Research

Current

  • The plastic deformation of amorphous solids is an active field of research. Differently from crystals, they do not present topological defects to guide our physical intuitions about deformation. However, it has been proposed that the irreversible deformation of amorphous solids is due to the movements of small groups of particles. We thus investigate the microscopic dynamics of model amorphous solids subject to stress to understand the nature of these structures.

  • We extend this study as a function of the density of the solid. Our model materials experiment a change of phase when the density is decreased, in such a way that they loose rigidity (and they are no longer solids) at a critical density, in the so called jamming point. This jamming (unjamming) transition has been proposed to be shared by a wide variety of amorphous materials, from crystals to foams or emulsion, and has constituted a conceptual frame for many studies concerning this class of materials.

  • Given that granular materials interact only via contact forces, it has long been acknowledged that their structural features are important to understand their physical properties. We consider the grains and their contacts as a set of nodes and links making up a graph or network. The field of complex networks then offers a great variety of tools to investigate the topology of these abstractly defined structures.

  • The problem of clogging in the flow of particulate materials is important from practical and fundamental reasons. From the practical point of view, many industrial processes depend on the transport of particulates, from mining to agriculture.

  • From a more basic perspective, the flow of discrete particles differs from that of familiar fluids and is not currently understood under a comprehensive theoretical frame.

  • Considering as a model the flow from a silo, one can tune the flow from continuous to intermitent to jammed just by reducing the size of the outlet. The dynamics of particle movements differ widely in each of these states. The jammed state (the transition in this case is obscured by the complicated dynamics) is reached when (for small outlet sizes) an arch of particles blocks the exit. One of our goals is to understand this arching process.

  • Curiously, the clogging in silos has been found to share similarities with the "flow" of pedestrians in crowd dynamics studies. In this case pedestrians are the discrete units that exit a room under different conditions. When pedestrians have relatively small velocities, they exit the room in a continuous flow. Under panic, instead (when they are forced to move fast) the flow is intermittent and clogging can develop.

Past

My thesis research focused in the study of dense granular materials from a “microscopic” point of view, modelling the individual movements and the interactions of the components of the system by means of molecular dynamics simulations (DEM method). To this end I wrote a DEM code from scratch (in C language) and used it through my research to analyse the complex behaviour of granular materials in different geometries. The main topics that I have studied are silo discharge in a wide range of aperture size, silo clogging, pile formation, tapped deposits, sheared flow and isotropic compression.

In the geometry of a 2D silo discharged by gravity I have analysed the influence of the aperture size in macroscopic variables such as the flow and the density of the material, obtaining very good agreement with experiments. The existence of an intermittent flow region and a region of clogging for orifices which size is similar to that of the grains have also been studied and found to agree with experiments.

I have as well studied the silo from a microscopic point of view, considering the movement of the grains as a process of self-diffusion. Starting from the very beginning of the discharge process I have calculated the distribution function of the grains velocities and the mean squared displacement. From these measurements it is possible to asses the existence of a transitory during which the diffusion of grains can be considered anomalous: fat tails appear in the distribution functions of the velocities and the mean squared displacement is not linear in time. This transitory evolves rapidly to a stationary state characterized by gaussian distribution functions and MSD linear in time. The transition to the stationary state is slower for orifices which size is similar to that of the grains.

I have studied tapped deposits of grains focusing in the process of compaction and in the analysis of the structures that arise inside the medium as functions of the applied excitation. In this work the statistical properties of these structures (or bridges) was related to properties of the packing, such as the density.

Pile formation is a much studied process in granular media since the final state is known to depend on details such as whether grains are poured from an extended or a punctual source. I have studied the formation of piles in the presence of a thin vertical wall (two semi-piles) and how the ordering induced by this wall influences the stress dip usually found in piles.

The last part of my thesis consisted in the analysis of the structural properties of granular packing using the concepts and methods of complex networks, considering grains as nodes and contacts among them as edges. The system was a bidisperse medium in two dimensions, isotropically compressed by four moving walls. The structural properties were analysed during the jamming transition and it was found that the topology of the system changes dramatically at the transition point. In particular, it was found that the number of triangles (closed loops formed by three edges) behave as an order parameter: it grows following a power law of the distance to the transition point.