58 research outputs found
Static triplet correlations in glass-forming liquids: A molecular dynamics study
We present a numerical evaluation of the three-point static correlations
functions of the Kob-Andersen Lennard-Jones binary mixture and of its purely
repulsive, Weeks-Chandler-Andersen variant. In the glassy regime, the two
models possess a similar pair structure, yet their dynamics differ markedly.
The static triplet correlation functions S^(3) indicate that the local ordering
is more pronounced in the Lennard-Jones model, an observation consistent with
its slower dynamics. A comparison of the direct triplet correlation functions
c^(3) reveals that these structural differences are due, to a good extent, to
an amplification of the small discrepancies observed at the pair level. We
demonstrate the existence of a broad, positive peak at small wave-vectors and
angles in c^(3). In this portion of k-space, slight, systematic differences
between the models are observed, revealing "genuine" three-body contributions
to the triplet structure. The possible role of the low-k features of c^(3) and
the implications of our results for dynamic theories of the glass transition
are discussed.Comment: 9 pages, 8 figures, contribution to the JCP Special Issue on the
Glass Transitio
Locally preferred structures and many-body static correlations in viscous liquids
We investigate the influence of static correlations beyond the pair level on
the dynamics of selected model glass-formers. We compare the pair structure,
angular distribution functions, and statistics of Voronoi polyhedra of two
well-known Lennard-Jones mixtures as well as of the corresponding
Weeks-Chandler-Andersen variants, in which the attractive part of the potential
is truncated. By means of the Voronoi construction we identify the atomic
arrangements corresponding to the locally preferred structures of the models.
We find that the growth of domains formed by interconnected locally preferred
structures signals the onset of the slow dynamics regime and allows to
rationalize the different dynamic behaviors of the models. At low temperature,
the spatial extension of the structurally correlated domains, evaluated at
fixed relaxation time, increases with the fragility of the models and is
systematically reduced by truncating the attractions. In view of these results,
proper inclusion of many-body static correlations in theories of the glass
transition appears crucial for the description of the dynamics of fragile
glass-formers.Comment: 9 pages, 8 figures, added two tables, minor revisions to the tex
Non-linear dynamic response of glass-forming liquids to random pinning
We use large scale computer simulations of a glass-forming liquid in which a
fraction c of the particles has been permanently pinned. We find that the
relaxation dynamics shows an exponential dependence on c. This result can be
rationalized by means of a simple theoretical Ansatz and we discuss its
implication for thermodynamic theories for the glass-transition. For
intermediate and low temperatures we find that the slowing down of the dynamics
due to the pinning saturates and that the cooperativity decreases with
increasing c, results which indicate that in glass-forming liquids there is a
dynamic crossover at which the shape of the relaxing entities changes
Cluster and reentrant anomalies of nearly Gaussian core particles
We study through integral equation theory and numerical simulations the
structure and dynamics of fluids composed of ultrasoft, nearly Gaussian
particles. Namely, we explore the fluid phase diagram of a model in which
particles interact via the generalized exponential potential u(r)=\epsilon
exp[-(r/\sigma)^n], with a softness exponent n slightly larger than 2. In
addition to the well-known anomaly associated to reentrant melting, the
structure and dynamics of the fluid display two additional anomalies, which are
visible in the isothermal variation of the structure factor and diffusivity.
These features are correlated to the appearance of dimers in the fluid phase
and to the subsequent modification of the cluster structure upon compression.
We corroborate these results through an analysis of the local minima of the
potential energy surface, in which clusters appear as much tighter
conglomerates of particles. We find that reentrant melting and clustering
coexist for softness exponents ranging from 2^+ up to values relevant for the
description of amphiphilic dendrimers, i.e., n=3.Comment: 10 pages, 8 figure
Hopping and microscopic dynamics of ultrasoft particles in cluster crystals
We have investigated the slow dynamics of ultrasoft particles in crystalline
cluster phases, where point particles interact through the generalized
exponential potential u(r) = \epsilon \exp[-(r/\sigma)^n], focusing on the
cluster fcc phase of this model with n=4. In an effort to elucidate how the
mechanisms of mass transport depend on the microscopic dynamics and in order to
mimic a realistic scenario in a related experiment we have performed molecular
dynamics, Brownian dynamics, and Monte Carlo simulations. In molecular dynamics
simulations the diffusion of particles proceeds through long-range jumps,
guided by strong correlations in the momentum direction. In Monte Carlo and
Brownian dynamics simulations jump events are short-ranged, reflecting the
purely configurational nature of the dynamics. In contrast to what was found in
models of glass-forming liquids, the effect of Newtonian and stochastic
microscopic dynamics on the long-time relaxation cannot be accounted for by a
temperature-independent rescaling of the time units. From the obvious
qualitative discrepancies in the short time behavior between the three
simulation methods and the non-trivial difference in jump length distributions,
long time relaxation, and dynamic heterogeneity, we learn that a more complex
modeling of the dynamics in realistic systems of ultrasoft colloids is
required.Comment: 12 pages, 18 figures, added results of Brownian dynamics simulation
Structure and dynamics of coupled viscous liquids
We perform Monte-Carlo simulations to analyse the structure and microscopic
dynamics of a viscous Lennard-Jones liquid coupled to a quenched reference
configuration of the same liquid. The coupling between the two replicas is
introduced via a field epsilon conjugate to the overlap Q between the two
particle configurations. This allows us to study the evolution of various
static and dynamic correlation functions across the (epsilon, T) equilibrium
phase diagram. As the temperature is decreased, we identify increasingly marked
precursors of a first-order phase transition between a low-Q and a high-Q phase
induced by the field epsilon. We show in particular that both static and
dynamic susceptibilities have a maximum at a temperature-dependent value of the
coupling field, which defines a `Widom line'. We also show that, in the
high-overlap regime, diffusion and structural relaxation are strongly decoupled
because single particle motion mostly occurs via discrete hopping on the sites
defined by the reference configuration. These results, obtained using
conventional numerical tools, provide encouraging signs that an equilibrium
phase transition exists in coupled viscous liquids, but also demonstrate that
important numerical challenges must be overcome to obtain more conclusive
numerical evidence.Comment: 14 pages, 8 figures. Accepted for publication in Molecular Physics
(Special Issue in honour of J.-P. Hansen
Models and algorithms for the next generation of glass transition studies
Successful computer studies of glass-forming materials need to overcome both
the natural tendency to structural ordering and the dramatic increase of
relaxation times at low temperatures. We present a comprehensive analysis of
eleven glass-forming models to demonstrate that both challenges can be
efficiently tackled using carefully designed models of size polydisperse
supercooled liquids together with an efficient Monte Carlo algorithm where
translational particle displacements are complemented by swaps of particle
pairs. We study a broad range of size polydispersities, using both discrete and
continuous mixtures, and we systematically investigate the role of particle
softness, attractivity and non-additivity of the interactions. Each system is
characterized by its robustness against structural ordering and by the
efficiency of the swap Monte Carlo algorithm. We show that the combined
optimisation of the potential's softness, polydispersity and non-additivity
leads to novel computer models with excellent glass-forming ability. For such
models, we achieve over ten orders of magnitude gain in the equilibration
timescale using the swap Monte Carlo algorithm, thus paving the way to
computational studies of static and thermodynamic properties under experimental
conditions. In addition, we provide microscopic insights into the performance
of the swap algorithm which should help optimizing models and algorithms even
further.Comment: 22 pages, 15 fig
Dynamic arrest of colloids in porous environments: disentangling crowding and confinement
Using numerical simulations we study the slow dynamics of a colloidal
hard-sphere fluid adsorbed in a matrix of disordered hard-sphere obstacles. We
calculate separately the contributions to the single-particle dynamic
correlation functions due to free and trapped particles. The separation is
based on a Delaunay tessellation to partition the space accessible to the
centres of fluid particles into percolating and disconnected voids. We find
that the trapping of particles into disconnected voids of the matrix is
responsible for the appearance of a nonzero long-time plateau in the
single-particle intermediate scattering functions of the full fluid. The
subdiffusive exponent , obtained from the logarithmic derivative of the
mean-squared displacement, is observed to be essentially unaffected by the
motion of trapped particles: close to the percolation transition, we determined
for both the full fluid and the particles moving in the
percolating void. Notably, the same value of is found in single-file
diffusion and is also predicted by mode-coupling theory along the
diffusion-localisation line. We also reveal subtle effects of dynamic
heterogeneity in both the free and the trapped component of the fluid
particles, and discuss microscopic mechanisms that contribute to this
phenomenon.Comment: 18 pages, 12 figures, minor change
Impact of random obstacles on the dynamics of a dense colloidal fluid
Using molecular dynamics simulations we study the slow dynamics of a
colloidal fluid annealed within a matrix of obstacles quenched from an
equilibrated colloidal fluid. We choose all particles to be of the same size
and to interact as hard spheres, thus retaining all features of the porous
confinement while limiting the control parameters to the packing fraction of
the matrix, {\Phi}m, and that of the fluid, {\Phi}f. We conduct detailed
investigations on several dynamic properties, including the tagged-particle and
collective intermediate scattering functions, the mean-squared displacement,
and the van Hove function. We show the confining obstacles to profoundly impact
the relaxation pattern of various quantifiers pertinent to the fluid. Varying
the type of quantifier (tagged-particle or collective) as well as {\Phi}m and
{\Phi}f, we unveil both discontinuous and continuous arrest scenarios.
Furthermore, we discover subdiffusive behavior and demonstrate its close
connection to the matrix structure. Our findings partly confirm the various
predictions of a recent extension of mode-coupling theory to the
quenched-annealed protocol.Comment: 16 pages, 20 figures, minor revision
Exploring the jamming transition over a wide range of critical densities
We numerically study the jamming transition of frictionless polydisperse
spheres in three dimensions. We use an efficient thermalisation algorithm for
the equilibrium hard sphere fluid and generate amorphous jammed packings over a
range of critical jamming densities that is about three times broader than in
previous studies. This allows us to reexamine a wide range of structural
properties characterizing the jamming transition. Both isostaticity and the
critical behavior of the pair correlation function hold over the entire range
of jamming densities. At intermediate length scales, we find a weak, smooth
increase of bond orientational order. By contrast, distorted icosahedral
structures grow rapidly with increasing the volume fraction in both fluid and
jammed states. Surprisingly, at large scale we observe that denser jammed
states show stronger deviations from hyperuniformity, suggesting that the
enhanced amorphous ordering inherited from the equilibrium fluid competes with,
rather than enhances, hyperuniformity. Finally, finite size fluctuations of the
critical jamming density are considerably suppressed in the denser jammed
states, indicating an important change in the topography of the potential
energy landscape. By considerably stretching the amplitude of the critical
"J-line", our work disentangles physical properties at the contact scale that
are associated with jamming criticality, from those occurring at larger length
scales, which have a different nature.Comment: 19 pages, 11 figures, resubmission to SciPos
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