1,507 research outputs found
Renormalisation-theoretic analysis of non-equilibrium phase transitions II: The effect of perturbations on rate coefficients in the Becker-Doring equations
We study in detail the application of renormalisation theory to models of
cluster aggregation and fragmentation of relevance to nucleation and growth
processes. In particular, we investigate the Becker-Doring (BD) equations,
originally formulated to describe and analyse non-equilibrium phase
transitions, but more recently generalised to describe a wide range of
physicochemical problems. We consider here rate coefficients which depend on
the cluster size in a power-law fashion, but now perturbed by small amplitude
random noise. Power-law rate coefficients arise naturally in the theory of
surface-controlled nucleation and growth processes. The noisy perturbations on
these rates reflect the effect of microscopic variations in such mean-field
coefficients, thermal fluctuations and/or experimental uncertainties. In the
present paper we generalise our earlier work that identified the nine classes
into which all dynamical behaviour must fall by investigating how random
perturbations of the rate coefficients influence the steady-state and kinetic
behaviour of the coarse-grained, renormalised system. We are hence able to
confirm the existence of a set of up to nine universality classes for such BD
systems.Comment: 30 pages, to appear in J Phys A Math Ge
Renormalisation-theoretic analysis of non-equilibrium phase transitions I: The Becker-Doring equations with power law rate coefficients
We study in detail the application of renormalisation theory to models of
cluster aggregation and fragmentation of relevance to nucleation and growth
processes. We investigate the Becker-Dorging equations, originally formulated
to describe and analyse non-equilibrium phase transitions, and more recently
generalised to describe a wide range of physicochemical problems. In the
present paper we analyse how the systematic coarse-graining renormalisation of
the \BD system of equations affects the aggregation and fragmentation rate
coefficients. We consider the case of power-law size-dependent cluster rate
coefficients which we show lead to only three classes of system that require
analysis: coagulation-dominated systems, fragmentation-dominated systems and
those where coagulation and fragmentation are exactly balanced. We analyse the
late-time asymptotics associated with each class.Comment: 18 pages, to appear in J Phys A Math Ge
Evaluation of Synthetic and Semi- synthetic Culture Media for Endo-1,4-β- Glucanases Secretion by Trichoderma koningiopsis
AbstractThe actual demand of energy and the environmental concerns together with the reduced fossil fuel reserves have played an important role to convert the second generation bioethanol production into an attractive research area. To convert lignocellulosic biomass to bioethanol the cellulosic components must be hydrolyzed to fermentable sugars. Trichoderma fungi secrete large amounts of enzymes of industrial interest such as cellulases, able to degrade holocellulose in the saccharification of lignocellulosic biomass. In this work we evaluated endo-1.4-β-glucanases enzymatic secretion of Trichoderma koningiopsis from Misiones province, in synthetic medium, with carboxymethylcellulose as carbon source; and semi-synthetic medium, with pine sawdust as carbon source. Higher values of endo-1.4-β-glucanases were reached when the semi-synthetic medium was used. It could be concluded that pine sawdust seems to be a good candidate for utilization as carbon source in culture media aiming to obtain good enzyme secretion, being also an economic and easily available substrate
Mean field approximation of two coupled populations of excitable units
The analysis on stability and bifurcations in the macroscopic dynamics
exhibited by the system of two coupled large populations comprised of
stochastic excitable units each is performed by studying an approximate system,
obtained by replacing each population with the corresponding mean-field model.
In the exact system, one has the units within an ensemble communicating via the
time-delayed linear couplings, whereas the inter-ensemble terms involve the
nonlinear time-delayed interaction mediated by the appropriate global
variables. The aim is to demonstrate that the bifurcations affecting the
stability of the stationary state of the original system, governed by a set of
4N stochastic delay-differential equations for the microscopic dynamics, can
accurately be reproduced by a flow containing just four deterministic
delay-differential equations which describe the evolution of the mean-field
based variables. In particular, the considered issues include determining the
parameter domains where the stationary state is stable, the scenarios for the
onset and the time-delay induced suppression of the collective mode, as well as
the parameter domains admitting bistability between the equilibrium and the
oscillatory state. We show how analytically tractable bifurcations occurring in
the approximate model can be used to identify the characteristic mechanisms by
which the stationary state is destabilized under different system
configurations, like those with symmetrical or asymmetrical inter-population
couplings.Comment: 5 figure
Gradual transition from insulator to semimetal of CaEuB with increasing Eu concentration
The local environment of Eu (, ) in
CaEuB () is investigated by
means of electron spin resonance (ESR). For the spectra show
resolved \textit{fine} and \textit{hyperfine} structures due to the cubic
crystal \textit{electric} field and nuclear \textit{hyperfine} field,
respectively. The resonances have Lorentzian line shape, indicating an
\textit{insulating} environment for the Eu ions. For , as increases, the ESR lines broaden due to local
distortions caused by the Eu/Ca ions substitution. For , the lines broaden further and the spectra gradually change from
Lorentzian to Dysonian resonances, suggesting a coexistence of both
\textit{insulating} and \textit{metallic} environments for the Eu ions.
In contrast to CaGdB, the \textit{fine} structure is still
observable up to . For the \textit{fine} and
\textit{hyperfine} structures are no longer observed, the line width increases,
and the line shape is purely Dysonian anticipating the \textit{semimetallic}
character of EuB. This broadening is attributed to a spin-flip scattering
relaxation process due to the exchange interaction between conduction and
Eu electrons. High field ESR measurements for
reveal smaller and anisotropic line widths, which are attributed to magnetic
polarons and Fermi surface effects, respectively.Comment: Submitted to PR
Proper motions of the HH1 jet
We describe a new method for determining proper motions of extended objects,
and a pipeline developed for the application of this method. We then apply this
method to an analysis of four epochs of [S~II] HST images of the HH~1 jet
(covering a period of ~yr).
We determine the proper motions of the knots along the jet, and make a
reconstruction of the past ejection velocity time-variability (assuming
ballistic knot motions). This reconstruction shows an "acceleration" of the
ejection velocities of the jet knots, with higher velocities at more recent
times. This acceleration will result in an eventual merging of the knots in
~yr and at a distance of from the outflow source, close to
the present-day position of HH~1.Comment: 12 pages, 8 figure
Numerical Modeling of Eta Carinae Bipolar Outflows
In this paper, we present two-dimensional gas dynamic simulations of the
formation and evolution of the eta-Car bipolar outflows. Adopting the
interacting nonspherical winds model, we have carried out high-resolution
numerical simulations, which include explicitly computed time-dependent
radiative cooling, for different possible scenarios of the colliding winds. In
our simulations, we consider different degrees of non-spherical symmetry for
the pre-outburst wind and the great eruption of the 1840s presented by the
eta-Car wind. From these models, we obtain important differences in the shape
and kinematical properties of the Homunculus structure. In particular, we find
an appropriate combination of the wind parameters (that control the degree of
non-spherical symmetry) and obtain numerical experiments that best match both
the observed morphology and the expansion velocity of the eta-Car bipolar
shell. In addition, our numerical simulations show the formation of a bipolar
nebula embedded within the Homunculus (the little Homunculus) developed from a
secondary eruptive event suffered by the star in the 1890s, and also the
development of tenuous, high velocity ejections in the equatorial region that
result from the impact of the eruptive wind of the 1840s with the pre-outburst
wind and that could explain some of the high speed features observed in the
equatorial ejecta. The models were, however, unable to produce equatorial
ejections associated to the second eruptive event.Comment: 33 pages, 9 figures, accepted by the Astrophysical Journa
Alternative model of the Antonov problem
Astrophysical systems will never be in a real Thermodynamic equilibrium: they
undergo an evaporation process due to the fact that the gravity is not able to
confine the particles. Ordinarily, this difficulty is overcome by enclosing the
system in a rigid container which avoids the evaporation. We proposed an
energetic prescription which is able to confine the particles, leading in this
way to an alternative version of the Antonov isothermal model which unifies the
well-known isothermal and polytropic profiles. Besides of the main features of
the isothermal sphere model: the existence of the gravitational collapse and
the energetic region with a negative specific heat, this alternative model has
the advantage that the system size naturally appears as a consequence of the
particles evaporation.Comment: RevTex4, 9 pages, 10 figures, Version Submitted to PR
Charge-Fluctuation-Induced Non-analytic Bending Rigidity
In this Letter, we consider a neutral system of mobile positive and negative
charges confined on the surface of curved films. This may be an appropriate
model for: i) a highly charged membrane whose counterions are confined to a
sheath near its surface; ii) a membrane composed of an equimolar mixture of
anionic and cationic surfactants in aqueous solution. We find that the charge
fluctuations contribute a non-analytic term to the bending rigidity that varies
logarithmically with the radius of curvature. This may lead to spontaneous
vesicle formation, which is indeed observed in similar systems.Comment: Revtex, 9 pages, no figures, submitted to PR
Charge Fluctuations on Membrane Surfaces in Water
We generalize the predictions for attractions between over-all neutral
surfaces induced by charge fluctuations/correlations to non-uniform systems
that include dielectric discontinuities, as is the case for mixed charged lipid
membranes in an aqueous solution. We show that the induced interactions depend
in a non-trivial way on the dielectric constants of membrane and water and show
different scaling with distance depending on these properties. The generality
of the calculations also allows us to predict under which dielectric conditions
the interaction will change sign and become repulsive
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