Simple Model for Wet Granular Materials with Liquid Clusters

We propose a simple phenomenological model for wet granular media to take into account many particle interaction through liquid in the funicular state as well as two-body cohesive force by a liquid bridge in the pendular state. In the wet granular media with small liquid content, liquid forms a bridge at each contact point, which induces two-body cohesive force due to the surface tension. As the liquid content increases, some liquid bridges merge, and more than two grains interact through a single liquid cluster. In our model, the cohesive force acts between the grains connected by a liquid-gas interface. As the liquid content increases, the number of grains that interact through the liquid increases, but the liquid-gas interface may decrease when liquid clusters are formed. Due to this competition, our model shows that the shear stress has a maximum as a function of the liquid-content.Comment: 6 pages, 8 figures. Discussion is updated. Accepted for publication in EP

Internal relaxation time in immersed particulate materials

We study the dynamics of the solid to liquid transition for a model material made of elastic particles immersed in a viscous fluid. The interaction between particle surfaces includes their viscous lubrication, a sharp repulsion when they get closer than a tuned steric length and their elastic deflection induced by those two forces. We use Soft Dynamics to simulate the dynamics of this material when it experiences a step increase in the shear stress and a constant normal stress. We observe a long creep phase before a substantial flow eventually establishes. We find that the typical creep time relies on an internal relaxation process, namely the separation of two particles driven by the applied stress and resisted by the viscous friction. This mechanism should be relevant for granular pastes, living cells, emulsions and wet foams

Ahaggar

Le Pays (P. Rognon) L’Ahaggar (ou Hoggar en arabe) est le nom historique du pays contrôlé jadis par la Confédération des Touaregs Kel Ahaggar. Ce territoire correspond à une énorme « boutonnière » de socle précambrien, parsemée de quelques massifs volcaniques tertiaires et quaternaires, et délimitée au nord, à l’est et au sud par les falaises, imposantes ou dégradées, des reliefs de côtes des grès primaires de l’« enceinte tassilienne ». Au-delà, les plateaux des Tassilis appartiennent à une ..

Ahaggar

Le Pays L’Ahaggar (ou Hoggar en arabe) est le nom historique du pays contrôlé jadis par la Confédération des Touaregs Kel Ahaggar. Ce territoire correspond à une énorme « boutonnière » de socle précambrien, parsemée de quelques massifs volcaniques tertiaires et quaternaires, et délimitée au nord, à l’est et au sud par les falaises, imposantes ou dégradées, des reliefs de côtes des grès primaires de l’« enceinte tassilienne ». Au-delà, les plateaux des Tassilis appartiennent à une famille de p..

Rheophysics of dense granular materials : Discrete simulation of plane shear flows

We study the steady plane shear flow of a dense assembly of frictional, inelastic disks using discrete simulation and prescribing the pressure and the shear rate. We show that, in the limit of rigid grains, the shear state is determined by a single dimensionless number, called inertial number I, which describes the ratio of inertial to pressure forces. Small values of I correspond to the quasi-static regime of soil mechanics, while large values of I correspond to the collisional regime of the kinetic theory. Those shear states are homogeneous, and become intermittent in the quasi-static regime. When I increases in the intermediate regime, we measure an approximately linear decrease of the solid fraction from the maximum packing value, and an approximately linear increase of the effective friction coefficient from the static internal friction value. From those dilatancy and friction laws, we deduce the constitutive law for dense granular flows, with a plastic Coulomb term and a viscous Bagnold term. We also show that the relative velocity fluctuations follow a scaling law as a function of I. The mechanical characteristics of the grains (restitution, friction and elasticity) have a very small influence in this intermediate regime. Then, we explain how the friction law is related to the angular distribution of contact forces, and why the local frictional forces have a small contribution to the macroscopic friction. At the end, as an example of heterogeneous stress distribution, we describe the shear localization when gravity is added.Comment: 24 pages, 19 figure

Soft Dynamics simulation: 2. Elastic spheres undergoing a T1 process in a viscous fluid

Robust empirical constitutive laws for granular materials in air or in a viscous fluid have been expressed in terms of timescales based on the dynamics of a single particle. However, some behaviours such as viscosity bifurcation or shear localization, observed also in foams, emulsions, and block copolymer cubic phases, seem to involve other micro-timescales which may be related to the dynamics of local particle reorganizations. In the present work, we consider a T1 process as an example of a rearrangement. Using the Soft dynamics simulation method introduced in the first paper of this series, we describe theoretically and numerically the motion of four elastic spheres in a viscous fluid. Hydrodynamic interactions are described at the level of lubrication (Poiseuille squeezing and Couette shear flow) and the elastic deflection of the particle surface is modeled as Hertzian. The duration of the simulated T1 process can vary substantially as a consequence of minute changes in the initial separations, consistently with predictions. For the first time, a collective behaviour is thus found to depend on another parameter than the typical volume fraction in particles.Comment: 11 pages - 5 figure

Computer simulation of model cohesive powders: Plastic consolidation, structural changes and elasticity under isotropic loads

The quasistatic behavior of a simple 2D model of a cohesive powder under isotropic loads is investigated by Discrete Element simulations. The loose packing states, as studied in a previous paper, undergo important structural changes under growing confining pressure P, while solid fraction \Phi irreversibly increases by large amounts. The system state goes through three stages, with different forms of the plastic consolidation curve \Phi(P*), under growing reduced pressure P* = Pa/F0, defined with adhesion force F0 and grain diameter a. In the low-confinement regime (I), plastic compaction is negligible, and the structure is influenced by the assembling process. The following stage (regime II) is independent of initial conditions. The void ratio varies linearly with log P, as described in the engineering literature. In the last stage of compaction (III), a maximum solid fraction is approached, and properties of cohesionless granular packs are retrieved. Under consolidation, while the fractal range of density correlations decreases, force patterns reorganize, and elastic moduli increase by a large factor. Plastic deformation events correspond to very small changes in the network topology, while the denser regions tend to move like rigid bodies. Elastic properties are dominated by the bending of thin junctions in loose systems. For growing RR those tend to reduce to particle chains, the folding of which, rather than tensile ruptures, controls plastic compaction

Change over time of COVID-19 hospital presentation in Northern Italy

none40After the first autochthonous case described on February 19, also in Italy the Severe Acute Respiratory Syndrome CoronaVirus 2 (SARSCoV-2) infection rapidly circulated, mainly in the Northern regions of the country. The earliest reports on Coronavirus disease-19 (COVID-19) have described worldwide a high prevalence of severe respiratory illness [1]. A suggestive feature of COVID-19 has been a rapid progression of the respiratory impairment, leading to acute respiratory distress syndrome (ARDS) and often requiring ventilation support [2]. To date, whether clinical features at hospital presentation and outcome of COVID-19 have changed over the outbreak course is unknown. We explored this issue in a multicenter cohort of patients hospitalized for COVID-19 in Northern Italy.mixedPatti G.; Mennuni M.; Della Corte F.; Spinoni E.; Sainaghi P. P.; COVID-UPO Clinical Team; Azzolina D; Hayden E; Rognon A; Grisafi L; Colombo C; Lio V; Pirisi M; Vaschetto R; Aimaretti G; Krengli M; Avanzi GC; Balbo PE; Capponi A; Castello LM; Bellan M; Malerba M; Garavelli PL; Zeppegno P; Savoia P; Chichino G; Olivieri C; Re R; Maconi A; Comi C; Roveta A; Bertolotti M; Carriero A; Betti M; Mussa M; Borrè S; Cantaluppi V; Cantello R; Bobbio F; GavellI F.Patti, G.; Mennuni, M.; Della Corte, F.; Spinoni, E.; Sainaghi, P. P.; COVID-UPO Clinical, Team; Azzolina, D; Hayden, E; Rognon, A; Grisafi, L; Colombo, C; Lio, V; Pirisi, M; Vaschetto, R; Aimaretti, G; Krengli, M; Avanzi, Gc; Balbo, Pe; Capponi, A; Castello, Lm; Bellan, M; Malerba, M; Garavelli, Pl; Zeppegno, P; Savoia, P; Chichino, G; Olivieri, C; Re, R; Maconi, A; Comi, C; Roveta, A; Bertolotti, M; Carriero, A; Betti, M; Mussa, M; Borrè, S; Cantaluppi, V; Cantello, R; Bobbio, F; Gavelli, F