367 research outputs found
Pressure screening and fluctuations at the bottom of a granular column
We report sets of precise and reproducible measurements on the static
pressure at the bottom of a granular column. We make a quantitative analysis of
the pressure saturation when the column height is increased. We evidence a
great sensitivity of the measurements with the global packing fraction and the
eventual presence of shear bands at the boundaries. We also show the limit of
the classical Janssen model and discuss these experimental results under the
scope of recently proposed theoretical frameworks.Comment: 17 pages, Latex, 8 eps figures, to appear in the European Physical
Journal B (1999
Asymptotic network models of subwavelength metamaterials formed by closely packed photonic and phononic crystals
We demonstrate that photonic and phononic crystals consisting of closely
spaced inclusions constitute a versatile class of subwavelength metamaterials.
Intuitively, the voids and narrow gaps that characterise the crystal form an
interconnected network of Helmholtz-like resonators. We use this intuition to
argue that these continuous photonic (phononic) crystals are in fact
asymptotically equivalent, at low frequencies, to discrete capacitor-inductor
(mass-spring) networks whose lumped parameters we derive explicitly. The
crystals are tantamount to metamaterials as their entire acoustic branch, or
branches when the discrete analogue is polyatomic, is squeezed into a
subwavelength regime where the ratio of wavelength to period scales like the
ratio of period to gap width raised to the power 1/4; at yet larger wavelengths
we accordingly find a comparably large effective refractive index. The fully
analytical dispersion relations predicted by the discrete models yield
dispersion curves that agree with those from finite-element simulations of the
continuous crystals. The insight gained from the network approach is used to
show that, surprisingly, the continuum created by a closely packed hexagonal
lattice of cylinders is represented by a discrete honeycomb lattice. The
analogy is utilised to show that the hexagonal continuum lattice has a
Dirac-point degeneracy that is lifted in a controlled manner by specifying the
area of a symmetry-breaking defect
Super-Arrhenius dynamics for sub-critical crack growth in disordered brittle media
Taking into account stress fluctuations due to thermal noise, we study
thermally activated irreversible crack growth in disordered media. The
influence of material disorder on sub-critical growth of a single crack in
two-dimensional brittle elastic material is described through the introduction
of a rupture threshold distribution. We derive analytical predictions for crack
growth velocity and material lifetime in agreement with direct numerical
calculations. It is claimed that crack growth process is inhibited by disorder:
velocity decreases and lifetime increases with disorder. More precisely,
lifetime is shown to follow a super-Arrhenius law, with an effective
temperature theta - theta_d, where theta is related to the thermodynamical
temperature and theta_d to the disorder variance.Comment: Submitted to Europhysics Letter
Imaging the stick-slip peeling of an adhesive tape under a constant load
Using a high speed camera, we study the peeling dynamics of an adhesive tape
under a constant load with a special focus on the so-called stick-slip regime
of the peeling. It is the first time that the very fast motion of the peeling
point is imaged. The speed of the camera, up to 16000 fps, allows us to observe
and quantify the details of the peeling point motion during the stick and slip
phases: stick and slip velocities, durations and amplitudes. First, in contrast
with previous observations, the stick-slip regime appears to be only transient
in the force controlled peeling. Additionally, we discover that the stick and
slip phases have similar durations and that at high mean peeling velocity, the
slip phase actually lasts longer than the stick phase. Depending on the mean
peeling velocity, we also observe that the velocity change between stick and
slip phase ranges from a rather sudden to a smooth transition. These new
observations can help to discriminate between the various assumptions used in
theoretical models for describing the complex peeling of an adhesive tape. The
present imaging technique opens the door for an extensive study of the velocity
controlled stick-slip peeling of an adhesive tape that will allow to understand
the statistical complexity of the stick-slip in a stationary case
Modal expansion for plasmonic resonators in the time domain
We study the electromagnetic field scattered by a metallic nanoparticle with
dispersive material parameters placed in a homogeneous medium in a low
frequency regime. We use asymptotic analysis and spectral theory to diagonalise
a singular integral operator, which allows us to write the field inside and
outside the particle in the form of a complete and orthogonal modal expansion.
We find the eigenvalues of the volume operator to be associated, via a
non-linear relation, to the resonant frequencies of the problem. We prove that
all resonances lie in a bounded region near the origin. Finally we use complex
analysis to compute the Fourier transform of the scattered field and obtain its
modal expansion in the time domain
Fracture Surfaces as Multiscaling Graphs
Fracture paths in quasi-two-dimenisonal (2D) media (e.g thin layers of
materials, paper) are analyzed as self-affine graphs of height as a
function of length . We show that these are multiscaling, in the sense that
order moments of the height fluctuations across any distance
scale with a characteristic exponent that depends nonlinearly on the order of
the moment. Having demonstrated this, one rules out a widely held conjecture
that fracture in 2D belongs to the universality class of directed polymers in
random media. In fact, 2D fracture does not belong to any of the known kinetic
roughening models. The presence of multiscaling offers a stringent test for any
theoretical model; we show that a recently introduced model of quasi-static
fracture passes this test.Comment: 4 pages, 5 figure
Green's function probe of a static granular piling
We present an experiment which aim is to investigate the mechanical
properties of a static granular assembly. The piling is an horizontal 3D
granular layer confined in a box, we apply a localized extra force at the
surface and the spatial distribution of stresses at the bottom is obtained (the
mechanical Green's function). For different types of granular media, we observe
a linear pressure response which profile shows one peak centered at the
vertical of the point of application. The peak's width increases linearly when
increasing the depth. This green function seems to be in -at least- qualitative
agreement with predictions of elastic theory.Comment: 9 pages, 3 .eps figures, submitted to PR
Discrepancy between sub-critical and fast rupture roughness: a cumulant analysis
We study the roughness of a crack interface in a sheet of paper. We
distinguish between slow (sub-critical) and fast crack growth regimes. We show
that the fracture roughness is different in the two regimes using a new method
based on a multifractal formalism recently developed in the turbulence
literature. Deviations from monofractality also appear to be different in both
regimes
Confined granular packings: structure, stress, and forces
The structure and stresses of static granular packs in cylindrical containers
are studied using large-scale discrete element molecular dynamics simulations
in three dimensions. We generate packings by both pouring and sedimentation and
examine how the final state depends on the method of construction. The vertical
stress becomes depth-independent for deep piles and we compare these stress
depth-profiles to the classical Janssen theory. The majority of the tangential
forces for particle-wall contacts are found to be close to the Coulomb failure
criterion, in agreement with the theory of Janssen, while particle-particle
contacts in the bulk are far from the Coulomb criterion. In addition, we show
that a linear hydrostatic-like region at the top of the packings unexplained by
the Janssen theory arises because most of the particle-wall tangential forces
in this region are far from the Coulomb yield criterion. The distributions of
particle-particle and particle-wall contact forces exhibit
exponential-like decay at large forces in agreement with previous studies.Comment: 11 pages, 11 figures, submitted to PRE (v2) added new references,
fixed typo
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