10,856 research outputs found
Internal avalanches in models of granular media
We study the phenomenon of internal avalanching within the context of
recently introduced lattice models of granular media. The avalanche is produced
by pulling out a grain at the base of the packing and studying how many grains
have to rearrange before the packing is once more stable. We find that the
avalanches are long-ranged, decaying as a power-law. We study the distriution
of avalanches as a function of the density of the packing and find that the
avalanche distribution is a very sensitive structural probe of the system.Comment: 12 pages including 9 eps figures, LaTeX. To appear in Fractal
Turbulent-like fluctuations in quasistatic flow of granular media
We analyze particle velocity fluctuations in a simulated granular system
subjected to homogeneous quasistatic shearing. We show that these fluctuations
share the following scaling characteristics of fluid turbulence in spite of
their different physical origins: 1) Scale-dependent probability distribution
with non-Guassian broadening at small time scales; 2) Power-law spectrum,
reflecting long-range correlations and the self-affine nature of the
fluctuations; 3) Superdiffusion with respect to the mean background flow
Strain versus stress in a model granular material: a Devil's staircase
The series of equilibrium states reached by disordered packings of rigid,
frictionless discs in two dimensions, under gradually varying stress, are
studied by numerical simulations. Statistical properties of trajectories in
configuration space are found to be independent of specific assumptions ruling
granular dynamics, and determined by geometry only. A monotonic increase in
some macroscopic loading parameter causes a discrete sequence of
rearrangements. For a biaxial compression, we show that, due to the statistical
importance of such events of large magnitudes, the dependence of the resulting
strain on stress direction is a Levy flight in the thermodynamic limit.Comment: REVTeX, 4 pages, 5 included PostScript figures. New version altered
throughout text, very close to published pape
Force distribution in a scalar model for non-cohesive granular material
We study a scalar lattice model for inter-grain forces in static,
non-cohesive, granular materials, obtaining two primary results. (i) The
applied stress as a function of overall strain shows a power law dependence
with a nontrivial exponent, which moreover varies with system geometry. (ii)
Probability distributions for forces on individual grains appear Gaussian at
all stages of compression, showing no evidence of exponential tails. With
regard to both results, we identify correlations responsible for deviations
from previously suggested theories.Comment: 16 pages, 9 figures, Submitted to PR
Two-photon optics of Bessel-Gaussian modes
In this paper we consider geometrical two-photon optics of Bessel-Gaussian
modes generated in spontaneous parametric down-conversion of a Gaussian pump
beam. We provide a general theoretical expression for the orbital angular
momentum (OAM) spectrum and Schmidt number in this basis and show how this may
be varied by control over the radial degree of freedom, a continuous parameter
in Bessel-Gaussian modes. As a test we first implement a back-projection
technique to classically predict, by experiment, the quantum correlations for
Bessel-Gaussian modes produced by three holographic masks, a blazed axicon,
binary axicon and a binary Bessel function. We then proceed to test the theory
on the down-converted photons using the binary Bessel mask. We experimentally
quantify the number of usable OAM modes and confirm the theoretical prediction
of a flattening in the OAM spectrum and a concomitant increase in the OAM
bandwidth. The results have implications for the control of dimensionality in
quantum states.Comment: 8 pages, 10 figure
The Regulatory Functions of Calcium and the Potential Role of Calcium in Mediating Gravitational Responses in Cells and Tissues
The hypothesis that calcium plays an important part in regulating cellular response to gravity and to other environmental stimuli is explored. Topics covered include the role of calmodulin and other proteins, gravitropic responses, bone demineralization during space flight, and intracellular communication
Defect-mediated turbulence in systems with local deterministic chaos
We show that defect-mediated turbulence can exist in media where the
underlying local dynamics is deterministically chaotic. While many of the
characteristics of defect-mediated turbulence, such as the exponential decay of
correlations and a squared Poissonian distribution for the number of defects,
are identical to those seen in oscillatory media, the fluctuations in the
number of defects differ significantly. The power spectra suggest the existence
of underlying correlations that lead to a different and non-universal scaling
structure in chaotic media.Comment: 4 pages, 5 figure
Scaling Laws of Stress and Strain in Brittle Fracture
A numerical realization of an elastic beam lattice is used to obtain scaling
exponents relevant to the extent of damage within the controlled, catastrophic
and total regimes of mode-I brittle fracture. The relative fraction of damage
at the onset of catastrophic rupture approaches a fixed value in the continuum
limit. This enables disorder in a real material to be quantified through its
relationship with random samples generated on the computer.Comment: 4 pages and 6 figure
Mobile particles in an immobile environment: Molecular Dynamics simulation of a binary Yukawa mixture
Molecular dynamics computer simulations are used to investigate thedynamics
of a binary mixture of charged (Yukawa) particles with a size-ratio of 1:5. We
find that the system undergoes a phase transition where the large particles
crystallize while the small particles remain in a fluid-like (delocalized)
phase. Upon decreasing temperature below the transition, the small particles
become increasingly localized on intermediate time scales. This is reflected in
the incoherent intermediate scattering functions by the appearance of a plateau
with a growing height. At long times, the small particles show a diffusive
hopping motion. We find that these transport properties are related to
structural correlations and the single-particle potential energy distribution
of the small particles.Comment: 7 pages, 5 figure
Internal states of model isotropic granular packings. III. Elastic properties
In this third and final paper of a series, elastic properties of numerically
simulated isotropic packings of spherical beads assembled by different
procedures and subjected to a varying confining pressure P are investigated. In
addition P, which determines the stiffness of contacts by Hertz's law, elastic
moduli are chiefly sensitive to the coordination number, the possible values of
which are not necessarily correlated with the density. Comparisons of numerical
and experimental results for glass beads in the 10kPa-10MPa range reveal
similar differences between dry samples compacted by vibrations and lubricated
packings. The greater stiffness of the latter, in spite of their lower density,
can hence be attributed to a larger coordination number. Voigt and Reuss bounds
bracket bulk modulus B accurately, but simple estimation schemes fail for shear
modulus G, especially in poorly coordinated configurations under low P.
Tenuous, fragile networks respond differently to changes in load direction, as
compared to load intensity. The shear modulus, in poorly coordinated packings,
tends to vary proportionally to the degree of force indeterminacy per unit
volume. The elastic range extends to small strain intervals, in agreement with
experimental observations. The origins of nonelastic response are discussed. We
conclude that elastic moduli provide access to mechanically important
information about coordination numbers, which escape direct measurement
techniques, and indicate further perspectives.Comment: Published in Physical Review E 25 page
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