577 research outputs found
The Geometrical Structure of Disordered Sphere Packings
The three dimensional structure of large packings of monosized spheres with
volume fractions ranging between 0.58 and 0.64 has been studied with X-ray
Computed Tomography. We search for signatures of organization, we classify
local arrangements and we explore the effects of local geometrical constrains
on the global packing. This study is the largest and the most accurate
empirical analysis of disordered packings at the grain-scale to date with over
140,000 sphere coordinates mapped. We discuss topological and geometrical ways
to characterize and classify these systems, and discuss implications that local
geometry can have on the mechanisms of formation of these amorphous structures.Comment: 15 pages; 16 figure
Investigating the Geometrical Structure of Disordered Sphere Packings
Bead packs of up to 150,000 mono-sized spheres with packing densities ranging
from 0.58 to 0.64 have been studied by means of X-ray Computed Tomography.
These studies represent the largest and the most accurate description of the
structure of disordered packings at the grain-scale ever attempted. We
investigate the geometrical structure of such packings looking for signatures
of disorder. We discuss ways to characterize and classify these systems and the
implications that local geometry can have on densification dynamics.Comment: 3 figures, 9 page
Local and Global relations between the number of contacts and density in monodisperse sphere packs
The topological structure resulting from the network of contacts between
grains (\emph{contact network}) is studied for large samples of monosized
spheres with densities (fraction of volume occupied by the spheres) ranging
from 0.59 to 0.64. We retrieve the coordinates of each bead in the pack and we
calculate the average coordination number by using three different methods. We
show that, in the range of density investigated, the coordination number is
larger than 4 and it increases with the packing fraction. At local level we
also observe a positive correlation between local packing fraction and number
of neighbors. We discover a dependence between the local densities of
configurations with few neighbors in contact and the global sample-denities.
This might indicate that local configurations with small number of neighbors
are able to deform plastically when the sample is compactifying.
PACS: 45.70.-n, Granular Systems; 45.70.Cc, Static sandpiles; Granular
Compaction.Comment: 10 pages, 6 figure
Experimental investigation of the mechanical stiffness of periodic framework-patterned elastomers
Recent advances in the cataloguing of three-dimensional nets mean a systematic search for framework structures with specific properties is now feasible. Theoretical arguments about the elastic deformation of frameworks suggest characteristics of mechanically isotropic networks. We explore these concepts on both isotropic and anisotropic networks by manufacturing porous elastomers with three different periodic net geometries. The blocks of patterned elastomers are subjected to a range of mechanical tests to determine the dependence of elastic moduli on geometric and topological parameters. We report results from axial compression experiments, three-dimensional X-ray computed tomography imaging and image-based finite-element simulations of elastic properties of framework-patterned elastomers
Effects of Synaptic and Myelin Plasticity on Learning in a Network of Kuramoto Phase Oscillators
Models of learning typically focus on synaptic plasticity. However, learning
is the result of both synaptic and myelin plasticity. Specifically, synaptic
changes often co-occur and interact with myelin changes, leading to complex
dynamic interactions between these processes. Here, we investigate the
implications of these interactions for the coupling behavior of a system of
Kuramoto oscillators. To that end, we construct a fully connected,
one-dimensional ring network of phase oscillators whose coupling strength
(reflecting synaptic strength) as well as conduction velocity (reflecting
myelination) are each regulated by a Hebbian learning rule. We evaluate the
behavior of the system in terms of structural (pairwise connection strength and
conduction velocity) and functional connectivity (local and global
synchronization behavior). We find that for conditions in which a system
limited to synaptic plasticity develops two distinct clusters both structurally
and functionally, additional adaptive myelination allows for functional
communication across these structural clusters. Hence, dynamic conduction
velocity permits the functional integration of structurally segregated
clusters. Our results confirm that network states following learning may be
different when myelin plasticity is considered in addition to synaptic
plasticity, pointing towards the relevance of integrating both factors in
computational models of learning.Comment: 39 pages, 15 figures This work is submitted in Chaos: An
Interdisciplinary Journal of Nonlinear Scienc
Reading Imagined Letter Shapes from the Mind's Eye Using Real-time 7 Tesla fMRI
We present a 7 Tesla fMRI proof-of-concept study of the first letter speller BCI that decodes imagined letter shapes from activity patterns in early visual cortical areas. New tools are developed to enable real-time population receptive field retinotopic mapping for encoding and decoding. Using two different letter shapes (H and T), classification performance of generated activity patterns during imagery reaches 80% accuracy in each individual. Using a denoising autoencoder, recognizable letter shapes could be reconstructed and displayed as feedback to participants in the scanner
Mechanical characterization of partially crystallized sphere packings
We study grain-scale mechanical and geometrical features of partially crystallized packings of frictional spheres, produced experimentally by a vibrational protocol. By combining x-ray computed tomography, 3D image analysis, and discrete element method simulations, we have access to the 3D structure of internal forces. We investigate how the network of mechanical contacts and intergranular forces change when the packing structure evolves from amorphous to near perfect crystalline arrangements. We compare the behavior of the geometrical neighbors (quasicontracts) of a grain to the evolution of the mechanical contacts. The mechanical coordination number Zm is a key parameter characterizing the crystallization onset. The high fluctuation level of Zm and of the force distribution in highly crystallized packings reveals that a geometrically ordered structure still possesses a highly random mechanical backbone similar to that of amorphous packings
An invariant distribution in static granular media
We have discovered an invariant distribution for local packing configurations
in static granular media. This distribution holds in experiments for packing
fractions covering most of the range from random loose packed to random close
packed, for beads packed both in air and in water. Assuming only that there
exist elementary cells in which the system volume is subdivided, we derive from
statistical mechanics a distribution that is in accord with the observations.
This universal distribution function for granular media is analogous to the
Maxwell-Boltzmann distribution for molecular gasses.Comment: 4 pages 3 figure
Extracting Structural Information of a Heteropolymer from Force-Extension Curves
We present a theory for the reverse analysis on the sequence information of a
single H/P two-letter random hetero-polymer (RHP) from its force-extension(f-z)
curves during quasi static stretching. Upon stretching of a self-assembled RHP,
it undergoes several structural transitions. The typical elastic response of a
hetero-polymeric globule is a set of overlapping saw-tooth patterns. With
consideration of the height and the position of the overlapping saw-tooth
shape, we analyze the possibility of extracting the binding energies of the
internal domains and the corresponding block sizes of the contributing
conformations.Comment: 5 figures 7 page
Minkowski tensors and local structure metrics: Amorphous and crystalline sphere packings
Robust and sensitive tools to characterise local structure are essential for investigations of granular or particulate matter. Often local structure metrics derived from the bond network are used for this purpose, in particular Steinhardt's bond-orientational order parameters ql . Here we discuss an alternative method, based on the robust characterisation of the shape of the particles' Voronoi cells, by Minkowski tensors and derived anisotropy measures. We have successfully applied these metrics to quantify structural changes and the onset of crystallisation in random sphere packs. Here we specifically discuss the expectation values of these metrics for simple crystalline unimodal packings of spheres, consisting of single spheres on the points of a Bravais lattice. These data provide an important reference for the discussion of anisotropy values of disordered structures that are typically of relevance in granular systems. This analysis demonstrates that, at least for sufficiently high packing fractions above φ > 0.61, crystalline sphere packs exist whose Voronoi cells are more anisotropic with respect to a volumetric moment tensor than the average value of Voronoi cell anisotropy in random sphere packs
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