465 research outputs found
Numerical study of the glass-glass transition in short-ranged attractive colloids
We report extensive numerical simulations in the {\it glass} region for a
simple model of short-ranged attractive colloids, the square well model. We
investigate the behavior of the density autocorrelation function and of the
static structure factor in the region of temperatures and packing fractions
where a glass-glass transition is expected according to theoretical
predictions. We strengthen our observations by studying both waiting time and
history dependence of the numerical results. We provide evidence supporting the
possibility that activated bond-breaking processes destabilize the attractive
glass, preventing the full observation of a sharp glass-glass kinetic
transition.Comment: 15 pages, 9 figures; Proceedings of "Structural Arrest Transitions in
Colloidal Systems with Short-Range Attractions", Messina, Italy, December
2003 (submitted to J. Phys.: Condens. Matt.
Viscoelasticity and Stokes-Einstein relation in repulsive and attractive colloidal glasses
We report a numerical investigation of the visco-elastic behavior in models
for steric repulsive and short-range attractive colloidal suspensions, along
different paths in the attraction-strength vs packing fraction plane. More
specifically, we study the behavior of the viscosity (and its frequency
dependence) on approaching the repulsive glass, the attractive glass and in the
re-entrant region where viscosity shows a non monotonic behavior on increasing
attraction strength. On approaching the glass lines, the increase of the
viscosity is consistent with a power-law divergence with the same exponent and
critical packing fraction previously obtained for the divergence of the density
fluctuations. Based on mode-coupling calculations, we associate the increase of
the viscosity with specific contributions from different length scales. We also
show that the results are independent on the microscopic dynamics by comparing
newtonian and brownian simulations for the same model. Finally we evaluate the
Stokes-Einstein relation approaching both glass transitions, finding a clear
breakdown which is particularly strong for the case of the attractive glass.Comment: 12 pages; sent to J. Chem. Phy
Crystallization of hard-sphere glasses
We study by molecular dynamics the interplay between arrest and
crystallization in hard spheres. For state points in the plane of volume
fraction () and polydispersity (), we delineate states that spontaneously crystallize from those that do
not. For noncrystallizing (or precrystallization) samples we find
isodiffusivity lines consistent with an ideal glass transition at , independent of . Despite this, for , crystallization
occurs at . This happens on time scales for which the system is
aging, and a diffusive regime in the mean square displacement is not reached;
by those criteria, the system is a glass. Hence, contrary to a widespread
assumption in the colloid literature, the occurrence of spontaneous
crystallization within a bulk amorphous state does not prove that this state
was an ergodic fluid rather than a glass.Comment: 4 pages, 3 figure
Hard Spheres: Crystallization and Glass Formation
Motivated by old experiments on colloidal suspensions, we report molecular
dynamics simulations of assemblies of hard spheres, addressing crystallization
and glass formation. The simulations cover wide ranges of polydispersity s
(standard deviation of the particle size distribution divided by its mean) and
particle concentration. No crystallization is observed for s > 0.07. For 0.02 <
s < 0.07, we find that increasing the polydispersity at a given concentration
slows down crystal nucleation. The main effect here is that polydispersity
reduces the supersaturation since it tends to stabilise the fluid but to
destabilise the crystal. At a given polydispersity (< 0.07) we find three
regimes of nucleation: standard nucleation and growth at concentrations in and
slightly above the coexistence region; "spinodal nucleation", where the free
energy barrier to nucleation appears to be negligible, at intermediate
concentrations; and, at the highest concentrations, a new mechanism, still to
be fully understood, which only requires small re-arrangement of the particle
positions. The cross-over between the second and third regimes occurs at a
concentration, around 58% by volume, where the colloid experiments show a
marked change in the nature of the crystals formed and the particle dynamics
indicate an "ideal" glass transition
Scaling of dynamics with the range of interaction in short-range attractive colloids
We numerically study the dependence of the dynamics on the range of
interaction for the short-range square well potential. We find that,
for small , dynamics scale exactly in the same way as thermodynamics,
both for Newtonian and Brownian microscopic dynamics. For interaction ranges
from a few percent down to the Baxter limit, the relative location of the
attractive glass line and the liquid-gas line does not depend on . This
proves that in this class of potentials, disordered arrested states (gels) can
be generated only as a result of a kinetically arrested phase separation.Comment: 4 pages, 4 figure
Multiple glass transitions in star polymer mixtures: Insights from theory and simulations
The glass transition in binary mixtures of star polymers is studied by mode
coupling theory and extensive molecular dynamics computer simulations. In
particular, we have explored vitrification in the parameter space of size
asymmetry and concentration of the small star polymers at
fixed concentration of the large ones. Depending on the choice of parameters,
three different glassy states are identified: a single glass of big polymers at
low and low , a double glass at high and low
, and a novel double glass at high and high which is
characterized by a strong localization of the small particles. At low
and high there is a competition between vitrification and phase
separation. Centered in the -plane, a liquid lake shows up
revealing reentrant glass formation. We compare the behavior of the dynamical
density correlators with the predictions of the theory and find remarkable
agreement between the two.Comment: 15 figures, to be published in Macromolecule
On polydispersity and the hard-sphere glass transition
We simulate the dynamics of polydisperse hard spheres at high packing
fractions, , with an experimentally-realistic particle size distribution
(PSD) and other commonly-used PSDs such as gaussian or top hat. We find that
the mode of kinetic arrest depends on the PSD's shape and not only on its
variance. For the experimentally-realistic PSD, the largest particles undergo
an ideal glass transition at while the smallest particles
remain mobile. Such species-specific localisation was previously observed only
in asymmetric binary mixtures. Our findings suggest that the recent observation
of ergodic behavior up to in a hard-sphere system is not
evidence for activated dynamics, but an effect of polydispersity
Effect of bond lifetime on the dynamics of a short-range attractive colloidal system
We perform molecular dynamics simulations of short-range attractive colloid
particles modeled by a narrow (3% of the hard sphere diameter) square well
potential of unit depth. We compare the dynamics of systems with the same
thermodynamics but different bond lifetimes, by adding to the square well
potential a thin barrier at the edge of the attractive well. For permanent
bonds, the relaxation time diverges as the packing fraction
approaches a threshold related to percolation, while for short-lived bonds, the
-dependence of is more typical of a glassy system. At intermediate
bond lifetimes, the -dependence of is driven by percolation at low
, but then crosses over to glassy behavior at higher . We also
study the wavevector dependence of the percolation dynamics.Comment: Revised; 9 pages, 9 figure
Crystallization Mechanism of Hard Sphere Glasses
In supercooled liquids, vitrification generally suppresses crystallization.
Yet some glasses can still crystallize despite the arrest of diffusive motion.
This ill-understood process may limit the stability of glasses, but its
microscopic mechanism is not yet known. Here we present extensive computer
simulations addressing the crystallization of monodisperse hard-sphere glasses
at constant volume (as in a colloid experiment). Multiple crystalline patches
appear without particles having to diffuse more than one diameter. As these
patches grow, the mobility in neighbouring areas is enhanced, creating dynamic
heterogeneity with positive feedback. The future crystallization pattern cannot
be predicted from the coordinates alone: crystallization proceeds by a sequence
of stochastic micro-nucleation events, correlated in space by emergent dynamic
heterogeneity.Comment: 4 pages 4 figures Accepted for publication in Phys. Rev. Lett., April
201
A random walk description of the heterogeneous glassy dynamics of attracting colloids
We study the heterogeneous dynamics of attractive colloidal particles close
to the gel transition using confocal microscopy experiments combined with a
theoretical statistical analysis. We focus on single particle dynamics and show
that the self part of the van Hove distribution function is not the Gaussian
expected for a Fickian process, but that it reflects instead the existence, at
any given time, of colloids with widely different mobilities. Our confocal
microscopy measurements can be described well by a simple analytical model
based on a conventional continuous time random walk picture, as already found
in several other glassy materials. In particular, the theory successfully
accounts for the presence of broad tails in the van Hove distributions that
exhibit exponential, rather than Gaussian, decay at large distance.Comment: 13 pages, 5 figs. Submitted to special issue "Classical and Quantum
Glasses" of J. Phys.: Condens. Matter; v2: response to refere
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