840 research outputs found
Stability at Random Close Packing
The requirement that packings of hard particles, arguably the simplest
structural glass, cannot be compressed by rearranging their network of contacts
is shown to yield a new constraint on their microscopic structure. This
constraint takes the form a bound between the distribution of contact forces
P(f) and the pair distribution function g(r): if P(f) \sim f^{\theta} and g(r)
\sim (r-{\sigma})^(-{\gamma}), where {\sigma} is the particle diameter, one
finds that {\gamma} \geq 1/(2+{\theta}). This bound plays a role similar to
those found in some glassy materials with long-range interactions, such as the
Coulomb gap in Anderson insulators or the distribution of local fields in
mean-field spin glasses. There is ground to believe that this bound is
saturated, offering an explanation for the presence of avalanches of
rearrangements with power-law statistics observed in packings
On the dependence of the avalanche angle on the granular layer thickness
A layer of sand of thickness h flows down a rough surface if the inclination
is larger than some threshold value theta which decreases with h. A tentative
microscopic model for the dependence of theta with h is proposed for rigid
frictional grains, based on the following hypothesis: (i) a horizontal layer of
sand has some coordination z larger than a critical value z_c where mechanical
stability is lost (ii) as the tilt angle is increased, the configurations
visited present a growing proportion $_s of sliding contacts. Instability with
respect to flow occurs when z-z_s=z_c. This criterion leads to a prediction for
theta(h) in good agreement with empirical observations.Comment: 6 pages, 2 figure
Theory of the Jamming Transition at Finite Temperature
A theory for the microscopic structure and the vibrational properties of soft
sphere glass at finite temperature is presented. With an effective potential,
derived here, the phase diagram and vibrational properties are worked out
around the Maxwell critical point at zero temperature and pressure .
Variational arguments and effective medium theory identically predict a
non-trivial temperature scale with
such that low-energy vibrational properties are hard-sphere like for , and zero-temperature soft-sphere like otherwise. However, due to
crossovers in the equation of state relating , , and the packing fraction
, these two regimes lead to four regions where scaling behaviors differ
when expressed in terms of and . Scaling predictions are presented
for the mean-squared displacement, characteristic frequency, shear modulus, and
characteristic elastic length in all regions of the phase diagram.Comment: 8 pages + 3 pages S
Geometric origin of excess low-frequency vibrational modes in amorphous solids
Glasses have a large excess of low-frequency vibrational modes in comparison
with crystalline solids. We show that such a feature is a necessary consequence
of the geometry generic to weakly connected solids. In particular, we analyze
the density of states of a recently simulated system, comprised of weakly
compressed spheres at zero temperature. We account for the observed a)
constancy of the density of modes with frequency, b) appearance of a
low-frequency cutoff, and c) power-law increase of this cutoff with
compression. We predict a length scale below which vibrations are very
different from those of a continuous elastic body.Comment: 4 pages, 2 figures. Argument rewritten, identical result
Theory for the density of interacting quasi-localised modes in amorphous solids
Quasi-localised modes appear in the vibrational spectrum of amorphous solids
at low-frequency. Though never formalised, these modes are believed to have a
close relationship with other important local excitations, including shear
transformations and two-level systems. We provide a theory for their frequency
density, , that establishes this link for
systems at zero temperature under quasi-static loading. It predicts two regimes
depending on the density of shear transformations (with
the additional stress needed to trigger a shear transformation). If
, and a finite fraction of quasi-localised modes form
shear transformations, whose amplitudes vanish at low frequencies. If
, and all quasi-localised modes form shear
transformations with a finite amplitude at vanishing frequencies. We confirm
our predictions numerically
How collective asperity detachments nucleate slip at frictional interfaces
Sliding at a quasi-statically loaded frictional interface can occur via
macroscopic slip events, which nucleate locally before propagating as rupture
fronts very similar to fracture. We introduce a novel microscopic model of a
frictional interface that includes asperity-level disorder, elastic interaction
between local slip events, and inertia. For a perfectly flat and homogeneously
loaded interface, we find that slip is nucleated by avalanches of asperity
detachments of extension larger than a critical radius governed by a
Griffith criterion. We find that after slip, the density of asperities at a
local distance to yielding presents a pseudo-gap , where is a non-universal exponent that depends on
the statistics of the disorder. This result makes a link between friction and
the plasticity of amorphous materials where a pseudo-gap is also present. For
friction, we find that a consequence is that stick-slip is an extremely slowly
decaying finite size effect, while the slip nucleation radius diverges as
a -dependent power law of the system size. We discuss how these
predictions can be tested experimentally
Dynamics of Strongly Deformed Polymers in Solution
Bead spring models for polymers in solution are nonlinear if either the
finite extensibility of the polymer, excluded volume effects or hydrodynamic
interactions between polymer segments are taken into account. For such models
we use a powerful method for the determination of the complete relaxation
spectrum of fluctuations at {\it steady state}. In general, the spectrum and
modes differ significantly from those of the linear Rouse model. For a tethered
polymer in uniform flow the differences are mainly caused by an inhomogeneous
distribution of tension along the chain and are most pronounced due to the
finite chain extensibility. Beyond the dynamics of steady state fluctuations we
also investigate the nonlinear response of the polymer to a {\em large sudden
change} in the flow. This response exhibits several distinct regimes with
characteristic decay laws and shows features which are beyond the scope of
single mode theories such as the dumbbell model.Comment: 7 pages, 3 figure
Contact line motion for partially wetting fluids
We study the flow close to an advancing contact line in the limit of small
capillary number. To take into account wetting effects, both long and
short-ranged contributions to the disjoining pressure are taken into account.
In front of the contact line, there is a microscopic film corresponding to a
minimum of the interaction potential. We compute the parameters of the contact
line solution relevant to the matching to a macroscopic problem, for example a
spreading droplet. The result closely resembles previous results obtained with
a slip model
Classification of the nickel-like silver spectrum (AgXX) from a fast capillary discharge plasma
Includes bibliographical references (page 25).A study of the Ni-like silver (AgXX) spectra in the 13:7-20:5 nm wavelength region using a plasma generated by a fast high power capillary discharge is reported. Forty-three AgXX transitions have been identified with the assistance of calculations performed using the Slater-Condon method with generalized least-squares fits of the energy parameters. The average difference between the measured transition wavelengths and the theoretical values is 0.0026 nm
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