5,696 research outputs found

    From one cell to the whole froth: a dynamical map

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    We investigate two and three-dimensional shell-structured-inflatable froths, which can be constructed by a recursion procedure adding successive layers of cells around a germ cell. We prove that any froth can be reduced into a system of concentric shells. There is only a restricted set of local configurations for which the recursive inflation transformation is not applicable. These configurations are inclusions between successive layers and can be treated as vertices and edges decorations of a shell-structure-inflatable skeleton. The recursion procedure is described by a logistic map, which provides a natural classification into Euclidean, hyperbolic and elliptic froths. Froths tiling manifolds with different curvature can be classified simply by distinguishing between those with a bounded or unbounded number of elements per shell, without any a-priori knowledge on their curvature. A new result, associated with maximal orientational entropy, is obtained on topological properties of natural cellular systems. The topological characteristics of all experimentally known tetrahedrally close-packed structures are retrieved.Comment: 20 Pages Tex, 11 Postscript figures, 1 Postscript tabl

    Packing structure of a two-dimensional granular system through the jamming transition

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    We have performed a novel experiment on granular packs composed of automatically swelling particles. By analyzing the Voronoi structure of packs going through the jamming transition, we show that the local configuration of a jamming pack is strikingly similar to that of a glass-forming liquid, both in terms of their universal area distribution and the process of defect annealing. Furthermore, we demonstrate that an unambiguous structural signature of the jamming transition can be obtained from the pair correlation functions of a pack. Our study provides insights into the structural properties of general jamming systems.Comment: 5 pages, 3 figure

    Electronic structure and the minimum conductance of a graphene layer on SiO2 from density-functional methods.

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    The effect of the SiO2_2 substrate on a graphene film is investigated using realistic but computationally convenient energy-optimized models of the substrate supporting a layer of graphene. The electronic bands are calculated using density-functional methods for several model substrates. This provides an estimate of the substrate-charge effects on the behaviour of the bands near EFE_F, as well as a variation of the equilibrium distance of the graphene sheet. A model of a wavy graphene layer is examined as a possible candidate for understanding the nature of the minimally conducting states in graphene.Comment: 6 pages, 5 figure

    Air Traffic Control Safety Indicators: What is Achievable?

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    European Air Traffic Control is extremely safe. The drawback to this safety record is that it is very difficult to estimate what the ‘underlying’ accident rate for mid-air collisions is now, or to detect any changes over time. The aim is to see if it possible to construct simple ATC safety indicators that correlate with this underlying accident rate. A perfect indicator would be simple to comprehend and capable of being calculated by a checklist process. An important concept is that of ‘system control’: the ability to determine the outcome against reasonably foreseen changes and variations of system parameters. A promising indicator is ‘Incident Not Resolved by ATC’, INRA, incidents in which the ground ATC defences have been ‘used up’. The key question is: if someone says he or she knows how to make a good estimate of the underlying accident rate, then how could this claim be tested? If it correlates very well with INRA, then what would be the argument for saying that it is a better indicator
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