880 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
Nematic phase in the J-J square lattice Ising model in an external field
The J-J Ising model in the square lattice in the presence of an
external field is studied by two approaches: the Cluster Variation Method (CVM)
and Monte Carlo simulations. The use of the CVM in the square approximation
leads to the presence of a new equilibrium phase, not previously reported for
this model: an Ising-nematic phase, which shows orientational order but not
positional order, between the known stripes and disordered phases. Suitable
order parameters are defined and the phase diagram of the model is obtained.
Monte Carlo simulations are in qualitative agreement with the CVM results,
giving support to the presence of the new Ising-nematic phase. Phase diagrams
in the temperature-external field plane are obtained for selected values of the
parameter which measures the relative strength of the
competing interactions. From the CVM in the square approximation we obtain a
line of second order transitions between the disordered and nematic phases,
while the nematic-stripes phase transitions are found to be of first order. The
Monte Carlo results suggest a line of second order nematic-disordered phase
transitions in agreement with the CVM results. Regarding the stripes-nematic
transitions, the present Monte Carlo results are not precise enough to reach
definite conclusions about the nature of the transitions.Comment: 13 pages, 10 figure
Approximation to the application of the Bougard process on the ceramics of Dolni Vestonice I
Este trabajo recoge la propuesta que realiza Bougard en su trabajo de 2011 sobre la forma
de producción de las figuras cerámicas que se encuentran en el yacimiento checo de Dolni
Vestonice I, cuya cronología se encuadra en el Gravetiense de Europa Central y la
importancia técnica del trato de la arcilla en una etapa del Paleolítico Superior con pocos
ejemplos parecidos. La configuración de la experimentación se realizará en el Laboratorio
Experimental de la Universidad Autónoma y con materiales de la Península IbéricaThis work reflects the proposal made by Bougard in his work in 2011 on the form of
production of the ceramic figures found in the Czech deposit of Dolni Vestonice I, whose
chronology is in the Gravetian of Central Europe and the technical importance of the
treatment Of the clay in a stage of the Upper Paleolithic with few similar examples. The
configuration of the experimentation will be carried out in the Experimental Laboratory
of the UAM and with materials of the Iberian Peninsul
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