192 research outputs found
Anomalous dynamical light scattering in soft glassy gels
We compute the dynamical structure factor S(q,tau) of an elastic medium where
force dipoles appear at random in space and in time, due to `micro-collapses'
of the structure. Various regimes are found, depending on the wave vector q and
the collapse time. In an early time regime, the logarithm of the structure
factor behaves as (q tau)^{3/2}, as predicted by Cipelletti et al. [1] using
heuristic arguments. However, in an intermediate time regime we rather obtain a
q tau)^{5/4} behaviour. Finally, the asymptotic long time regime is found to
behave as q^{3/2} tau. We also give a plausible scenario for aging, in terms of
a strain dependent energy barrier for micro-collapses. The relaxation time is
found to grow with the age t_w, quasi-exponentially at first, and then as
t_w^{4/5} with logarithmic corrections.Comment: 15 pages, 1 .eps figure. Submitted to EPJ-
Glassy effects in the swelling/collapse dynamics of homogeneous polymers
We investigate, using numerical simulations and analytical arguments, a
simple one dimensional model for the swelling or the collapse of a closed
polymer chain of size N, representing the dynamical evolution of a polymer in a
\Theta-solvent that is rapidly changed into a good solvent (swelling) or a bad
solvent (collapse). In the case of swelling, the density profile for
intermediate times is parabolic and expands in space as t^{1/3}, as predicted
by a Flory-like continuum theory. The dynamics slows down after a time \propto
N^2 when the chain becomes stretched, and the polymer gets stuck in metastable
`zig-zag' configurations, from which it escapes through thermal activation. The
size of the polymer in the final stages is found to grow as \sqrt{\ln t}. In
the case of collapse, the chain very quickly (after a time of order unity)
breaks up into clusters of monomers (`pearls'). The evolution of the chain then
proceeds through a slow growth of the size of these metastable clusters, again
evolving as the logarithm of time. We enumerate the total number of metastable
states as a function of the extension of the chain, and deduce from this
computation that the radius of the chain should decrease as 1/\ln(\ln t). We
compute the total number of metastable states with a given value of the energy,
and find that the complexity is non zero for arbitrary low energies. We also
obtain the distribution of cluster sizes, that we compare to simple
`cut-in-two' coalescence models. Finally, we determine the aging properties of
the dynamical structure. The subaging behaviour that we find is attributed to
the tail of the distribution at small cluster sizes, corresponding to
anomalously `fast' clusters (as compared to the average). We argue that this
mechanism for subaging might hold in other slowly coarsening systems.Comment: 35 pages, 12 .ps figures. Submitted to EPJ
Glassy behavior of a homopolymer from molecular dynamics simulations
We study at- and out-of-equilibrium dynamics of a single homopolymer chain at
low temperature using molecular dynamics simulations. The main quantities of
interest are the average root mean square displacement of the monomers below
the theta point, and the structure factor, as a function of time. The
observation of these quantities show a close resemblance to those measured in
structural glasses and suggest that the polymer chain in its low temperature
phase is in a glassy phase, with its dynamics dominated by traps. In
equilibrium, at low temperature, we observe the trapping of the monomers and a
slowing down of the overall motion of the polymer as well as non-exponential
relaxation of the structure factor. In out-of-equilibrium, at low temperatures,
we compute the two-time quantities and observe breaking of ergodicity in a
range of waiting times, with the onset of aging.Comment: 11 pages, 4 figure
Dynamic first-order phase transition in kinetically constrained models of glasses
We show that the dynamics of kinetically constrained models of glass formers
takes place at a first-order coexistence line between active and inactive
dynamical phases. We prove this by computing the large-deviation functions of
suitable space-time observables, such as the number of configuration changes in
a trajectory. We present analytic results for dynamic facilitated models in a
mean-field approximation, and numerical results for the Fredrickson-Andersen
model, the East model, and constrained lattice gases, in various dimensions.
This dynamical first-order transition is generic in kinetically constrained
models, and we expect it to be present in systems with fully jammed states.Comment: 4.1 pages, 3 figure
Spatial correlations in the relaxation of the Kob-Andersen model
We describe spatio-temporal correlations and heterogeneities in a kinetically
constrained glassy model, the Kob-Andersen model. The kinetic constraints of
the model alone induce the existence of dynamic correlation lengths, that
increase as the density increases, in a way compatible with a
double-exponential law. We characterize in detail the trapping time correlation
length, the cooperativity length, and the distribution of persistent clusters
of particles. This last quantity is related to the typical size of blocked
clusters that slow down the dynamics for a given density.Comment: 7 pages, 6 figures, published version (title has changed
Ethane-beta-Sultam Modifies the Activation of the Innate Immune System Induced by Intermittent Ethanol Administration in Female Adolescent Rats
Intermittent ethanol abuse or ‘binge drinking’ during adolescence induces neuronal damage, which may be associated with cognitive dysfunction. To investigate the neurochemical processes involved, rats were administered either 1 g/kg or 2 g/kg ethanol in a ‘binge drinking’ regime. After only 3 weeks, significant activation of phagocytic
cells in the peripheral (alveolar macrophages) and the hippocampal brain region (microglia cells) was present,as exemplified by increases in the release of pro-inflammatory cytokines in the macrophages and of iNOS in the microglia. This was associated with neuronal loss in the hippocampus CA1 region. Daily supplementation with a taurine prodrug, ethane-β-sultam, 0.028 g/kg, during the intermittent ethanol loading regime, supressed the release of the pro-inflammatory cytokines and of reactive nitrogen species, as well as neuronal loss, particularly in the rats administered the lower dose of ethanol, 1 g/kg. Plasma, macrophage and hippocampal taurine levels increased
marginally after ethane-β-sultam supplementation. The ‘binge drinking’ ethanol rats administered 1 g/kg ethanol showed increased latencies to those of the control rats in their acquisition of spacial navigation in the Morris Water
Maze, which was normalised to that of the controls values after ethane-β-sultam administration.
Such results confirm that the administration of ethane-β-sultam to binge drinking rats reduces neuroinflammation in both the periphery and the brain, suppresses neuronal loss, and improved working memory of rats in a water maze
study
Finite-size effects and intermittency in a simple aging system
We study the intermittent dynamics and the fluctuations of the dynamic
correlation function of a simple aging system. Given its size and its
coherence length , the system can be divided into independent
subsystems, where , and is the dimension of space.
Each of them is considered as an aging subsystem which evolves according to
an activated dynamics between energy levels.
We compute analytically the distribution of trapping times for the global
system, which can take power-law, stretched-exponential or exponential forms
according to the values of and the regime of times considered. An effective
number of subsystems at age , , can be defined, which
decreases as increases, as well as an effective coherence length,
, where characterizes the trapping
times distribution of a single subsystem. We also compute the probability
distribution functions of the time intervals between large decorrelations,
which exhibit different power-law behaviours as increases (or
decreases), and which should be accessible experimentally.
Finally, we calculate the probability distribution function of the two-time
correlator.
We show that in a phenomenological approach, where is replaced by the
effective number of subsystems , the same qualitative behaviour
as in experiments and simulations of several glassy systems can be obtained.Comment: 15 pages, 6 figures, published versio
Injected Power Fluctuations in 1D Dissipative Systems
Using fermionic techniques, we compute exactly the large deviation function
(ldf) of the time-integrated injected power in several one-dimensional
dissipative systems of classical spins. The dynamics are T=0 Glauber dynamics
supplemented by an injection mechanism, which is taken as a Poissonian flipping
of one particular spin. We discuss the physical content of the results,
specifically the influence of the rate of the Poisson process on the properties
of the ldf.Comment: 18 pages, 8 figure
Embedding a Native State into a Random Heteropolymer Model: The Dynamic Approach
We study a random heteropolymer model with Langevin dynamics, in the
supersymmetric formulation. Employing a procedure similar to one that has been
used in static calculations, we construct an ensemble in which the affinity of
the system for a native state is controlled by a "selection temperature" T0. In
the limit of high T0, the model reduces to a random heteropolymer, while for
T0-->0 the system is forced into the native state. Within the Gaussian
variational approach that we employed previously for the random heteropolymer,
we explore the phases of the system for large and small T0. For large T0, the
system exhibits a (dynamical) spin glass phase, like that found for the random
heteropolymer, below a temperature Tg. For small T0, we find an ordered phase,
characterized by a nonzero overlap with the native state, below a temperature
Tn \propto 1/T0 > Tg. However, the random-globule phase remains locally stable
below Tn, down to the dynamical glass transition at Tg. Thus, in this model,
folding is rapid for temperatures between Tg and Tn, but below Tg the system
can get trapped in conformations uncorrelated with the native state. At a lower
temperature, the ordered phase can also undergo a dynamical glass transition,
splitting into substates separated by large barriers.Comment: 19 pages, revtex, 6 figure
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