1,507 research outputs found
Continuum limit of amorphous elastic bodies (III): Three dimensional systems
Extending recent numerical studies on two dimensional amorphous bodies, we
characterize the approach of elastic continuum limit in three dimensional
(weakly polydisperse) Lennard-Jones systems. While performing a systematic
finite-size analysis (for two different quench protocols) we investigate the
non-affine displacement field under external strain, the linear response to an
external delta force and the low-frequency harmonic eigenmodes and their
density distribution. Qualitatively similar behavior is found as in two
dimensions. We demonstrate that the classical elasticity description breaks
down below an intermediate length scale , which in our system is
approximately 23 molecular sizes. This length characterizes the correlations of
the non-affine displacement field, the self-averaging of external noise with
distance from the source and gives the lower wave length bound for the
applicability of the classical eigenfrequency calculations. We trace back the
"Boson-peak" of the density of eigenfrequencies (obtained from the velocity
auto-correlation function) to the inhomogeneities on wave lengths smaller than
.Comment: 27 pages, 11 figures, submitted to Phys. Rev.
Dynamical Monte Carlo Study of Equilibrium Polymers (II): The Role of Rings
We investigate by means of a number of different dynamical Monte Carlo
simulation methods the self-assembly of equilibrium polymers in dilute,
semidilute and concentrated solutions under good-solvent conditions. In our
simulations, both linear chains and closed loops compete for the monomers,
expanding on earlier work in which loop formation was disallowed. Our findings
show that the conformational properties of the linear chains, as well as the
shape of their size distribution function, are not altered by the formation of
rings. Rings only seem to deplete material from the solution available to the
linear chains. In agreement with scaling theory, the rings obey an algebraic
size distribution, whereas the linear chains conform to a Schultz--Zimm type of
distribution in dilute solution, and to an exponentional distribution in
semidilute and concentrated solution. A diagram presenting different states of
aggregation, including monomer-, ring- and chain-dominated regimes, is given
Melting and Pressure-Induced Amorphization of Quartz
It has recently been shown that amorphization and melting of ice were
intimately linked. In this letter, we infer from molecular dynamics simulations
on the SiO2 system that the extension of the quartz melting line in the
metastable pressure-temperature domain is the pressure-induced amorphization
line. It seems therefore likely that melting is the physical phenomenon
responsible for pressure induced amorphization. Moreover, we show that the
structure of a "pressure glass" is similar to that of a very rapidly (1e+13 to
1e+14 kelvins per second) quenched thermal glass.Comment: 9 pages, 4 figures, LaTeX2
Compaction dynamics of a granular media under vertical tapping
We report new experimental results on granular compaction under consecutive
vertical taps. The evolution of the mean volume fraction and of the mean
potential energy of a granular packing presents a slow densification until a
final steady-state, and is reminiscent to usual relaxation in glasses via a
stretched exponential law. The intensity of the taps seems to rule the
characteristic time of the relaxation according to an Arrhenius's type relation
>. Finally, the analysis of the vertical volume fraction profile reveals an
almost homogeneous densification in the packing.Comment: 7 pages, 4 figures, to appear in Europhysics Letter
Driven activation versus thermal activation
Activated dynamics in a glassy system undergoing steady shear deformation is
studied by numerical simulations. Our results show that the external driving
force has a strong influence on the barrier crossing rate, even though the
reaction coordinate is only weakly coupled to the nonequilibrium system. This
"driven activation" can be quantified by introducing in the Arrhenius
expression an effective temperature, which is close to the one determined from
the fluctuation-dissipation relation. This conclusion is supported by
analytical results for a simplified model system.Comment: 5 pages, 3 figure
First-order transition in small-world networks
The small-world transition is a first-order transition at zero density of
shortcuts, whereby the normalized shortest-path distance undergoes a
discontinuity in the thermodynamic limit. On finite systems the apparent
transition is shifted by . Equivalently a ``persistence
size'' can be defined in connection with finite-size
effects. Assuming , simple rescaling arguments imply that
. We confirm this result by extensive numerical simulation in one to
four dimensions, and argue that implies that this transition is
first-order.Comment: 4 pages, 3 figures, To appear in Europhysics Letter
Discrete aqueous solvent effects and possible attractive forces
We study discrete solvent effects on the interaction of two parallel charged
surfaces in ionic aqueous solution. These effects are taken into account by
adding a bilinear non-local term to the free energy of Poisson-Boltzmann
theory. We study numerically the density profile of ions between the two
plates, and the resulting inter-plate pressure. At large plate separations the
two plates are decoupled and the ion distribution can be characterized by an
effective Poisson-Boltzmann charge that is smaller than the nominal charge. The
pressure is thus reduced relative to Poisson-Boltzmann predictions. At plate
separations below ~2 nm the pressure is modified considerably, due to the
solvent mediated short-range attraction between ions in the the system. For
high surface charges this contribution can overcome the mean-field repulsion
giving rise to a net attraction between the plates.Comment: 12 figures in 16 files. 19 pages. Submitted to J. Chem. Phys., July
200
A repulsive reference potential reproducing the dynamics of a liquid with attractions
A well-known result of liquid state theory is that the structure of dense
fluids is mainly determined by repulsive forces. The WCA potential, which cuts
intermolecular potentials at their minima, is therefore often used as a
reference. However, this reference gives quite wrong results for the viscous
dynamics of the Kob-Andersen binary Lennard-Jones liquid [Berthier and Tarjus,
Phys. Rev. Lett. 103, 170601 (2009)]. We show that repulsive inverse-power law
potentials provide a useful reference for this liquid by reproducing its
structure, dynamics, and isochoric heat capacity
Universal velocity distributions in an experimental granular fluid
We present experimental results on the velocity statistics of a uniformly
heated granular fluid, in a quasi-2D configuration. We find the base state, as
measured by the single particle velocity distribution , to be universal
over a wide range of filling fractions and only weakly dependent on all other
system parameters. There is a consistent overpopulation in the distribution's
tails, which scale as . More
importantly, the high probability central region of , at low velocities,
deviates from a Maxwell-Boltzmann by a second order Sonine polynomial with a
single adjustable parameter, in agreement with recent theoretical analysis of
inelastic hard spheres driven by a stochastic thermostat. To our knowledge,
this is the first time that Sonine deviations have been measured in an
experimental system.Comment: 13 pages, 15 figures, with minor corrections, submitted to Phys. Rev.
Electrostatic attraction between cationic-anionic assemblies with surface compositional heterogeneities
Electrostatics plays a key role in biomolecular assembly. Oppositely charged
biomolecules, for instance, can co-assembled into functional units, such as DNA
and histone proteins into nucleosomes and actin-binding protein complexes into
cytoskeleton components, at appropriate ionic conditions. These
cationic-anionic co-assemblies often have surface charge heterogeneities that
result from the delicate balance between electrostatics and packing
constraints. Despite their importance, the precise role of surface charge
heterogeneities in the organization of cationic-anionic co-assemblies is not
well understood. We show here that co-assemblies with charge heterogeneities
strongly interact through polarization of the domains. We find that this leads
to symmetry breaking, which is important for functional capabilities, and
structural changes, which is crucial in the organization of co-assemblies. We
determine the range and strength of the attraction as a function of the
competition between the steric and hydrophobic constraints and electrostatic
interactions.Comment: JCP June/200
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