Evidence of Mechanically Activated Processes in Slow Granular Flows

Reddy, K. Anki; Forterre, Yoël; Pouliquen, Olivier

doi:10.1103/physrevlett.106.108301

Cited by 155 publications

(221 citation statements)

References 29 publications

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“…Nevertheless, this local description falls short of describing non-parallel flows where the streamlines are far from parallel and also quasi-static regimes close to the "liquid-solid" transition [5]. Nonlocal effects, where the stress not only depends on the strain rate but also on its spatial variations, have been recently reported both experimentally and numerically [6][7][8]. These effects may arise from plastic rearrangements inducing long range correlations and cooperative behaviors.…”

Section: Introductionmentioning

confidence: 99%

Local rheological measurements in the granular flow around an intruder

et al. 2016

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The rheological properties of granular matter within a two-dimensional flow around a moving disk is investigated experimentally. Using a combination of photoelastic and standard tessellation techniques, the strain and stress tensors are estimated at the grain scale in the time-averaged flow field around a large disk pulled at constant velocity in an assembly of smaller disks. On the one hand, one observes inhomogeneous shear rate and strongly localized shear stress and pressure fields. On the other hand, a significant dilation rate, which has the same magnitude as the shear strain rate, is reported. Significant deviations are observed with local rheology that justify the need of searching for a non-local rheology.

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Section: Introductionmentioning

confidence: 99%

Local rheological measurements in the granular flow around an intruder

et al. 2016

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“…We related quantitatively the effective viscosity to the inverse of the stress modulation rate and have shown that the viscosity decreases significantly when approaching the dynamical threshold. Note that in spite of resemblances, this phenomenon is a priori different from another fluidization process occurring when a granular packing is placed in contact with a fluidized shear band [38,39]. In the last case, theoretical analysis and numerical simulations show that the induced creeping process comes from a non-local stress relaxation, from the flowing part to the material bulk [40][41][42][43].…”

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confidence: 98%

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

et al. 2015

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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].

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“…Force chains are a network of intergranular contacts in the bulk medium, and granular hydrostatic pressure acts on the intruder surface along the force chains. [13][14][15] The rotation of intruder yields additional intergranular motions, which facilitate breaking of the force chains, [16][17][18][19] thereby reducing the vertical drag forces. The local surface speed induced by rotation increases with the radial distance from the intruder centerline, so the grains easily slip in the outer region.…”

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confidence: 99%

Reduction of granular drag inspired by self-burrowing rotary seeds

et al. 2017

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We present quantitative measurements and mat hematical analysis of the granular drag reduction by rotation, as motivated by the digging of Erodium and Pelargonium seeds. The seeds create a motion to dig into soil before germination using their moisture-responsive awns, which are originally helical shaped but reversibly deform to a linear configuration in a humid environment. We show that the rotation greatly lowers the resistance of soil against penetration because grain rearrangements near the intruder change the force chain network. We find a general correlation for the drag reduction by relative slip, leading to a mathematical model for the granular drag of a rotating intruder. In addition to shedding light on the mechanics of a rotating body in granular media, this work can guide us to design robots working in granular media with enhanced maneuverability.

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Evidence of Mechanically Activated Processes in Slow Granular Flows

Cited by 155 publications

References 29 publications

Local rheological measurements in the granular flow around an intruder

Local rheological measurements in the granular flow around an intruder

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

Reduction of granular drag inspired by self-burrowing rotary seeds

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