User:Jaksic/Documents/Misc
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Granular Materials
Granular materials are large assemblies of solid macroscopic particles characterized by a loss of energy whenever the particles interact. If they are noncohesive, the forces between them are strictly repulsive. The particles are usually surrounded by a fluid, most often air, which may play a role in the dynamics of the systems. The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations. Thus, the lower size limit for grains in granular material is about 1 μm. Examples of such materials include sand, stones, soil, ores, pharmaceuticals, and variety of chemicals. Powders are a special class of granular material due to their small particle size, which makes them more cohesive and more easily suspended in a gas. Granular materials are commercially important in applications as diverse as pharmaceutical industry, agriculture, and energy production. Granular materials are ubiquitous in nature and are the second-most manipulated material in industry (the first one is water).
At the root of the unique status of granular materials are two characteristic: ordinary temperature plays no role, and the interactions between grains are dissipative because of static friction and the inelasticity of collisions. There are no long-range interactions between individual grains or between individual grains and the walls of a confining container. Depending on the average energy of the individual grains they may exhibit the properties of solids, liquids, or gases. When the average energy of the individual grains is low and the grains are fairly stationary relative to each other, the granular material acts like a solid. When the granular matter is driven and energy is fed into the system (such as by shaking) such that the grains are not in constant contact with each other, the granular material is said to fluidize and enter a liquid-like state. If the granular material is driven harder such that contacts between the grains become highly infrequent, the material enters a gaseous state.
Yet despite this seeming simplicity, a granular material behaves differently from any of the other familiar forms of matter - solids, liquids, or gases. For instance one can cite internal stress fluctuations, strain localization, non-Newtonian rheology, spontaneous clusterization, size segregation or spatial pattern creations. All these phenomena have no equivalent in classical solid- or liquid-state physics. Granular materials dissipate energy quickly, so techniques of statistical mechanics that assume conservation of energy are of limited use. Therefore, granular material should be considered an additional state of matter in its own right. Attempts toward understanding and controlling both static and dynamic properties of granular materials are thus of highest interest to many fields of physics, applied sciences and engineering.
We are generally interested in understanding of the cooperative dynamics of powder and relationship between the macroscopic behavior of granular materials and their microstructures.
Upward penetration of grains through a granular medium
We study experimentally the creeping penetration of guest (percolating) grains through densely packed granular media in two dimensions. The evolution of the system of the guest grains during the penetration is studied by image analysis. To quantify the changes in the internal structure of the packing we use Voronoi tessellation and certain shape factor which is a clear indicator of the presence of different underlying substructures (domains). We first consider the impact of the effective gravitational acceleration on upward penetration of grains. It is found that the higher effective gravity increases the resistance to upward penetration and enhances structural organization in the system of the percolating grains. We also focus our attention on the dependence of the structural rearrangements of percolating grains on some parameters like the polydispersity and the initial packing fraction of the host granular system. It is found that anisotropy of penetration is larger in the monodisperse case than in the bidisperse one, for the same value of packing fraction of host medium. Compaction of initial host granular packing also increases anisotropy of penetration of guest grains. When a binary mixture of large and small guest grains is penetrated into host granular medium, we observe a size segregation patterns.
