122 research outputs found
Pacman Percolation and the Glass Transition
We investigate via Monte Carlo simulations the kinetically constrained
Kob-Andersen lattice glass model showing that, contrary to current
expectations, the relaxation process and the dynamical heterogeneities seems to
be characterized by different time scales. Indeed, we found that the relaxation
time is related to a reverse percolation transition, whereas the time of
maximum heterogeneity is related to the spatial correlation between particles.
This investigation leads to a geometrical interpretation of the relaxation
processes and of the different observed time scales.Comment: 12 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1109.428
Spatial correlations of elementary relaxation events in glass-forming liquids
The dynamical facilitation scenario, by which localized relaxation events
promote nearby relaxation events in an avalanching process, has been suggested
as the key mechanism connecting the microscopic and the macroscopic dynamics of
structural glasses. Here we investigate the statistical features of this
process via the numerical simulation of a model structural glass. First we show
that the relaxation dynamics of the system occurs through particle jumps that
are irreversible, and that cannot be decomposed in smaller irreversible events.
Then we show that each jump does actually trigger an avalanche. The
characteristic of this avalanche change on cooling, suggesting that the
relaxation dynamics crossovers from a noise dominated regime where jumps do not
trigger other relaxation events, to a regime dominated by the facilitation
process, where a jump trigger more relaxation events.Comment: 8 pages, 6 figure
Particle jumps in structural glasses
Particles in structural glasses rattle around temporary equilibriumpositions,
that seldom change through a process which is much faster than the relaxation
time, known as particle jump. Since the relaxation of the system is due to the
accumulation of many such jumps, it could be possible to connect the single
particle short time motion to the macroscopic relaxation by understanding the
features of the jump dynamics. Here we review recent results in this research
direction, clarifying the features of particles jumps that have been understood
and those that are still under investigation, and examining the role of
particle jumps in different theories of the glass transition.Comment: 10 pages, 4 figures, Review articl
Cage Size and Jump Precursors in Glass-Forming Liquids: Experiment and Simulations
Glassy dynamics is intermittent, as particles suddenly jump out of the cage
formed by their neighbours, and heterogeneous, as these jumps are not uniformly
distributed across the system. Relating these features of the dynamics to the
diverse local environments explored by the particles is essential to
rationalize the relaxation process. Here we investigate this issue
characterizing the local environment of a particle with the amplitude of its
short time vibrational motion, as determined by segmenting in cages and jumps
the particle trajectories. Both simulations of supercooled liquids and
experiments on colloidal suspensions show that particles in large cages are
likely to jump after a small time-lag, and that, on average, the cage enlarges
shortly before the particle jumps. At large time-lags, the cage has essentially
a constant value, which is smaller for longer-lasting cages. Finally, we
clarify how this coupling between cage size and duration controls the average
behaviour and opens the way to a better understanding of the relaxation process
in glass--forming liquids.Comment: Letter, 4 figure
Glassy dynamics of a polymer monolayer on a heterogeneous disordered substrate
We present molecular dynamics simulations of a polymer monolayer on randomly functionalized
surfaces that are characterized by different fractions of weakly and strongly attractive sites. We show
that the dynamics slow-down upon cooling resembles that of a strong glass-forming liquid. Indeed, the
mean-square displacements show an increasingly lasting subdiffusive behaviour before the diffusive
regime, with signs of Fickian yet not Gaussian diffusion, and the dynamic correlation functions exhibit a
stretched exponential decay. The glassy dynamics of this relatively dilute system is dominated by the
interaction of the polymer with the substrate and becomes more marked when the substrate composition is
heterogeneous. Accordingly, the estimated glass transition temperature shows a non-monotic dependence
on surface composition, in agreement with previous results for the activation energy and with an analysis
of the potential energy landscape experienced by the polymer beads. Our findings are relevant to the
description of polymer–surface adhesion and friction and the development of polymer nanocomposites with
tailored structural and mechanical properties
Cage-jump motion reveals universal dynamics and non-universal structural features in glass forming liquids
The sluggish and heterogeneous dynamics of glass forming liquids is
frequently associated to the transient coexistence of two phases of particles,
respectively with an high and low mobility. In the absence of a dynamical order
parameter that acquires a transient bimodal shape, these phases are commonly
identified empirically, which makes difficult investigating their relation with
the structural properties of the system. Here we show that the distribution of
single particle diffusivities can be accessed within a Continuous Time Random
Walk description of the intermittent motion, and that this distribution
acquires a transient bimodal shape in the deeply supercooled regime, thus
allowing for a clear identification of the two coexisting phase. In a simple
two-dimensional glass forming model, the dynamic phase coexistence is
accompanied by a striking structural counterpart: the distribution of the
crystalline-like order parameter becomes also bimodal on cooling, with
increasing overlap between ordered and immobile particles. This simple
structural signature is absent in other models, such as the three-dimesional
Kob-Andersen Lennard-Jones mixture, where more sophisticated order parameters
might be relevant. In this perspective, the identification of the two dynamical
coexisting phases opens the way to deeper investigations of structure-dynamics
correlations.Comment: Published in the J. Stat. Mech. Special Issue "The Role of Structure
in Glassy and Jammed Systems
Dynamical Correlation Length and Relaxation Processes in a Glass Former
We investigate the relaxation process and the dynamical heterogeneities of
the kinetically constrained Kob--Anderson lattice glass model, and show that
these are characterized by different timescales. The dynamics is well described
within the diffusing defect paradigm, which suggest to relate the relaxation
process to a reverse--percolation transition. This allows for a geometrical
interpretation of the relaxation process, and of the different timescales
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