159 research outputs found
Three-dimensional patchy lattice model: ring formation and phase separation
We investigate the structural and thermodynamic properties of a model of
particles with patches of type and patches of type . Particles
are placed on the sites of a face centered cubic lattice with the patches
oriented along the nearest neighbor directions. The competition between the
self-assembly of chains, rings and networks on the phase diagram is
investigated by carrying out a systematic investigation of this class of
models, using an extension of Wertheim's theory for associating fluids and
Monte Carlo numerical simulations. We varied the ratio
of the interaction between patches and
, , and between patches, (
is set to ) as well as the relative position of the patches, i.e., the
angle between the (lattice) directions of the patches. We found
that both and ( or ) have a
profound effect on the phase diagram. In the empty fluid regime () the
phase diagram is re-entrant with a closed miscibility loop. The region around
the lower critical point exhibits unusual structural and thermodynamic behavior
determined by the presence of relatively short rings. The agreement between the
results of theory and simulation is excellent for but
deteriorates as decreases, revealing the need for new theoretical
approaches to describe the structure and thermodynamics of systems dominated by
small rings.Comment: 26 pages, 10 figure
Crossover Scales at the Critical Points of Fluids with Electrostatic Interactions
Criticality in a fluid of dielectric constant D that exhibits Ising-type
behavior is studied as additional electrostatic (i.e., ionic) interactions are
turned on. An exploratory perturbative calculation is performed for small
ionicity as measured by the ratio of the electrostatic energy to the strength
of the short-range nonionic (i.e., van der Waals) interactions in the uncharged
fluid.
With the aid of distinct transformations for the short-range and for the
Coulombic interactions, an effective Hamiltonian with coefficients depending on
the ionicity is derived at the Debye-Hueckel limiting-law level for a fully
symmetric model. The crossover between classical (mean-field) and Ising
behavior is then estimated using a Ginzburg criterion. This indicates that the
reduced crossover temperature depends only weakly on the ionicity (and on the
range of the nonionic potentials); however, the trends do correlate with the,
much stronger, dependence observed experimentally.Comment: 25 pages, 4 figure; submitted to J. Chem. Phy
The effect of polydispersity on the ordering transition of adsorbed self-assembled rigid rods
Extensive Monte Carlo simulations were carried out to investigate the nature
of the ordering transition of a model of adsorbed self-assembled rigid rods on
the bonds of a square lattice [Tavares et. al., Phys. Rev E 79, 021505 (2009)].
The polydisperse rods undergo a continuous ordering transition that is found to
be in the two-dimensional Ising universality class, as in models where the rods
are monodisperse. This finding is in sharp contrast with the recent claim that
equilibrium polydispersity changes the nature of the phase transition in this
class of models [L`opez et. al., Phys. Rev E 80, 040105(R)(2009)].Comment: 19 pages, 5 figure
The nature of the ordered phase of the confined self-assembled rigid rod model
We investigate the nature of the ordered phase and the orientational
correlations between adjacent layers of the confined three-dimensional
self-assembled rigid rod model, on the cubic lattice. We find that the ordered
phase at finite temperatures becomes uniaxial in the thermodynamic limit, by
contrast to the ground state (partial) order where the orientation of the
uncorrelated layers is perpendicular to one of the three lattice directions.
The increase of the orientational correlation between layers as the number of
layers increases suggests that the unconfined model may also exhibit uniaxial
ordering at finite temperatures.Comment: 8 Pates, 10 Figures. Submitted to J. Chem. Phys.8 pages (10 figures
Pathways to folding, nucleation events and native geometry
We perform extensive Monte Carlo simulations of a lattice model and the Go
potential to investigate the existence of folding pathways at the level of
contact cluster formation for two native structures with markedly different
geometries. Our analysis of folding pathways revealed a common underlying
folding mechanism, based on nucleation phenomena, for both protein models.
However, folding to the more complex geometry (i.e. that with more non-local
contacts) is driven by a folding nucleus whose geometric traits more closely
resemble those of the native fold. For this geometry folding is clearly a more
cooperative process.Comment: Accepted in J. Chem. Phy
Newspaper covers as display windows in a pandemic season: analysis of three Portuguese daily newspapers
Neste artigo, que é uma síntese de um estudo realizado no MediaLab CIES-Iscte , parte-se da ideia de pandemia enquanto evento mediático e da mediatização como aspeto estruturante da narrativa jornalística, para propor três modelos de montras jornalísticas – de saldos, de produto e de diversidade em que se considera as características e critérios de noticiabilidade adotados por cada um dos veículos analisados. A análise englobou três dimensões: a distribuição do conteúdo no espaço físico e hierárquico do jornal (manchete, centro e periferia); as marcas, que podem ser palavras destacadas nos títulos e chamadas; e os recursos imagéticos utilizados.
A pesquisa teve em conta 90 capas dos jornais Correio da Manhã, Jornal de Notícias e Público publicadas durante os primeiros 30 dias de cobertura da pandemia em Portugal, divididos em três ciclos: o primeiro, entre a divulgação dos primeiros casos identificados de COVID-19 em Portugal (3 de março de 2020) e a declaração do estado de alerta em Portugal (12 de março); o segundo, desde o encerramento das escolas (13 de março) à véspera da notícia do decreto presidencial que autoriza ao governo decretar estado de emergência (18 de março); o terceiro, da repercussão do decreto presidencial (19 de março) até à véspera da aprovação do decreto de emergência na Assembleia da República (1 de abril).
O estudo conclui que as narrativas nas capas dos jornais são diferentes entre si e afetadas pelos modelos de montras, os quais refletem a dicotomia entre os interesses jornalísticos e os interesses comerciais, questionando se o público-modelo que norteia a construção das diferentes montras é apenas um, entre vários possíveis, “retrato-robô”, para fazer «vender» jornais, ou um claro identificador sociocultural dos públicos da imprensa escrita portuguesa?info:eu-repo/semantics/publishedVersio
Interfacial motion in flexo- and order-electric switching between nematic filled states
We consider a nematic liquid crystal, in coexistence with its isotropic
phase, in contact with a substrate patterned with rectangular grooves. In such
a system, the nematic phase may fill the grooves without the occurrence of
complete wetting. There may exist multiple (meta)stable filled states, each
characterised by the type of distortion (bend or splay) in each corner of the
groove and by the shape of the nematic-isotropic interface, and additionally
the plateaux that separate the grooves may be either dry or wet with a thin
layer of nematic. Using numerical simulations, we analyse the dynamical
response of the system to an externally- applied electric field, with the aim
of identifying switching transitions between these filled states. We find that
order-electric coupling between the fluid and the field provides a means of
switching between states where the plateaux between grooves are dry and states
where they are wet by a nematic layer, without affecting the configuration of
the nematic within the groove. We find that flexoelectric coupling may change
the nematic texture in the groove, provided that the flexoelectric coupling
differentiates between the types of distortion at the corners of the substrate.
We identify intermediate stages of the transitions, and the role played by the
motion of the nematic-isotropic interface. We determine quantitatively the
field magnitudes and orientations required to effect each type of transition.Comment: 14 pages, 12 fig
The condensation and ordering of models of empty liquids
We consider a simple model consisting of particles with four bonding sites
("patches"), two of type A and two of type B, on the square lattice, and
investigate its global phase behavior by simulations and theory. We set the
interaction between B patches to zero and calculate the phase diagram as the
ratio between the AB and the AA interactions, {\epsilon}^\star_AB, varies. In
line with previous work, on three-dimensional off-lattice models, we show that
the liquid-vapor phase diagram exhibits a re-entrant or "pinched" shape for the
same range of \epsilon^\star_AB, suggesting that the ratio of the energy scales
- and the corresponding empty fluid regime - is independent of the
dimensionality of the system and of the lattice structure. In addition, the
model exhibits an order-disorder transition that is ferromagnetic in the
re-entrant regime. The use of low-dimensional lattice models allows the
simulation of sufficiently large systems to establish the nature of the
liquid-vapor critical points and to describe the structure of the liquid phase
in the empty fluid regime, where the size of the "voids" increases as the
temperature decreases. We have found that the liquid-vapor critical point is in
the 2D Ising universality class, with a scaling region that decreases rapidly
as the temperature decreases. The results of simulations and theoretical
analysis suggest that the line of order-disorder transitions intersects the
condensation line at a multicritical point at zero temperature and density, for
patchy particle models with a re-entrant, empty fluid, regime.Comment: 29 pagres, 9 figure
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