1,033 research outputs found
Observation of a tricritical wedge filling transition in the 3D Ising model
In this Letter we present evidences of the occurrence of a tricritical
filling transition for an Ising model in a linear wedge. We perform Monte Carlo
simulations in a double wedge where antisymmetric fields act at the top and
bottom wedges, decorated with specific field acting only along the wegde axes.
A finite-size scaling analysis of these simulations shows a novel critical
phenomenon, which is distinct from the critical filling. We adapt to
tricritical filling the phenomenological theory which successfully was applied
to the finite-size analysis of the critical filling in this geometry, observing
good agreement between the simulations and the theoretical predictions for
tricritical filling.Comment: 5 pages, 3 figure
Density functional theory study of the nematic-isotropic transition in an hybrid cell
We have employed the Density Functional Theory formalism to investigate the
nematic-isotropic capillary transitions of a nematogen confined by walls that
favor antagonist orientations to the liquid crystal molecules (hybrid cell). We
analyse the behavior of the capillary transition as a function of the
fluid-substrate interactions and the pore width. In addition to the usual
capillary transition between isotropic-like to nematic-like states, we find
that this transition can be suppressed when one substrate is wet by the
isotropic phase and the other by the nematic phase. Under this condition the
system presents interface-like states which allow to continuously transform the
nematic-like phase to the isotropic-like phase without undergoing a phase
transition. Two different mechanisms for the disappearance of the capillary
transition are identified. When the director of the nematic-like state is
homogeneously planar-anchored with respect to the substrates, the capillary
transition ends up in a critical point. This scenario is analogous to the
observed in Ising models when confined in slit pores with opposing surface
fields which have critical wetting transitions. When the nematic-like state has
a linearly distorted director field, the capillary transition continuously
transforms in a transition between two nematic-like states.Comment: 31 pages, 10 figures, submitted to J. Chem. Phy
Phenological and molecular studies on the introduced seaweed Dictyota cyanoloma (Dictyotales, Phaeophyceae) along the Mediterranean coast of the Iberian Peninsula
Dictyota cyanoloma, a distinctive brown algal species characterized by a blue-iridescent margin, was recently reported as an introduced species in the Mediterranean Sea but little is known about its distribution dynamics, morphological plasticity and genetic structure. In this integrative study, we evaluate its past and present occurrence along the Mediterranean Iberian coast, assess the species' phenology in Palamos (Girona, Spain) and analyze the haplotype diversity by sequencing 49 individuals from nine sampling sites for different chloroplast and mitochondrial DNA regions. Although D. cyanoloma currently occurs along all the Mediterranean Iberian coasts (in 19 of 36 localities sampled, mostly in marinas and harbour environments), we were not able to find any herbarium material of this species (at BCN-Phyc and MA) predating the year 1987. In Palamos, D. cyanoloma is present all through the year, with a maximum development in winter and a minimum in summer. Fertile specimens are absent during summer (July and August). Sporophytes are dominant from January to June and gametophytes were found only in February, March and June. Information about the antheridia, which has never been described before, is provided. Two chloroplast and three mitochondrial haplotypes were observed, indicating that multiple introductions of D. cyanoloma occurred in the study area. Additionally, the genetic structure suggests that spread did not occur through simple advancing wave fronts but by several longdistance dispersal events. Further studies employing microsatellite markers could potentially offer a better resolution to unravel expansion and colonisation dynamics of D. cyanoloma in the Mediterranean Sea
The community of Cystoseira brachycarpa J. Agardh emend. Giaccone (Fucales, Phaeophyceae) in a shallow hydrothermal vent area of the Aeolian Islands (Tyrrhenian Sea, Italy)
A Cystoseira brachycarpa community from a vent area off Panarea Island (Italy) was investigated in two sites at different pH
values. At low pH, species richness and coverage were low and the community displayed a reduced reproductive capacity.
Conversely, at normal pH, dense canopies of fertile C. brachycarpa were found
Molecular-dynamics ensembles: Fluctuations and correlations near the phase transitions
Computer simulations of liquid and solid systems very close to the melting-freezing transition zone have been performed for the microcanonical, canonical, and isothermal-isobaric molecular-dynamics ensembles. Temperature, pressure, and density fluctuations were studied over long evolution times, and graphical and analytical statistical-error methods were used to investigate correlations in the data. The Nosé-Hoover (NH) method combined with the Toxvaerd algorithm is proposed as a correct method of obtaining the true fluctuation and correlation of the thermodynamic variables in the system, because the temperature and/or pressure constraints in the NH method do not affect the dynamical evolution of the system, and because the fifth-order Toxvaerd algorithm gives very accurate behavior for the correlations, as has been shown in recent studies.Comisión Interministerial de Ciencia y Tecnología PB92-052
Temperature and pressure constraints near the freezing point
The isothermal-isobaric ensemble molecular-dynamics method MD(T,p,N) proposed by Nosé and Hoover is used to study the fluctuations in a two-dimensional Lennard-Jones fluid, close to the freezing point. The T and p constraints in this method do not affect the dynamical behavior of the system, since spontaneous fluctuations in the density allow the system to freeze and melt just as do the T and p fluctuations in the microcanonical ensemble MD(E,V,N) close to the melting zone
Isothermal molecular-dynamics calculations
We have performed long-time runs of molecular-dynamics computer simulations of a two-dimensional Lennard-Jones system, without any scaling procedure. The thermodynamic properties show spontaneous fluctuations except when the system is far from the melting zon
Is the alpine divide becoming more permeable to biological invasions? - Insights on the invasion and establishment of the Walnut Husk Fly, Rhagoletis completa (Diptera: Tephritidae) in Switzerland
The Walnut Husk Fly, Rhagoletis completa Cresson (Diptera: Tephritidae), is native to North America (Midwestern US and north-eastern Mexico) and has invaded several European countries in the past decades by likely crossing the alpine divide separating most parts of Switzerland from Italy. Here, we determined its current distribution in Switzerland by sampling walnuts (Juglans regia L.) in ecologically and climatically distinct regions along potential invasion corridors. R. completa was found to be firmly established in most low altitude areas of Switzerland where walnuts thrive, but notably not a single parasitoid was recovered from any of the samples. Infested fruit was recovered in 42 of the 71 localities that were surveyed, with mean fruit infestation rate varying greatly among sites. The incidence of R. completa in Switzerland is closely related to meteorological mean spring temperature patterns influencing growing season length, but not to winter temperatures, reflecting survival potential during hibernation. Importantly, areas in which the fly is absent correspond with localities where the mean spring temperatures fall below 7°C. Historical data records show that the natural cold barrier around the Alpine divide in the central Swiss Alps corresponding to such minimal temperatures has shrunk significantly from a width of more than 40 km before 1990 to around 20 km after 2000. We hypothesize on possible invasion/expansion routes along alpine valleys, dwell on distribution patterns in relation to climate, and outline future research needs as the incursion of R. completa into Switzerland; and, more recently, other European countries, such as Germany, Austria, France and Slovenia, represent an example of alien species that settle first in the Mediterranean Basin and from there become invasive by crossing the Alp
Breakdown of the mean-field approximation in a wealth distribution model
One of the key socioeconomic phenomena to explain is the distribution of
wealth. Bouchaud and M\'ezard have proposed an interesting model of economy
[Bouchaud and M\'ezard (2000)] based on trade and investments of agents. In the
mean-field approximation, the model produces a stationary wealth distribution
with a power-law tail. In this paper we examine characteristic time scales of
the model and show that for any finite number of agents, the validity of the
mean-field result is time-limited and the model in fact has no stationary
wealth distribution. Further analysis suggests that for heterogeneous agents,
the limitations are even stronger. We conclude with general implications of the
presented results.Comment: 11 pages, 3 figure
La Termogènesi als calorímetres per conducció: característiques dinàmiques i possibilitats deconvolutives
Es descriuen les característiques generals dels dispositius calorimètrics, les possibilitats de tractament
mitjançant models i els mètodes per aproximar-se a la termogènesi o dissipació instantània al si de la cèllula laboratori.
La descripció dinàmica es fa dins de l'espai freqüencial,
que permet introduir de manera natural diferents límits
de freqüència segons les característiques dels dispositius
i del procés que vol estudiar-se.
Per al tractament global dels calorímetres s'utilitza
una escala relativa de temps i de freqüència. Això permet
tractar sistemàticament les condicions imposades per les
tècniques deconvolutives, l'aparellatge experimental i els
propis fenòmens físics.General features of flow or conduction calorimeters are
briefly described together with several methods to obtain the thermogenesis or instantaneous power dissipated inside the laboratory cell. The possibi l ities of solvable models are also discussed. The dynamic behaviour of the calorimeter is readily described in frequency space, thus allowing the intrcduction of several frequential limits depending on the characteristics both of the device and of the phenomenon under study. A systematic treatment of conduction calorimeters may be attained if relative scales, i n time (t/τ1) and frequency
(θ τ1), are considered. Now, the efficiency of a given deconvolutive technique and the limits imposed by the experimental device itself or by the physical phenomena studied are easily evaluated
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