Sébastien Kiesgen de Richter

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

In 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 η0 = GτR, where η0 is the low shear viscosity of the suspension (Newtonian plateau), G is the shear modulus (modulus of rigidity) and τR is the longest relaxation time. The two macroscopic parameters, G and τR, 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 σf and γc through the relation G = σf/γc where σf is the frictional stress and γc the critical strain corresponding to the onset of contact breaking. In a second step we show that the relaxation time τR, determined by mechanical spectroscopy, matches precisely with the decorrelation time τD, derived independently from local optical measurements. We show that, similarly to the plateau viscosity η0, both are controlled by the dimensionless Peclet number Pelub in that τR ∝ 1/Pelub and τD ∝ 1/Pelub where Pelub = σlub/σf has been defined in a previous article as the ratio of the lubrication stress, induced by vibrations, and the frictional stress [Hanotin et al., Phys. Rev. Lett., 2012, 108, 198301].

Shear-banding and Taylor-Couette instability in thixotropic yield stress fluids

We consider the flow of thixotropic yield stress fluids between two concentric cylinders. To account for the fluid thixotropy, we use Houskas model [Houska, Ph.D. thesis, Czech Technical University, Prague, 1981] with a single structural parameter driven by a kinetic equation. Because of the yield stress and the geometric inhomogeneity of the stress, only a part of the material in the gap may flow. Depending on the breakdown rate of the structural parameter, the constitutive relation can lead to a nonmonotonic flow curve. This nonmonotonic behavior is known to induce a discontinuity in the slope of the velocity profile within the flowing material, called shear banding. Thus, for fragile structures, a shear-banded flow characterized by a very sharp transition between the flowing and the static regions may be observed. For stronger structures, the discontinuity disappears and a smooth transition between the flowing and the static regions is observed. The consequences of the thixotropy on the linear stability of the azimuthal flow are studied in a large range of parameters. Although the thixotropy allows shear banding in the base flow, it does not modify fundamentally the linear stability of the Couette flow compared to a simple yield stress fluid. The apparent shear-thinning behavior depends on the thixotropic parameters of the fluid and the results about the onset of the Taylor vortices in shear-thinning fluids are retrieved. Nevertheless, the shear banding modifies the stratification of the viscosity in the flowing zone such that the critical conditions are mainly driven by the width of the flowing region.

Bulk and local rheology in a dense and vibrated granular suspension

In this paper, we investigate experimentally the dynamics of particles in dense granular suspensions when both shear and external vibrations are applied. We study in detail how vibrations affect particle reorganization at the local scale and modify the apparent rheology. The nonlocal nature of the rheology when no vibrations are applied is evidenced, in agreement with previous numerical studies from the literature. It is also shown that vibrations induce structural reorganizations, which tend to homogenize the system and cancel the nonlocal properties.

Rheological behavior of vibrated bimodal granular suspensions: a free volume approach

In this paper, we investigate the dependence of the viscosity of vibrated bidisperse granular suspensions of spherical particles on the relative fraction of the two populations. At the same total volume fraction ϕ, the viscosity of the bidisperse suspension is lower than that of a monodisperse suspension. A unified viscosity model is derived using an effective free volume approach based on the definition of an equivalent mean diameter and a maximum packing fraction, both depending on the fraction of large particles ξ. The resulting model accounts for the viscosity of the suspending fluid ηf, the amplitude A, and the frequency f of the vibrations, for a constant ratio of the diameters of the beads λ = 5.3 and a fixed value of the total volume fraction ϕ ≈ 0.61. It led to a general equation describing both the rheology of monodisperse and bidisperse suspensions.

Overlapping Histogram method for testing Edward's Statistical Mechanics of Powders

We present a new way to test the applicability of Edward’s statistical mechanics of powders using overlapping volume histograms. We use this method to analyze data from numerical simulations and experiments of granular compaction. The volume histograms do indeed have the form predicted by the theory. This is a nontrivial success for the theory. We are also able to extract the entropy for a wide range of volumes.