ABSTRACT
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From the beach to the sandbox, granular experiments begin at an early age. Even to a child, it is clear that sand behaves differently than ordinary fluids and solids. When poured, sand flows like a fluid, but it can form a pile. When stationary, it supports the weight of a person like a solid, but patterns can form on the surface like frozen ripples on water (Figure 2.1). Dissipation and purely repulsive
interactions between grains cause these effects. In an ordinary fluid or solid, the molecules interact elastically through both long-range attraction and short-range repulsion. When two water molecules collide, the total shared kinetic and potential energy is conserved. When two macroscopic grains interact, energy is lost from the shared kinetic and potential energy. The lost energy flows to internal degrees of freedom as heat or radiates away as sound waves. Either way the sum of the translational kinetic energy and potential energy of the particles decreases during every interaction. Water molecules attract, leading to surface tension, solidification, and elastic behavior. Non-cohesive grains like dry sand only repel, leading to diffuse non-self-leveling surfaces, weak solids, and plastic behavior, Figure 2.1a. A strong confining pressure can mitigate some of these differences.
