Discrete Material Media Under Vertical Perturbation
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Discrete Material Media Under Vertical Perturbation José Damas1, Iker Zurigel1 and Diego Maza1 1 Physics and Applied Mathematic Department, Universidad de Navarra, Irunlarrea S/N, Pamplona, Navarra, Spain. ABSTRACT In this work we present new experimental results about the packing fraction of a granular monolayer excited by taps in the gravity direction. The role of large amplitude of tapping is discussed. Also, the relationship between packing fraction and the number of particles in the sample is analyzed. INTRODUCTION Granular materials are large aggregates of macroscopic particles, interacting between them through dissipative contact forces, although occasionally, the existence of interstitial fluid introduces some degree of cohesion. Granular media are ubiquitous in industries such as pharmacy, mines, agriculture, etc. and are the second most important manipulating material just behind the water in humankind. This kind of materials are mainly stored in silos or piles, rotated and vibrated in big horns, mixed in drums or tumblers, etc. For this reason it is important to understand the physics behind the behaviour displayed by these materials, in particular, one of their most basic and important properties, the granular packing. In nature, temperature plays an important role at molecular scales, allowing the media to explore the landscape of energies (phase space) around certain equilibrium state. However, in the case granular materials, the energy scale introduced by its typical size is so small that one may consider them as “athermal” (mg>>kT). This means that every static configuration reached by any granular ensemble is intrinsically in a metastable state, and an external input of energy is necessary to explore nearby configurations. Indeed, granular materials are a canonical example of out of equilibrium systems [1]. Some years ago, S. Edwards et al [2], introduced the idea of apply a gently perturbation to a metastable granular configuration to allow the system explore the landscape of spatial configurations compatible with a certain volume V. Hence, in this framework, volume plays a similar role of energy in a thermal system and makes it (or the packing fraction, ) a state variable for certain statistical description. Simultaneously, “the Chicago group” [3] developed an experimental setup to verify the behaviour of the packing fraction against a tapping perturbation. They found that the packing fraction have a monotonous decay against an increasing amplitude of the external excitation. Nevertheless, some numerical results [4] suggested that if the applied perturbation is sufficiently high, the packing fraction increases after reaching a minimum ( min). In order to confirm this, we performed new experiments and found that the packing fraction is not monotonous [5]. Accordingly, it is possible to find two different configurations with the same packing fraction but using different perturbations. This result implies that another “state” variable must be considered to complete a statistical description. Recent n
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