287 research outputs found
Molecular dynamics simulations of the Johari-Goldstein relaxation in a molecular liquid
Molecular dynamics simulations (mds) were carried out to investigate the
reorientational motion of a rigid (fixed bond length), asymmetric diatomic
molecule in the liquid and glassy states. In the latter the molecule reorients
via large-angle jumps, which we identify with the Johari-Goldstein (JG)
dynamics. This relaxation process has a broad distribution of relaxation times,
and at least deeply in the glass state, the mobility of a given molecule
remains fixed over time; that is, there is no dynamic exchange among molecules.
Interestingly, the JG relaxation time for a molecule does not depend on the
local density, although the non-ergodicity factor is weakly correlated with the
packing efficiency of neighboring molecules. In the liquid state the frequency
of the JG process increases significantly, eventually subsuming the slower
alpha-relaxation. This evolution of the JG-motion into structural relaxation
underlies the correlation of many properties of the JG- and alpha-dynamics.Comment: 12 pages, 6 figure
Equilibrium and non-equilibrium fluctuations in a glass-forming liquid
Glass-forming liquids display strong fluctuations -- dynamical
heterogeneities -- near their glass transition. By numerically simulating a
binary Weeks-Chandler-Andersen liquid and varying both temperature and
timescale, we investigate the probability distributions of two kinds of local
fluctuations in the non-equilibrium (aging) regime and in the equilibrium
regime; and find them to be very similar in the two regimes and across
temperatures. We also observe that, when appropriately rescaled, the integrated
dynamic susceptibility is very weakly dependent on temperature and very similar
in both regimes.Comment: v1: 5 pages, 4 figures v2: 5 pages, 4 figures. Now includes results
at three temperatures, two of them above T_{MCT} and one below T_{MCT}; and
more extensive discussion of connections to experiment
Origin of the slow dynamics and the aging of a soft glass
We study by light microscopy a soft glass consisting of a compact arrangement
of polydisperse elastic spheres. We show that its slow and non-stationary
dynamics results from the unavoidable small fluctuations of temperature, which
induce intermittent local mechanical shear in the sample, because of thermal
expansion and contraction. Temperature-induced shear provokes both reversible
and irreversible rearrangements whose amplitude decreases with time, leading to
an exponential slowing down of the dynamics with sample age.Comment: published in PRL 97, 238301, 200
Binary mixture of hard disks as a model glass former: Caging and uncaging
I have proposed a measure for the cage effect in glass forming systems. A
binary mixture of hard disks is numerically studied as a model glass former. A
network is constructed on the basis of the colliding pairs of disks. A rigidity
matrix is formed from the isostatic (rigid) sub--network, corresponding to a
cage. The determinant of the matrix changes its sign when an uncaging event
occurs. Time evolution of the number of the uncaging events is determined
numerically. I have found that there is a gap in the uncaging timescales
between the cages involving different numbers of disks. Caging of one disk by
two neighboring disks sustains for a longer time as compared with other cages
involving more than one disk. This gap causes two--step relaxation of this
system
Investigation of -dependent dynamical heterogeneity in a colloidal gel by x-ray photon correlation spectroscopy
We use time-resolved X-Photon Correlation Spectroscopy to investigate the
slow dynamics of colloidal gels made of moderately attractive carbon black
particles. We show that the slow dynamics is temporally heterogeneous and
quantify its fluctuations by measuring the variance of the instantaneous
intensity correlation function. The amplitude of dynamical fluctuations has a
non-monotonic dependence on scattering vector , in stark contrast with
recent experiments on strongly attractive colloidal gels [Duri and Cipelletti,
\textit{Europhys. Lett.} \textbf{76}, 972 (2006)]. We propose a simple scaling
argument for the -dependence of fluctuations in glassy systems that
rationalizes these findings.Comment: Final version published in PR
Expansion for -Core Percolation
The physics of -core percolation pertains to those systems whose
constituents require a minimum number of connections to each other in order
to participate in any clustering phenomenon. Examples of such a phenomenon
range from orientational ordering in solid ortho-para mixtures to
the onset of rigidity in bar-joint networks to dynamical arrest in
glass-forming liquids. Unlike ordinary () and biconnected ()
percolation, the mean field -core percolation transition is both
continuous and discontinuous, i.e. there is a jump in the order parameter
accompanied with a diverging length scale. To determine whether or not this
hybrid transition survives in finite dimensions, we present a expansion
for -core percolation on the -dimensional hypercubic lattice. We show
that to order the singularity in the order parameter and in the
susceptibility occur at the same value of the occupation probability. This
result suggests that the unusual hybrid nature of the mean field -core
transition survives in high dimensions.Comment: 47 pages, 26 figures, revtex
Quantitative Theory of a Relaxation Function in a Glass-Forming System
We present a quantitative theory for a relaxation function in a simple
glass-forming model (binary mixture of particles with different interaction
parameters). It is shown that the slowing down is caused by the competition
between locally favored regions (clusters) which are long lived but each of
which relaxes as a simple function of time. Without the clusters the relaxation
of the background is simply determined by one typical length which we deduce
from an elementary statistical mechanical argument. The total relaxation
function (which depends on time in a nontrivial manner) is quantitatively
determined as a weighted sum over the clusters and the background. The
`fragility' in this system can be understood quantitatively since it is
determined by the temperature dependence of the number fractions of the locally
favored regions.Comment: 4 pages, 5 figure
Length scale dependence of dynamical heterogeneity in a colloidal fractal gel
We use time-resolved dynamic light scattering to investigate the slow
dynamics of a colloidal gel. The final decay of the average intensity
autocorrelation function is well described by , with and
decreasing from 1.5 to 1 with increasing . We show that the dynamics is not
due to a continuous ballistic process, as proposed in previous works, but
rather to rare, intermittent rearrangements. We quantify the dynamical
fluctuations resulting from intermittency by means of the variance
of the instantaneous autocorrelation function, the analogous of
the dynamical susceptibility studied in glass formers. The amplitude
of is found to grow linearly with . We propose a simple --yet
general-- model of intermittent dynamics that accounts for the dependence
of both the average correlation functions and .Comment: Revised and improved, to appear in Europhys. Let
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