A two-species model of a two-dimensional sandpile surface: a case of asymptotic roughening

We present and analyze a model of an evolving sandpile surface in (2 + 1) dimensions where the dynamics of mobile grains ({\rho}(x, t)) and immobile clusters (h(x, t)) are coupled. Our coupling models the situation where the sandpile is flat on average, so that there is no bias due to gravity. We find anomalous scaling: the expected logarithmic smoothing at short length and time scales gives way to roughening in the asymptotic limit, where novel and non-trivial exponents are found.Comment: 7 Pages, 6 Figures; Granular Matter, 2012 (Online

Slow dynamics, aging, and glassy rheology in soft and living matter

We explore the origins of slow dynamics, aging and glassy rheology in soft and living matter. Non-diffusive slow dynamics and aging in materials characterised by crowding of the constituents can be explained in terms of structural rearrangement or remodelling events that occur within the jammed state. In this context, we introduce the jamming phase diagram proposed by Liu and Nagel to understand the ergodic-nonergodic transition in these systems, and discuss recent theoretical attempts to explain the unusual, faster-than-exponential dynamical structure factors observed in jammed soft materials. We next focus on the anomalous rheology (flow and deformation behaviour) ubiquitous in soft matter characterised by metastability and structural disorder, and refer to the Soft Glassy Rheology (SGR) model that quantifies the mechanical response of these systems and predicts aging under suitable conditions. As part of a survey of experimental work related to these issues, we present x-ray photon correlation spectroscopy (XPCS) results of the aging of laponite clay suspensions following rejuvenation. We conclude by exploring the scientific literature for recent theoretical advances in the understanding of these models and for experimental investigations aimed at testing their predictions.Comment: 22 pages, 5 postscript figures; invited review aricle, to appear in special issue on soft matter in Solid State Communication

On Self-Organized Criticality and Synchronization in Lattice Models of Coupled Dynamical Systems

Lattice models of coupled dynamical systems lead to a variety of complex behaviors. Between the individual motion of independent units and the collective behavior of members of a population evolving synchronously, there exist more complicated attractors. In some cases, these states are identified with self-organized critical phenomena. In other situations, with clusterization or phase-locking. The conditions leading to such different behaviors in models of integrate-and-fire oscillators and stick-slip processes are reviewed.Comment: 41 pages. Plain LaTeX. Style included in main file. To appear as an invited review in Int. J. Modern Physics B. Needs eps

Response properties in a model for granular matter

We investigate the response properties of granular media in the framework of the so-called {\em Random Tetris Model}. We monitor, for different driving procedures, several quantities: the evolution of the density and of the density profiles, the ageing properties through the two-times correlation functions and the two-times mean-square distance between the potential energies, the response function defined in terms of the difference in the potential energies of two replica driven in two slightly different ways. We focus in particular on the role played by the spatial inhomogeneities (structures) spontaneously emerging during the compaction process, the history of the sample and the driving procedure. It turns out that none of these ingredients can be neglected for the correct interpretation of the experimental or numerical data. We discuss the problem of the optimization of the compaction process and we comment on the validity of our results for the description of granular materials in a thermodynamic framework.Comment: 22 pages, 35 eps files (21 figures

Temperature and humidity within a mobile barchan sand dune, implications for microbial survival

International audienc

Onset of Granular Flows by Local and Global Forcing

This thesis focuses on the onset of granular flows and memory effects in granular materials under local and global forcing conditions. Global flows are induced in a shear cell of Taylor-Couette type by moving a boundary wall. We find that how a granular shear flow starts depends strongly on the prior shear direction. We observe that when the shear direction is reversed, the material goes through a transient period during which the material compacts, the shear force is small, and the shear band is wide. Three dimensional confocal imaging of particle rearrangements during shear reversal shows that bulk and surface flows are comparable. Local flows are induced by forcing a rod into a fluid immersed granular bed with various preparation methods. Particle rearrangements are observed in 3D by confocal microscopy and by moving a laser sheet through the sample. Image analysis indicates that rearrangements spread farthest not directly under the penetrometer but in a ring around the penetrometer. In addition, the direction of preformed stress chains in the material influences the particle rearrangements. Material compressed from one side exhibits anisotropic particle rearrangements under penetrometer testing

Resolution of grain scale interactions using the Discrete Element Method

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006.Includes bibliographical references (p. 201-221).Granular materials are an integral part of many engineering systems. Currently, a popular tool for numerically investigating granular systems is the Discrete Element Method (DEM). Nearly all implementations of the DEM, however, use spheres to represent particles despite mounting evidence showing that shape at multiple scales (sphericity, angularity, and friction) plays a role in granular material behavior. This thesis contributes a new non-spherical representation to model particles as ellipsoidal bodies. This is validated and benchmarked against current representations and is shown to have attractive computational efficiency and numerical stability. A numerical study of the formation of heaps using spheres and ellipsoids both validates the ellipsoid representation and illustrates shape-induced behavioral differences. Resolution of shape is extended by a new algorithm for a hierarchical, multi-scale representation of convex particle surface characteristics. Two applications are offered: (1) a micro-asperity model is used to demonstrate pair-wise interlocking, and (2) a surface-based cohesive contact law is validated using a series of virtual numerical pull-off tests, which agree well with experimental findings. An explicit quadrature algorithm based on quaternion rotation is developed and shown to more accurately determine rotational orientation with less computational effort than other common algorithms for integrating finite rotations.(cont.) Finally, a contact resolution algorithm between discrete elements and a polyhedral boundary is developed and shown to scale in O(M + N) versus common algorithms with scaling of O(NM), where N is the number of discrete elements and M the number of faces on the polyhedral boundary. These developments are illustrated with numerical studies to simulate the blending kinetics of cohesive, micron-scale pharmaceutical powders in V-shaped and cylindrical bench-scale blenders.by Scott Matthew Johnson.Ph.D

Strongly Correlated Random Interacting Processes

The focus of the workshop was to discuss the recent developments and future research directions in the area of large scale random interacting processes, with main emphasis in models where local microscopic interactions either produce strong correlations at macroscopic levels, or generate non-equilibrium dynamics. This report contains extended abstracts of the presentations, which featured research in several directions including selfinteracting random walks, spatially growing processes, strongly dependent percolation, spin systems with long-range order, and random permutations