106 research outputs found
Closed-loop liquid-vapor equilibrium in a one-component system
We report Monte Carlo simulations that show closed-loop liquid-vapor
equilibrium in a pure substance. As far as we know, this is the first time that
such a topology of the phase diagram has been found for one-component systems.
This finding has been achieved on a two-dimensional lattice model for patchy
particles that can form network fluids. We have considered related models with
a slightly different patch distribution in the order to understand the features
of the distribution of patches on the surface of the particles that make
possible the presence of the closed-loop liquid-vapor equilibrium, and its
relation with the phase diagram containing the so-called empty liquids.
Finally we discuss the likelihood of finding the closed-loop liquid-vapor
equilibria on related models for three dimensional models of patchy particles
in the continuum, and speculate on the possible relationship between the
mechanism behind the closed-loop liquid vapor equilibrium of our simple lattice
model and the salt-induced reentrant condensation found in complex systems.Comment: 5 pages (two columns); 7 Figures (Submitted to Physical Review
Phase behaviour of the confined lattice gas Lebwohl-Lasher model
The phase behaviour of the Lebwohl-Lasher lattice gas model (one of the
simplest representations of a nematogenic fluid) confined in a slab is
investigated by means of extensive Monte Carlo simulations. The model is known
to yield a first order gas-liquid transition in both the 2D and 3D limits, that
is coupled with an orientational order-disorder transition. This latter
transition happens to be first order in the 3D limit and it shares some
characteristic features with the continuous defect mediated
Berezinskii-Kosterlitz-Thouless transition in 2D. In this work we will analyze
in detail the behaviour of this system taking full advantage of the lattice
nature of the model and the particular symmetry of the interaction potential,
which allows for the use of efficient cluster algorithms.Comment: 6 pages, 5 figure
Phase behaviour of attractive and repulsive ramp fluids: integral equation and computer simulation studies
Using computer simulations and a thermodynamically self consistent integral
equation we investigate the phase behaviour and thermodynamic anomalies of a
fluid composed of spherical particles interacting via a two-scale ramp
potential (a hard core plus a repulsive and an attractive ramp) and the
corresponding purely repulsive model. Both simulation and integral equation
results predict a liquid-liquid de-mixing when attractive forces are present,
in addition to a gas-liquid transition. Furthermore, a fluid-solid transition
emerges in the neighbourhood of the liquid-liquid transition region, leading to
a phase diagram with a somewhat complicated topology. This solidification at
moderate densities is also present in the repulsive ramp fluid, thus preventing
fluid-fluid separation.Comment: 29 pages, 10 figure
Theory and simulation of the confined Lebwohl-Lasher model
We discuss the Lebwohl-Lasher model of nematic liquid crystals in a confined
geometry, using Monte Carlo simulation and mean-field theory. A film of
material is sandwiched between two planar, parallel plates that couple to the
adjacent spins via a surface strength . We consider the cases where
the favoured alignments at the two walls are the same (symmetric cell) or
different (asymmetric or hybrid cell). In the latter case, we demonstrate the
existence of a {\it single} phase transition in the slab for all values of the
cell thickness. This transition has been observed before in the regime of
narrow cells, where the two structures involved correspond to different
arrangements of the nematic director. By studying wider cells, we show that the
transition is in fact the usual isotropic-to-nematic (capillary) transition
under confinement in the case of antagonistic surface forces. We show results
for a wide range of values of film thickness, and discuss the phenomenology
using a mean-field model.Comment: 40 pages 19 figures (preprint format). Part of the text and some
figures were modified. New figure was include
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
Reply to "Comment on 'Effect of polydispersity on the ordering transition of adsorbed self- assembled rigid rods'"
We comment on the nature of the ordering transition of a model of equilibrium polydisperse rigid rods on the square lattice, which is reported by Lopez et al. to exhibit random percolation criticality in the canonical ensemble, in sharp contrast to (i) our results of Ising criticality for the same model in the grand canonical ensemble [Phys. Rev. E 82, 061117 (2010)] and (ii) the absence of exponent(s) renormalization for constrained systems with logarithmic specific-heat anomalies predicted on very general grounds by Fisher [Phys. Rev. 176, 257 (1968)]
Bistability in a self-assembling system confined by elastic walls: Exact results in a one-dimensional lattice model
© 2015 AIP Publishing LLC. The impact of confinement on self-assembly of particles interacting with short-range attraction and long-range repulsion potential is studied for thermodynamic states corresponding to local ordering of clusters or layers in the bulk. Exact and asymptotic expressions for the local density and for the effective potential between the confining surfaces are obtained for a one-dimensional lattice model introduced by J. Pękalski et al. [J. Chem. Phys. 138, 144903 (2013)]. The simple asymptotic formulas are shown to be in good quantitative agreement with exact results for slits containing at least 5 layers. We observe that the incommensurability of the system size and the average distance between the clusters or layers in the bulk leads to structural deformations that are different for different values of the chemical potential μ. The change of the type of defects is reflected in the dependence of density on μ that has a shape characteristic for phase transitions. Our results may help to avoid misinterpretation of the change of the type of defects as a phase transition in simulations of inhomogeneous systems. Finally, we show that a system confined by soft elastic walls may exhibit bistability such that two system sizes that differ approximately by the average distance between the clusters or layers are almost equally probable. This may happen when the equilibrium separation between the soft boundaries of an empty slit corresponds to the largest stress in the confined self-assembling system.Peer Reviewe
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