Experimental evidence of ageing and slow restoration of the weak-contact configuration in tilted 3D granular packings

Granular packings slowly driven towards their instability threshold are studied using a digital imaging technique as well as a nonlinear acoustic method. The former method allows us to study grain rearrangements on the surface during the tilting and the latter enables to selectively probe the modifications of the weak-contact fraction in the material bulk. Gradual ageing of both the surface activity and the weak-contact reconfigurations is observed as a result of repeated tilt cycles up to a given angle smaller than the angle of avalanche. For an aged configuration reached after several consecutive tilt cycles, abrupt resumption of the on-surface activity and of the weak-contact rearrangements occurs when the packing is subsequently inclined beyond the previous maximal tilting angle. This behavior is compared with literature results from numerical simulations of inclined 2D packings. It is also found that the aged weak-contact configurations exhibit spontaneous restoration towards the initial state if the packing remains at rest for tens of minutes. When the packing is titled forth and back between zero and near-critical angles, instead of ageing, the weak-contact configuration exhibits "internal weak-contact avalanches" in the vicinity of both the near-critical and zero angles. By contrast, the stronger-contact skeleton remains stable

Heterogeneous dynamics of a granular pack under vertical tapping

The dynamics of a tri-dimensional dense granular packing under gravity and vertical tapping is numerically investigated in a stationary state. A slowing-down of the dynamics is observed close to the bottom of the packing with a correlation length of 5–6d. In this perturbed zone, the self-intermediate scattering function, decaying slower than exponentially, suggests the existence of a heterogeneous dynamics. A characteristic time scale of the dynamics depending on the distance from the bottom of the packing is extracted. Our numerical results open the way to a systematic study of the influence of external driving force and of boundary conditions on the heterogeneity of the dynamics of granular packings under gravity

Rheology of fiber suspensions using MRI

The suspensions of non-Brownian fibers are of interest for many applications. Although many studies concerning suspensions are available in the literature, most of them concern suspensions of spherical particles. In this paper, global and local rheology of fiber suspensions are explored near the jamming transition. A critical volume fraction is extracted from the experimental data. The value of this critical volume fraction is in agreement with the expected value of the concentration of rigid rods above which the isotropic phase becomes unstable. Moreover, non-reversible effects of the shearing are observed in flow curves because of the non-Brownian behavior of the studied fibers

Dynamics of vibrated granular suspensions probed by mechanical spectroscopy and diffusing wave spectroscopy measurements

International audienceIn this paper, we investigate the dynamics of vibrated granular suspensions by mechanical spectroscopy and multi-speckle diffusing wave spectroscopy (MSDWS), with the aim of relating microscopic dynamical mechanisms, at a grain scale, to the resulting macroscopic rheological behavior of the samples. Rheological experiments reveal that the samples exhibit a Maxwellian behavior at low frequencies leading to eta(0) G tau(R), where eta(0) is the low shear viscosity of the suspension (Newtonian plateau), G is the shear modulus (modulus of rigidity) and tau(R) is the longest relaxation time. The two macroscopic parameters, G and tau(R), of the Maxwell model can be related to structural parameters in order to link microscopic and macroscopic levels. To do so, in a first step, we show that the macroscopic parameter G is related to the structural parameters sigma(f) and gamma(c) through the relation G = sigma(f)/gamma(c) where sf is the frictional stress and gc the critical strain corresponding to the onset of contact breaking. In a second step we show that the relaxation time tau(R), determined by mechanical spectroscopy, matches precisely with the decorrelation time tau(D), derived independently from local optical measurements. We show that, similarly to the plateau viscosity eta(0) both are controlled by the dimensionless Peclet number Pelub in that tau(R) alpha 1/Pe(lub) and tau(D) alpha 1/Pelub where Pelub sigma(lub)/sigma(f) has been defined in a previous article as the ratio of the lubrication stress, induced by vibrations, and the frictional stres

Two-state model to describe the rheological behavior of vibrated granular matter

International audienceIn this paper, we present a model aimed at predicting the rheological response of a 3D dry granular system to nonstationary mechanical solicitations, subjected or not to vibrations. This model is based on a phenomenological two-state approach related to the inherent bimodal behavior of chain forces in granular packing. It is set up from a kinetic equation describing the dynamics of the contact network. To allow experimental assessment, the kinetic equation is transformed into a differential constitutive equation, relating stress to strain, from which rheological properties can be derived. Its integration allows predicting and describing several rheological behaviors, in stationary and nonstationary conditions, including viscous (Newtonian) and frictional (Coulombian) regimes, as well as elastic linear (Hookean and Maxwellian) and nonlinear behaviors. Despite its simplicity, since it involves only three independent parameters, the model is in very close agreement with experiments. Moreover, within experimental errors, the values of these parameters are independent of the type of test used to determine them, evidence of the self-consistency of the model

Vibration-induced compaction of granular suspensions

We investigate the compaction dynamics of vibrated granular suspensions using both digital imaging technique and MRI measurements. Starting from initialy loose packings, our experimental data suggest the existence of two stages in the compaction dynamics: a fast stage at short times where a rising compaction front propagates through the granular suspension and a slow stage at large times where the packing compacts slowly and homogeneously. The compaction dynamics in each stage can be well fitted to usual stretched exponential laws with stretching exponents equal to 2 and 0.45, respectively. The transition time between these two stages, τ c , depends on the fluid viscosity, vibration intensity and grain diameter. We show that τ c −1 and the velocity of the front decrease roughly linearly with the lubrication Peclet number, Pe lub related to the competition between the lubrication stress induced by vibrations and the granular pressure

Using good vibrations: Melting and controlled shear jamming of dense granular suspensions

International audienceFlows of suspensions can be blocked when the suspended particles are densely packed. This makes their formulation and their transport challenging in the industry. In this paper, we study the impact of vibrations on the behavior of dense granular suspensions prepared at a volume fraction above their jamming volume fraction but below the particle assembly random close packing. Vibrations are shown to have a strong effect on their rheological properties and to tune their transition from solidlike to liquidlike behavior. We study suspensions of rough silica particles in a Newtonian fluid. In the absence of vibrations, they have a solidlike behavior: they flow only above a yield stress. Particles are confined by the liquid interface, and the yield stress is of the frictional origin. When vibrations are applied, the yield stress vanishes to give rise to a liquidlike pseudo-Newtonian behavior at a low shear rate. Using shear-reversal experiments, we show that these liquidlike vibrated suspensions of frictional particles behave like nonvibrated suspensions of frictionless particles. As the shear rate is increased, we observe a shear thickening of the vibrated suspensions, eventually leading to shear-jamming. The yield stress behavior is recovered, and vibrations have no more impact. We show that this shear thickening can be tuned by changing the vibration energy injected into the system. We, finally, propose a physical picture based on the competition between contact opening by vibration and contact formation by shear to account for these behaviors. In the framework of the Wyart and Cates [Phys. Rev. Lett. 112, 098302 (2014)] model, vibrations can be seen as introducing a thermal-like repulsive force, yielding a critical stress proportional to the vibration stress introduced by Hanotin et al. [J. Rheol. 59, 253–273 (2015)

Rheology of fiber suspensions using MRI

The suspensions of non-Brownian fibers are of interest for many applications. Although many studies concerning suspensions are available in the literature, most of them concern suspensions of spherical particles. In this paper, global and local rheology of fiber suspensions are explored near the jamming transition. A critical volume fraction is extracted from the experimental data. The value of this critical volume fraction is in agreement with the expected value of the concentration of rigid rods above which the isotropic phase becomes unstable. Moreover, non-reversible effects of the shearing are observed in flow curves because of the non-Brownian behavior of the studied fibers