323 research outputs found
Thermal Fluctuations in a Lamellar Phase of a Binary Amphiphile-Solvent Mixture: A Molecular Dynamics Study
We investigate thermal fluctuations in a smectic A phase of an
amphiphile-solvent mixture with molecular dynamics simulations. We use an
idealized model system, where solvent particles are represented by simple
beads, and amphiphiles by bead-and-spring tetramers. At a solvent bead fraction
of 20 % and sufficiently low temperature, the amphiphiles self-assemble into a
highly oriented lamellar phase. Our study aims at comparing the structure of
this phase with the predictions of the elastic theory of thermally fluctuating
fluid membrane stacks [Lei et al., J. Phys. II 5, 1155 (1995)]. We suggest a
method which permits to calculate the bending rigidity and compressibility
modulus of the lamellar stack from the simulation data. The simulation results
are in reasonable agreement with the theory
Cross-contamination of oilseeds by insecticide residues during storage
Pesticide residues are found in oilseeds (rapeseed and sunflower) and crude oils: they are mainly organophosphate insecticides (pirimiophos-methyl, malathion when authorized) used in empty storage facilities and for direct application to stored cereal grain. Even if some secondary pests are found in stored oilseeds, French regulations do not allow use of these insecticides on stored oilseeds. These residues arise from cross-contamination from storage bins and grain handling equipment of grain stores, and not from illegal use. This uptake of insecticide residues from their storage environment by oilseeds may lead to residue contents that exceed regulatory limits. A three-year investigation in grain storage companies allowed us to follow the course of sunflower batches and rapeseed batches during storage seasons 2006-2007, 2007-2008 and 2008-2009, from reception at the storage facility to outloading. Each of these batches was sampled at outloading, and was analyzed for insecticide residues. Traceability of oilseeds established by grain-store managers allowed us to identify cross-contamination sources. The insecticides that were most commonly detected were pirimiophos-methyl, malathion, and dichlorvos (in sunflower 2006-2007), plus chlorpyriphos-methyl and deltamethrin. Pirimiophos-methyl was the most commonly detected active substance, and caused the most cases of non-accordance with regulatory levels in rapeseed. Cross-contamination could have occurred when cereal grains were treated upon receipt, when rapeseed was also delivered, especially when treatments were done systematically to the cereal grains. For sunflower, the main cross-contamination hazard resulted from treatment of cereals at the period of receipt or at their outloading, just before sunflower seeds batches were outloaded. Another situation led to cross-contamination, but generally at a lower extent: oilseeds stored in bins that contained previously treated cereals, or loaded in empty bins with handling equipment treated before the receipt of oilseeds.Keywords: Oilseed storage, Cross-contamination, Insecticide residues, Rapeseed, Sunflowe
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
Critical behavior of frustrated systems: Monte Carlo simulations versus Renormalization Group
We study the critical behavior of frustrated systems by means of Pade-Borel
resummed three-loop renormalization-group expansions and numerical Monte Carlo
simulations. Amazingly, for six-component spins where the transition is second
order, both approaches disagree. This unusual situation is analyzed both from
the point of view of the convergence of the resummed series and from the
possible relevance of non perturbative effects.Comment: RevTex, 10 pages, 3 Postscript figure
The critical behavior of frustrated spin models with noncollinear order
We study the critical behavior of frustrated spin models with noncollinear
order, including stacked triangular antiferromagnets and helimagnets. For this
purpose we compute the field-theoretic expansions at fixed dimension to six
loops and determine their large-order behavior. For the physically relevant
cases of two and three components, we show the existence of a new stable fixed
point that corresponds to the conjectured chiral universality class. This
contradicts previous three-loop field-theoretical results but is in agreement
with experiments.Comment: 4 pages, RevTe
Gromov-Hausdorff-like distance function defined in the aspect of Riemannian submanifold theory
In this paper, we discuss how a Gromov-Hausdorff-like distance function over
the space of all isometric classes of compact -Riemannian manifolds should
be defined in the aspect of the Riemannan submanifold theory, where .
The most important fact in this discussion is as follows. The Hausdorff
distance function between two spheres of mutually distinct radii isometrically
embedded into the hypebolic space of curvature converges to zero as
. The key in the construction of the Gromov-Hausdorff-like
distance function given in this paper is to define the distance of two
-isometric embeddings of distinct compact -Riemannian manifolds
into a higher dimensional Riemannian manifold by using the Hausdorff distance
function in the tangent bundle of order equipped with the Sasaki metric.
Furthermore, we show that the convergence of a sequence of compact Riemannian
manifolds with respect to this distance function coincides with the convergence
in the sense of R. S. Hamilton.Comment: 14 page
Astrochemical models of interstellar ices: History matters
Ice is ubiquitous in the interstellar medium. We model the formation of the
main constituents of interstellar ices, including H2O, CO2 , CO, and CH3 OH. We
strive to understand what physical or chemical parameters influence the final
composition of the ice and how they benchmark to what has already been
observed, with the aim of applying these models to the preparation and analysis
of JWST observations. We used the Nautilus gas-grain model, which computes the
gas and ice composition as a function of time for a set of physical conditions,
starting from an initial gas phase composition. All important processes
(gas-phase reactions, gas-grain interactions, and grain surface processes) are
included and solved with the rate equation approximation. We first ran an
astrochemical code for fixed conditions of temperature and density mapped in
the cold core L429-C to benchmark the chemistry. One key parameter was revealed
to be the dust temperature. When the dust temperature is higher than 12 K, CO2
will form efficiently at the expense of H2O, while at temperatures below 12 K,
it will not form. Whatever hypothesis we assumed for the chemistry (within
realistic conditions), the static simulations failed to reproduce the observed
trends of interstellar ices in our target core. In a second step, we simulated
the chemical evolution of parcels of gas undergoing different physical and
chemical situations throughout the molecular cloud evolution and starting a few
1e7 yr prior to the core formation (dynamical simulations). Our dynamical
simulations satisfactorily reproduce the main trends already observed for
interstellar ices. Moreover, we predict that the apparent constant ratio of
CO2/H2O observed to date is probably not true for regions of low AV , and that
the history of the evolution of clouds plays an essential role, even prior to
their formation.Comment: Accepted for publication in A&
Spin Stiffness of Stacked Triangular Antiferromagnets
We study the spin stiffness of stacked triangular antiferromagnets using both
heat bath and broad histogram Monte Carlo methods. Our results are consistent
with a continuous transition belonging to the chiral universality class first
proposed by Kawamura.Comment: 5 pages, 7 figure
Critical thermodynamics of three-dimensional chiral model for N > 3
The critical behavior of the three-dimensional -vector chiral model is
studied for arbitrary . The known six-loop renormalization-group (RG)
expansions are resummed using the Borel transformation combined with the
conformal mapping and Pad\'e approximant techniques. Analyzing the fixed point
location and the structure of RG flows, it is found that two marginal values of
exist which separate domains of continuous chiral phase transitions and where such
transitions are first-order. Our calculations yield and
. For the structure of RG flows is identical to
that given by the and 1/N expansions with the chiral fixed point
being a stable node. For the chiral fixed point turns out to be a
focus having no generic relation to the stable fixed point seen at small
and large . In this domain, containing the physical values and , phase trajectories approach the fixed point in a spiral-like
manner giving rise to unusual crossover regimes which may imitate varying
(scattered) critical exponents seen in numerous physical and computer
experiments.Comment: 12 pages, 3 figure
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