495 research outputs found
Formation of Chain-Folded Structures from Supercooled Polymer Melts
The formation of chain-folded structures from the melt is observed in
molecular dynamics simulations resembling the lamellae of polymer crystals.
Crystallization and subsequent melting temperatures are related linearly to the
inverse lamellar thickness. Analysis of the single chain conformations in the
crystal shows that most chains reenter the same lamella by tight backfolds.
Simulations are performed with a mesoscopic bead-spring model including a
specific angle bending potential. They demonstrate that chain stiffness alone,
without an attractive inter-particle potential, is a sufficient driving force
for the formation of chain-folded lamellae.Comment: 4 pages, 5 figure
Kinetic Monte Carlo simulations of the growth of polymer crystals
Based upon kinetic Monte Carlo simulations of crystallization in a simple
polymer model we present a new picture of the mechanism by which the thickness
of lamellar polymer crystals is constrained to a value close to the minimum
thermodynamically stable thickness, l_{min}. The free energetic costs of the
polymer extending beyond the edges of the previous crystalline layer and of a
stem being shorter than l_{min} provide upper and lower constraints on the
length of stems in a new layer. Their combined effect is to cause the crystal
thickness to converge dynamically to a value close to l_{min} where growth with
constant thickness then occurs. This description contrasts with those given by
the two dominant theoretical approaches. However, at small supercoolings the
rounding of the crystal profile does inhibit growth as suggested in Sadler and
Gilmer's entropic barrier model.Comment: 12 pages, 13 figures, revte
The physical determinants of the thickness of lamellar polymer crystals
Based upon kinetic Monte Carlo simulations of crystallization in a simple
polymer model we present a new picture of the mechanism by which the thickness
of lamellar polymer crystals is constrained to a value close to the minimum
thermodynamically stable thickness. This description contrasts with those given
by the two dominant theoretical approaches.Comment: 4 pages, 4 figures, revte
Theory of collective opinion shifts: from smooth trends to abrupt swings
We unveil collective effects induced by imitation and social pressure by
analyzing data from three different sources: birth rates, sales of cell phones
and the drop of applause in concert halls. We interpret our results within the
framework of the Random Field Ising Model, which is a threshold model for
collective decisions accounting both for agent heterogeneity and social
imitation. Changes of opinion can occur either abruptly or continuously,
depending on the importance of herding effects. The main prediction of the
model is a scaling relation between the height h of the speed of variation peak
and its width of the form h ~ w^{-kappa}, with kappa = 2/3 for well
connected populations. Our three sets of data are compatible with such a
prediction, with kappa ~ 0.62 for birth rates, kappa ~ 0.71 for cell phones and
kappa ~ 0.64 for clapping. In this last case, we in fact observe that some
clapping samples end discontinuously (w=0), as predicted by the model for
strong enough imitation.Comment: 11 pages, 8 figure
Nitrogen deprivation induces triacylglycerol accumulation, drug tolerance and hypervirulence in mycobacteria.
Mycobacteria share with other actinomycetes the ability to produce large quantities of triacylglycerol (TAG), which accumulate as intracytoplasmic lipid inclusions (ILI) also known as lipid droplets (LD). Mycobacterium tuberculosis (M. tb), the etiologic agent of tuberculosis, acquires fatty acids from the human host which are utilized to synthesize TAG, subsequently stored in the form of ILI to meet the carbon and nutrient requirements of the bacterium during long periods of persistence. However, environmental factors governing mycobacterial ILI formation and degradation remain poorly understood. Herein, we demonstrated that in the absence of host cells, carbon excess and nitrogen starvation promote TAG accumulation in the form of ILI in M. smegmatis and M. abscessus, used as surrogate species of M. tb. Based on these findings, we developed a simple and reversible in vitro model to regulate ILI biosynthesis and hydrolysis in mycobacteria. We also showed that TAG formation is tgs1 dependent and that lipolytic enzymes mediate TAG breakdown. Moreover, we confirmed that the nitrogen-deprived and ILI-rich phenotype was associated with an increased tolerance towards several drugs used for treating mycobacterial infections. Importantly, we showed that the presence of ILI substantially enhanced the bacterial burden and granuloma abundance in zebrafish embryos infected with lipid-rich M. abscessus as compared to embryos infected with lipid-poor M. abscessus, suggesting that ILI are actively contributing to mycobacterial virulence and pathogenesis
Two-color interferometer for the study of laser filamentation triggered electric discharges in air
International audienceWe present a space and time resolved interferometric plasma diagnostic for use on plasmas where neutral-bound electron contribution to the refractive index cannot be neglected. By recording simultaneously the plasma optical index at 532 and 1064 nm, we are able to extract independently the neutral and free electron density profiles. We report a phase resolution of 30 mrad, corresponding to a maximum resolution on the order of 4 × 10 22 m −3 for the electron density, and of 10 24 m −3 for the neutral density. The interferometer is demonstrated on centimeter-scale sparks triggered by laser filamentation in air with typical currents of a few tens of A
Pseudo-potentials and loading surfaces for an endochronic plasticity theory with isotropic damage
The endochronic theory, developed in the early 70s, allows the plastic
behavior of materials to be represented by introducing the notion of intrinsic
time. With different viewpoints, several authors discussed the relationship
between this theory and the classical theory of plasticity. Two major
differences are the presence of plastic strains during unloading phases and the
absence of an elastic domain. Later, the endochronic plasticity theory was
modified in order to introduce the effect of damage. In the present paper, a
basic endochronic model with isotropic damage is formulated starting from the
postulate of strain equivalence. Unlike the previous similar analyses, in this
presentation the formal tools chosen to formulate the model are those of convex
analysis, often used in classical plasticity: namely pseudopotentials,
indicator functions, subdifferentials, etc. As a result, the notion of loading
surface for an endochronic model of plasticity with damage is investigated and
an insightful comparison with classical models is made possible. A damage
pseudopotential definition allowing a very general damage evolution is given
Gluon mass generation in the PT-BFM scheme
In this article we study the general structure and special properties of the
Schwinger-Dyson equation for the gluon propagator constructed with the pinch
technique, together with the question of how to obtain infrared finite
solutions, associated with the generation of an effective gluon mass.
Exploiting the known all-order correspondence between the pinch technique and
the background field method, we demonstrate that, contrary to the standard
formulation, the non-perturbative gluon self-energy is transverse
order-by-order in the dressed loop expansion, and separately for gluonic and
ghost contributions. We next present a comprehensive review of several subtle
issues relevant to the search of infrared finite solutions, paying particular
attention to the role of the seagull graph in enforcing transversality, the
necessity of introducing massless poles in the three-gluon vertex, and the
incorporation of the correct renormalization group properties. In addition, we
present a method for regulating the seagull-type contributions based on
dimensional regularization; its applicability depends crucially on the
asymptotic behavior of the solutions in the deep ultraviolet, and in particular
on the anomalous dimension of the dynamically generated gluon mass. A
linearized version of the truncated Schwinger-Dyson equation is derived, using
a vertex that satisfies the required Ward identity and contains massless poles
belonging to different Lorentz structures. The resulting integral equation is
then solved numerically, the infrared and ultraviolet properties of the
obtained solutions are examined in detail, and the allowed range for the
effective gluon mass is determined. Various open questions and possible
connections with different approaches in the literature are discussed.Comment: 54 pages, 24 figure
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