related to small stress modulations in agreement with the theoretical prediction of a generic secular drift. We finally propose to extend this result more generally, to a large class of glassy systems.PACS numbers: 81.40. Lm, 83.80.Fg Numerous amorphous materials such as concentrated suspensions, colloidal glasses, foams or granular materials share common global features in their mechanical response to shear [1,2]. They are characterized by a yield stress below which the material appears as a solid [3,4]. As this behaviour is shared by so many different materials, several conceptual and theoretical frameworks emerged recently [5][6][7][8][9][10] to provide a quantitative basis for the phenomenology of soft glassy rheology (SGR) above and beyond the yield stress. Even though many parallel approaches exist, sometimes at different level of description, they all share either explicitly or implicitly, the underlying idea that mesoscopic collective processes triggered by thermal or mechanical activation, contribute to the material fluidity. The direct visualization of local plastic events and the associated complex avalanching dynamics is supported by many experimental [11][12][13][14] or numerical [15][16][17] studies. In the "solid phase" corresponding to a strong dynamical arrest, soft-glassy systems display ageing properties manifesting in a slow creep relaxation process [18][19][20][21]. Ageing properties stem from a remaining thermal activation providing the possibility to cross enthalpic or entropic barriers and progressively set the system into deeper local minima where mechanical solidity is reinforced. The existence of external mechanical noise was also proposed as a substitute for thermal activation. In this sense, the behaviour of these amorphous soft glassy solids is very close phenomenologically to molecular glass-formers obtained by thermal quenching [22]. Yet, the fact that such mechanical noise truly acts as an effective temperature is presently debated [23] and indeed, deep differences in the way thermal noise and mechanical fluctuations act in amorphous systems has been recently pointed out [24].

Rheology of sediment transported by a laminar flow

“…The μ ( I v ) rheology, however, indicates that ${\tau }_c^{\ast }$ does not represent a well-defined yield stress criterion for the granular surface, instead μ = μ s defines the interface with the quasistatic dense bed. This is consistent with observations of creep behavior in dry granular material in horizontal shear experiments [49–51]. Our results generally support recent studies calling for a modification to the classical bed-load transport framework; in particular, that the friction coefficient cannot be considered constant [21,22], and that the “bed-load active layer” is not constant but instead expands vertically in both directions as the shear stress τ increases [22].…”

“…Results cannot be directly compared at present, however, as the nonlocal models implement a boundary condition of no particle motion very far from the shear zone. The few experiments where creep has been quantified [27,49–51] used varying boundary conditions (from smooth to rough), but some boundary slip likely occurred for all conditions [36]. If creep is indeed driven by nonlocal dynamics, varying boundary conditions in nonlocal rheology models will allow for insightful comparisons with different experimental results that will enrich our understanding of the creep mechanism.…”

Ice flow dynamics forced by water pressure variations in subglacial granular beds

“…Geophysical Research Letters 10.1002/2016GL071579 [Pons et al, 2015]. This behavior is analogous to how granular materials on hillslopes beneath the angle of repose experience downslope movement due to episodic disturbances, e.g., wet-dry or freeze-thaw cycles [Roering et al, 2001].…”

“…Creep rates in geotechnical materials subjected to a constant deviatoric stress just below its ultimate failure strength are known to decay logarithmically with time [e.g., Kamb , ; Moore and Iverson , ; Nguyen et al ., ] or to lead to runaway failure [e.g., Mitchell and Soga , ]. Our experiments suggest that repeated perturbation of effective stress causes sustained internal grain rearrangement (Figure ), which has also recently been observed in laboratory experiments on simple granular materials [ Pons et al ., ]. This behavior is analogous to how granular materials on hillslopes beneath the angle of repose experience downslope movement due to episodic disturbances, e.g., wet‐dry or freeze‐thaw cycles [ Roering et al ., ].…”

Induced and endogenous acoustic oscillations in granular faults

“…Concerning the first issue, the effect of flow rate and vibration amplitude on the rheological properties of glass beads has been recently investigated in several works, showing non-monotonic flow curves [22,48] and critical behaviour [34]. The effect of mechanical fluctuations on the probe has been studied in [49], where a generic rheological model is proposed. Other recent works addressed the interesting issue of non-local rheology (see e.g.…”