711 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
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
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
Excess Vibrational Modes and the Boson Peak in Model Glasses
The excess low-frequency normal modes for two widely-used models of glasses
were studied at zero temperature. The onset frequencies for the anomalous modes
for both systems agree well with predictions of a variational argument, which
is based on analyzing the vibrational energy originating from the excess
contacts per particle over the minimum number needed for mechanical stability.
Even though both glasses studied have a high coordination number, most of the
additional contacts can be considered to be weak.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Scaling of phononic transport with connectivity in amorphous solids
The effect of coordination on transport is investigated theoretically using
random networks of springs as model systems. An effective medium approximation
is made to compute the density of states of the vibrational modes, their energy
diffusivity (a spectral measure of transport) and their spatial correlations as
the network coordination is varied. Critical behaviors are obtained as
where these networks lose rigidity. A sharp cross-over from a regime
where modes are plane-wave-like toward a regime of extended but
strongly-scattered modes occurs at some frequency , which
does not correspond to the Ioffe-Regel criterion. Above both the
density of states and the diffusivity are nearly constant. These results agree
remarkably with recent numerical observations of repulsive particles near the
jamming threshold \cite{ning}. The analysis further predicts that the length
scale characterizing the correlation of displacements of the scattered modes
decays as with frequency, whereas for
Rayleigh scattering is found with a scattering length . It is argued that this description applies to silica glass
where it compares well with thermal conductivity data, and to transverse
ultrasound propagation in granular matter
On the rigidity of a hard sphere glass near random close packing
We study theoretically and numerically the microscopic cause of the
mechanical stability of hard sphere glasses near their maximum packing. We show
that, after coarse-graining over time, the hard sphere interaction can be
described by an effective potential which is exactly logarithmic at the random
close packing . This allows to define normal modes, and to apply recent
results valid for elastic networks: mechanical stability is a non-local
property of the packing geometry, and is characterized by some length scale
which diverges at [1, 2]. We compute the scaling of the bulk and
shear moduli near , and speculate on the possible implications of these
results for the glass transition.Comment: 7 pages, 4 figures. Figure 4 had a wrong unit in abscissa, which was
correcte
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