88 research outputs found
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
Collapse Dynamics of a Homopolymer: Theory and Simulation
We present a scaling theory describing the collapse of a homopolymer chain in
poor solvent. At time t after the beginning of the collapse, the original
Gaussian chain of length N is streamlined to form N/g segments of length R(t),
each containing g ~ t monomers. These segments are statistical quantities
representing cylinders of length R ~ t^{1/2} and diameter d ~ t^{1/4}, but
structured out of stretched arrays of spherical globules. This prescription
incorporates the capillary instability. We compare the time-dependent structure
factor derived for our theory with that obtained from ultra-large-scale
molecular dynamics simulation with explicit solvent. This is the first time
such a detailed comparison of theoretical and simulation predictions of
collapsing chain structure has been attempted. The favorable agreement between
the theoretical and computed structure factors supports the picture of the
coarse-graining process during polymer collapse.Comment: 4 pages, 3 figure
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
First-order dynamical phase transition in models of glasses: an approach based on ensembles of histories
We investigate the dynamics of kinetically constrained models of glass
formers by analysing the statistics of trajectories of the dynamics, or
histories, using large deviation function methods. We show that, in general,
these models exhibit a first-order dynamical transition between active and
inactive dynamical phases. We argue that the dynamical heterogeneities
displayed by these systems are a manifestation of dynamical first-order phase
coexistence. In particular, we calculate dynamical large deviation functions,
both analytically and numerically, for the Fredrickson-Andersen model, the East
model, and constrained lattice gas models. We also show how large deviation
functions can be obtained from a Landau-like theory for dynamical fluctuations.
We discuss possibilities for similar dynamical phase-coexistence behaviour in
other systems with heterogeneous dynamics.Comment: 29 pages, 7 figs, final versio
Capillary condensation in disordered porous materials: hysteresis versus equilibrium behavior
We study the interplay between hysteresis and equilibrium behavior in
capillary condensation of fluids in mesoporous disordered materials via a
mean-field density functional theory of a disordered lattice-gas model. The
approach reproduces all major features observed experimentally. We show that
the simple van der Waals picture of metastability fails due to the appearance
of a complex free-energy landscape with a large number of metastable states. In
particular, hysteresis can occur both with and without an underlying
equilibrium transition, thermodynamic consistency is not satisfied along the
hysteresis loop, and out-of-equilibrium phase transitions are possible.Comment: 4 pages, 4 figure
Critical behavior of a fluid in a disordered porous matrix: An Ornstein-Zernike approach
Using a liquid-state approach based on Ornstein-Zernike equations, we study
the behavior of a fluid inside a porous disordered matrix near the liquid-gas
critical point.The results obtained within various standard approximation
schemes such as lowest-order -ordering and the mean-spherical
approximation suggest that the critical behavior is closely related to that of
the random-field Ising model (RFIM).Comment: 10 pages, revtex, to appear in Physical Review Letter
Complete genome sequence of Burkholderia phenoliruptrix BR3459a (CLA1), a heat-tolerant, nitrogen-fixing symbiont of mimosa flocculosa.
The genus Burkholderia represents a challenge to the fields of taxonomy and phylogeny and, especially, to the understanding of the contrasting roles as either opportunistic pathogens or bacteria with biotechnological potential. Few genomes of nonpathogenic strains, especially of diazotrophic symbiotic bacteria, have been sequenced to improve understanding of the genus. Here, we contribute with the complete genome sequence of Burkholderia phenoliruptrix strain BR3459a (CLA1), an effective diazotrophic symbiont of the leguminous tree Mimosa flocculosa Burkart, which is endemic to South America
Peripheral blood T-cell signatures from high-resolution immune phenotyping of γδ and αβ T-cells in younger and older subjects in the Berlin Aging Study II
Background Aging and latent infection with Cytomegalovirus (CMV) are thought
to be major factors driving the immune system towards immunosenescence,
primarily characterized by reduced amounts of naïve T-cells and increased
memory T-cells, potentially associated with higher morbidity and mortality.
The composition of both major compartments, γδ as well as αβ T-cells, is
altered by age and CMV, but detailed knowledge of changes to the γδ subset is
currently limited. Results Here, we have surveyed a population of 73 younger
(23–35 years) and 144 older (62–85 years) individuals drawn from the Berlin
Aging Study II, investigating the distribution of detailed differentiation
phenotypes of both γδ and αβ T-cells. Correlation of frequencies and absolute
counts of the identified phenotypes with age and the presence of CMV revealed
a lower abundance of Vδ2-positive and a higher amount of Vδ1-positive cells.
We found higher frequencies of late-differentiated and lower frequencies of
early-differentiated cells in the Vδ1+ and Vδ1-Vδ2-, but not in the Vδ2+
populations in elderly CMV-seropositive individuals confirming the association
of these Vδ2-negative cells with CMV-immunosurveillance. We identified the
highest Vδ1:Vδ2 ratios in the CMV-seropositive elderly. The observed increased
CD4:CD8 ratios in the elderly were significantly lower in CMV-seropositive
individuals, who also possessed a lower naïve and a larger late-differentiated
compartment of CD8+ αβ T-cells, reflecting the consensus in the literature.
Conclusions Our findings illustrate in detail the strong influence of CMV on
the abundance and differentiation pattern of γδ T-cells as well as αβ T-cells
in older and younger people. Mechanisms responsible for the phenotypic
alterations in the γδ T-cell compartment, associated both with the presence of
CMV and with age require further clarification
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