P. Mills

Bio: P. Mills is an academic researcher from University of Paris. The author has contributed to research in topics: Shear rate & Shear stress. The author has an hindex of 9, co-authored 24 publications receiving 437 citations.

Settling of a Suspension of Hard Spheres

Abstract: We derive the average sedimentation rate, in the non-Brownian limit, for monodispersed spheres in systems ranging from dilute to concentrated. A collection of monodisperse hard spheres suspended in a viscous fluid constitutes a single scaled system. A mean-field approach was used to determine the energy balance during the sedimentation process. The model gives very accurately the volume fraction dependence for the velocities of the two shocks like fronts separating the suspension from the clear liquid and from the sediment.

Mechanisms for acoustic absorption in dry and weakly wet granular media.

Abstract: The dissipation of an elastic wave in dry and wet glass bead packings is measured using multiple sound scattering. The interplay of a linear viscoelastic loss and a nonlinear frictional one is observed in dry media. The Mindlin model provides a qualitative description of the experiment, but fails to quantitatively account for the data due to grain roughness. In weakly wet media, we find that the dissipation is dominated by a linear viscous loss due to the liquid films trapped at the grain surface asperities. Adding more liquid enables us to form the capillary menisci but does not increase the energy loss.

Apparent viscosity and particle pressure of a concentrated suspension of non-Brownian hard spheres near the jamming transition

Abstract: We consider the steady shear flow of a homogeneous and dense assembly of hard spheres suspended in a Newtonian viscous fluid. In a first part, a mean-field approach based on geometric arguments is used to determine the viscous dissipation in a dense isotropic suspension of smooth hard spheres and the hydrodynamic contribution to the suspension viscosity. In a second part, we consider the coexistence of transient solid clusters coupled to regions with free flowing particles near the jamming transition. The fraction of particles in transient clusters is derived through the Landau-Ginzburg concepts for first-order phase transition with an order parameter corresponding to the proportion of “solid” contacts. A state equation for the fraction of particle-accessible volume is introduced to derive the average normal stresses and a constitutive law that relates the total shear stress to the shear rate. The analytical expression of the average normal stresses well accounts for numerical or experimental evaluation of the particle pressure and non-equilibrium osmotic pressure in a dense sheared suspension. Both the friction level between particles and the suspension dilatancy are shown to determine the singularity of the apparent shear viscosity and the flow stability near the jamming transition. The model further predicts a Newtonian behavior for a concentrated suspension of neutrally buoyant particles and no shear thinning behavior in relation with the shear liquefaction of transient solid clusters.

Rheology and structure of granular materials near the jamming transition

Abstract: The shear stress of non-cohesive granular material in the vicinity of the jamming transition is supposed to be connected to the formation of transient rigid clusters of particles. The characteristics of these transient clusters are investigated as a function of the imposed pressure, the solid volume fraction and the shear rate. This is responsible for an increase of the shear stress for a vanishing shear rate, which leads to an instability close to the jamming transition. We discuss the consequences for stick-slip motion and flows down an inclined plane, in agreement with the observations. Then, the oscillation of the granular material between two jam-flow states generates fast velocity fluctuations which result in a mean frictional force proportional to the mean velocity relative to the jammed state in the flow direction. Accordingly the velocity field in a simple shear flow is governed by a Brinkman equation and any symmetry break favours a strain localization. This analysis might be extended to the case of granular pastes.

Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality

Abstract: Three-dimensional nanofibrous aerogels (NFAs) that are both highly compressible and resilient would have broad technological implications for areas ranging from electrical devices and bioengineering to damping materials; however, creating such NFAs has proven extremely challenging. Here we report a novel strategy to create fibrous, isotropically bonded elastic reconstructed (FIBER) NFAs with a hierarchical cellular structure and superelasticity by combining electrospun nanofibres and the fibrous freeze-shaping technique. Our approach causes the intrinsically lamellar deposited electrospun nanofibres to assemble into elastic bulk aerogels with tunable densities and desirable shapes on a large scale. The resulting FIBER NFAs exhibit densities of >0.12 mg cm(-3), rapid recovery from deformation, efficient energy absorption and multifunctionality in terms of the combination of thermal insulation, sound absorption, emulsion separation and elasticity-responsive electric conduction. The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional NFAs for various applications.

Unifying suspension and granular rheology.

Abstract: Using an original pressure-imposed shear cell, we study the rheology of dense suspensions. We show that they exhibit a viscoplastic behavior similarly to granular media successfully described by a frictional rheology and fully characterized by the evolution of the friction coefficient μ and the volume fraction ϕ with a dimensionless viscous number I(v). Dense suspension and granular media are thus unified under a common framework. These results are shown to be compatible with classical empirical models of suspension rheology and provide a clear determination of constitutive laws close to the jamming transition.

Granular Media: Between Fluid and Solid

Abstract: Foreword 1. Introduction 2. Interactions at the grain level 3. The granular solid: statics and elasticity 4. The granular solid: plasticity 5. Granular gases 6. The granular liquid 7. Immersed granular media 8. Erosion and sediment transport 9. Geomorphology References Index.