80 research outputs found
Crystallisation in a granular material
The athermal and dissipative nature of packings of grains is still challenging our understanding of their compaction
as well as their crystallisation. For instance, some beads poured in a container get jammed in random disordered con gurations,
which cannot be denser than 64%, the random closed packing (RCP) limit. Remarkably it has been suggested that the
RCP bound is saturated with dense patterns of beads aggregated into polytetrahedral structures. Yet when a suitable vibration
is applied, a packing of beads might start to order and some regular patterns appear.
We present new experiments on the crystallisation of the packing of beads. By extending tapping techniques, we have
obtained packings with volume fractions φ ranging from the RCP to the crystal (φ = 0.74). Computing tomography has been
used to scan the internal structure of large packings (≈200,000 beads). Voronoi and Delaunay space partitions on the grain
centres were performed to characterise the structural rearrangements during the crystallisation. This allows us to describe
statistical properties of the local volume uctuations and the evolution of the densest patterns of beads.
In terms of statistical description, a parameter based on the volume uctuations discloses different regimes during the
transition. In terms of geometry, we con rm that polytetrahedral dense clusters are ubiquitous at the RCP. We describe some
intrinsic features of these clusters such as rings of tetrahedra and show how they disappear as the crystal grows.
This experiment enlightens how an athermal system jammed in a complex frustrated con guration is gradually converted
into a periodic crystal
Geometrical frustration in amorphous and partially crystallized packings of spheres
We study the persistence of a geometrically frustrated local order inside partially crystallized packings of equal-sized spheres. Measurements by x-ray tomography reveal previously unseen grain scale rearrangements occurring inside large three-dimensional packings as they crystallize. Three successive structural transitions are detected by a statistical description of the local volume fluctuations. These compaction regimes are related to the disappearance of densely packed tetrahedral patterns of beads. Amorphous packings of monodisperse spheres are saturated with these tetrahedral clusters at Bernal's limiting density (ϕ≈64%). But, no periodic lattice can be built upon these patterns; they are geometrically frustrated and are thus condemned to vanish while the crystallization occurs. Remarkably, crystallization-induced grain rearrangements can be interpreted in terms of the evolution of key topological features of these aggregates
FCS-MBFLEACH: Designing an Energy-Aware Fault Detection System for Mobile Wireless Sensor Networks
Wireless sensor networks (WSNs) include large-scale sensor nodes that are densely distributed over a geographical region that is completely randomized for monitoring, identifying, and analyzing physical events. The crucial challenge in wireless sensor networks is the very high dependence of the sensor nodes on limited battery power to exchange information wirelessly as well as the non-rechargeable battery of the wireless sensor nodes, which makes the management and monitoring of these nodes in terms of abnormal changes very difficult. These anomalies appear under faults, including hardware, software, anomalies, and attacks by raiders, all of which affect the comprehensiveness of the data collected by wireless sensor networks. Hence, a crucial contraption should be taken to detect the early faults in the network, despite the limitations of the sensor nodes. Machine learning methods include solutions that can be used to detect the sensor node faults in the network. The purpose of this study is to use several classification methods to compute the fault detection accuracy with different densities under two scenarios in regions of interest such as MB-FLEACH, one-class support vector machine (SVM), fuzzy one-class, or a combination of SVM and FCS-MBFLEACH methods. It should be noted that in the study so far, no super cluster head (SCH) selection has been performed to detect node faults in the network. The simulation outcomes demonstrate that the FCS-MBFLEACH method has the best performance in terms of the accuracy of fault detection, false-positive rate (FPR), average remaining energy, and network lifetime compared to other classification methods
DEM simulation of experimental dense granular packing
In this study we present numerical analysis performed on the experimental results of sphere packings of mono-sized hard sphere whose packing fraction spans across a wide range of 0.59<Φ<0.72. Using X-ray Computed Tomography (XCT), we have full access to the 3D structure of the granular packings. Numerical analysis performed on thr data provides the first experimental proofs of how densification affects local order parameters. Furthermore by combining Discrete Element Method (DEM) and the experimental results from XCT, we investigate how the intergranular forces change with the onset of crystallization
Measuring intergranular force in granular media
A new method is proposed to measure intergranular forces in granular geomaterial from time-lapsehigh-resolution X-ray computed tomographyimaging using a grain trackingapproachand discrete element metho
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
Onset of mechanical stability in random packings of frictional spheres
Using sedimentation to obtain precisely controlled packings of noncohesive
spheres, we find that the volume fraction of the loosest
mechanically stable packing is in an operational sense well defined by a limit
process. This random loose packing volume fraction decreases with decreasing
pressure and increasing interparticle friction coefficient . Using
X-ray tomography to correct for a container boundary effect that depends on
particle size, we find for rough particles in the limit a new lower
bound, .Comment: significantly revised, published versio
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
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