5,696 research outputs found
From one cell to the whole froth: a dynamical map
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
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.
The effect of the SiO 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
, 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?
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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