6,943 research outputs found
Nanoscale surface relaxation of a membrane stack
Recent measurements of the short-wavelength (~ 1--100 nm) fluctuations in
stacks of lipid membranes have revealed two distinct relaxations: a fast one
(decay rate of ~ 0.1 ns^{-1}), which fits the known baroclinic mode of bulk
lamellar phases, and a slower one (~ 1--10 \mu s^{-1}) of unknown origin. We
show that the latter is accounted for by an overdamped capillary mode,
depending on the surface tension of the stack and its anisotropic viscosity. We
thereby demonstrate how the dynamic surface tension of membrane stacks could be
extracted from such measurements.Comment: 4 page
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
The impact of step-down line extension on consumer-brand relationships: A risky strategy for luxury brands
This paper analyzes the role of the brand concept (luxury vs. non-luxury) in the impact of step-down line extension on consumer-brand relationships. A before-and-after pseudo-experimental study conducted on the Internet among BMW and Peugeot buyers shows that step-down line extension negatively influences the main variables of consumer-brand relationships (e.g., self-brand connections, brand attachment, brand trust and brand commitment) only for the luxury brand BMW. On the contrary, no dilution effects are found for the non-luxury brand Peugeot.vertical line extension, dilution effects; consumer-brand relationships;luxury brands; cars; PSL approach
Internal states of model isotropic granular packings. I. Assembling process, geometry and contact networks
This is the first paper of a series of three, reporting on numerical
simulation studies of geometric and mechanical properties of static assemblies
of spherical beads under an isotropic pressure. Frictionless systems assemble
in the unique random close packing (RCP) state in the low pressure limit if the
compression process is fast enough, slower processes inducing traces of
crystallization, and exhibit specific properties directly related to
isostaticity of the force-carrying structure. The different structures of
frictional packings assembled by various methods cannot be classified by the
sole density. While lubricated systems approach RCP densities and coordination
number z^*~=6 on the backbone in the rigid limit, an idealized "vibration"
procedure results in equally dense configurations with z^*~=4.5. Near neighbor
correlations on various scales are computed and compared to available
laboratory data, although z^* values remain experimentally inaccessible. Low
coordination packings have many rattlers (more than 10% of the grains carry no
force), which should be accounted for on studying position correlations, and a
small proportion of harmless "floppy modes" associated with divalent grains.
Frictional packings, however slowly assembled under low pressure, retain a
finite level of force indeterminacy, except in the limit of infinite friction.Comment: 29 pages. Published in Physical Review
Numerical study of tracers transport by a mesoscale convective system over West Africa
A three-dimensional cloud-resolving model is used to investigate the vertical
transport from the lower to the upper troposphere in a mesoscale convective
system (MCS) that occurred over Niger on 15 August 2004. The redistribution
of five passive tracers initially confined in horizontally homogeneous layers
is analyzed. The monsoon layer tracer (0–1.5 km) is the most efficiently
transported in the upper troposphere with concentrations 3 to 4 times higher
than the other tracers in the anvil. On the contrary the African Easterly Jet
tracer (~3 km) has the lowest contribution above 5 km. The vertical
profiles of the mid-troposphere tracers (4.5–10 km) in the MCS exhibit two
peaks: one in their initial layers, and the second one at 13–14 km altitude,
underlying the importance of mid-tropospheric air in feeding the upper
troposphere. Mid-tropospheric tracers also experience efficient transport by
convective downdrafts with a consequent increase of their concentrations at
the surface. The concentration of the upper troposphere–lower stratosphere
tracer exhibits strong gradients at the edge of the cloud, meaning almost no
entrainment of this tracer into the cloud. No downward transport from the
upper troposphere is simulated below 5 km. A proxy for lightning produced
NO<sub>x</sub> is transported preferentially in the forward anvil in the upper
troposphere. Additionally, lateral inflows significantly contribute to the
updraft and downdraft airflows emphasizing the three-dimensional structure of
the West African MCSs
Passive monitoring of anisotropy change associated with the Parkfield 2004 earthquake
International audienceWe investigate temporal variations in the polarization of surface waves determined using ambient seismic noise cross-correlations between station pairs at the time of the Mw 6.0 Parkfield earthquake of September 28, 2004. We use data recorded by the High Resolution Seismic Network's 3-component seismometers located along the San Andreas Fault. Our results show strong variations in azimuthal surface wave polarizations, Psi, for the paths containing station VARB, one of the closest stations to the San Andreas Fault, synchronous with the Parkfield earthquake. Concerning the other station pair, only smooth temporal variations of Y are observed. Two principal contributions to these changes in Y are identified and separated. They are: (1) slow and weak variations due to seasonal changes in the incident direction of seismic noise; and (2) strong and rapid rotations synchronous with the Parkfield earthquake for paths containing station VARB. Strong shifts in Y are interpreted in terms of changes in crack-induced anisotropy due to the co-seismic rotation of the stress field. Because these changes are only observed on paths containing station VARB, the anisotropic layer responsible for the changes is most likely localized around VARB in the shallow crust. These results suggest that the polarization of surface waves may be very sensitive to changes in the orientations of distributed cracks and that implementation of our technique on a routine basis may prove useful for monitoring stress changes deep within seismogenic zones. Citation: Durand, S., J. P. Montagner, P. Roux, F. Brenguier, R. M. Nadeau, and Y. Ricard (2011), Passive monitoring of anisotropy change associated with the Parkfield 2004 earthquake, Geophys. Res. Lett., 38, L13303, doi: 10.1029/2011GL047875
Rheophysics of dense granular materials : Discrete simulation of plane shear flows
We study the steady plane shear flow of a dense assembly of frictional,
inelastic disks using discrete simulation and prescribing the pressure and the
shear rate. We show that, in the limit of rigid grains, the shear state is
determined by a single dimensionless number, called inertial number I, which
describes the ratio of inertial to pressure forces. Small values of I
correspond to the quasi-static regime of soil mechanics, while large values of
I correspond to the collisional regime of the kinetic theory. Those shear
states are homogeneous, and become intermittent in the quasi-static regime.
When I increases in the intermediate regime, we measure an approximately linear
decrease of the solid fraction from the maximum packing value, and an
approximately linear increase of the effective friction coefficient from the
static internal friction value. From those dilatancy and friction laws, we
deduce the constitutive law for dense granular flows, with a plastic Coulomb
term and a viscous Bagnold term. We also show that the relative velocity
fluctuations follow a scaling law as a function of I. The mechanical
characteristics of the grains (restitution, friction and elasticity) have a
very small influence in this intermediate regime. Then, we explain how the
friction law is related to the angular distribution of contact forces, and why
the local frictional forces have a small contribution to the macroscopic
friction. At the end, as an example of heterogeneous stress distribution, we
describe the shear localization when gravity is added.Comment: 24 pages, 19 figure
Permeability of self-affine rough fractures
The permeability of two-dimensional fractures with self-affine fractal
roughness is studied via analytic arguments and numerical simulations. The
limit where the roughness amplitude is small compared with average fracture
aperture is analyzed by a perturbation method, while in the opposite case of
narrow aperture, we use heuristic arguments based on lubrication theory.
Numerical simulations, using the lattice Boltzmann method, are used to examine
the complete range of aperture sizes, and confirm the analytic arguments.Comment: 11 pages, 9 figure
An algorithm to calculate the transport exponent in strip geometries
An algorithm for solving the random resistor problem by means of the
transfer-matrix approach is presented. Preconditioning by spanning clusters
extraction both reduces the size of the conductivity matrix and speed up the
calculations.Comment: 17 pages, RevTeX2.1, HLRZ - 97/9
Frictionless bead packs have macroscopic friction, but no dilatancy
The statement of the title is shown by numerical simulation of homogeneously
sheared packings of frictionless, nearly rigid beads in the quasistatic limit.
Results coincide for steady flows at constant shear rate γ in the
limit of small γ and static approaches, in which packings are equilibrated
under growing deviator stresses. The internal friction angle ϕ, equal to
5.76 0.22 degrees in simple shear, is independent on the average pressure
P in the rigid limit. It is shown to stem from the ability of stable
frictionless contact networks to form stress-induced anisotropic fabrics. No
enduring strain localization is observed. Dissipation at the macroscopic level
results from repeated network rearrangements, like the effective friction
of a frictionless slider on a bumpy surface. Solid fraction Φ remains
equal to the random close packing value ≃ 0.64 in slowly or statically
sheared systems. Fluctuations of stresses and volume are observed to regress in
the large system limit, and we conclude that the same friction law for simple
shear applies in the large psystem limit if normal stress or density is
externally controlled. Defining the inertia number as I = γ m/(aP),
with m the grain mass and a its diameter, both internal friction
coefficient ∗ = tan ϕ and volume 1/Φ increase as
powers of I in the quasistatic limit of vanishing I, in which all mechanical
properties are determined by contact network geometry. The microstructure of
the sheared material is characterized with a suitable parametrization of the
fabric tensor and measurements of connectivity and coordination numbers
associated with contacts and near neighbors.Comment: 19 pages. Additional technical details may be found in v
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