158 research outputs found
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
Surface tension of the Widom-Rowlinson model
11 pags., 5 figs., 3 tabs.We consider the computation of the surface tension of the fluid-fluid interface for the Widom-Rowlinson [J. Chem. Phys. 52, 1670 (1970)] binary mixture from direct simulation of the inhomogeneous system. We make use of the standard mechanical route, in which the surface tension follows from the computation of the normal and tangential components of the pressure tensor of the system. In addition to the usual approach, which involves simulations of the inhomogeneous system in the canonical ensemble, we also consider the computation of the surface tension in an ensemble where the pressure perpendicular (normal) to the planar interface is kept fixed. Both approaches are seen to provide consistent values of the interfacial tension. The issue of the system-size dependence of the surface tension is addressed. In addition, simulations of the fluid-fluid coexistence properties of the mixture are performed in the semigrand canonical ensemble. Our results are compared with existing data of the Widom-Rowlinson mixture and are also examined in the light of the vapor-liquid equilibrium of the thermodynamically equivalent one-component penetrable sphere model. © 2007 American Institute of Physics.Financial support is due to the Spanish Dirección General de Investigación Project Nos. FIS2004-06627-C02-01
E.d.M. and FIS2004-02954-C03-01 N.G.A. and from
Universidad de Huelva and Junta de Andalucía. Additional
funding from the Dirección General de Universidades e Investigación Comunidad de Madrid, Spain under the
MOSSNOHO-CM program Grant No. S0505/ESP/0299
and from the Engineering and Physical Sciences EPSRC of
the UK Grant Nos. GR/N20317, GR/N03358, GR/N35991,
GR/R09497, and EP/E016340, the Joint Research Equipment Initiative JREI GR/M94427, and the Royal Society Wolfson Foundation refurbishment grant is also acknowledged. Finally we are grateful to the Royal Society for the
award of a International Short Visit grant which has facilitated the collaborative work
Pattern formation in binary fluid mixtures induced by short-range competing interactions
Molecular dynamics simulations and integral equation calculations of a simple
equimolar mixture of diatomic molecules and monomers interacting via attractive
and repulsive short-range potentials show the existence of pattern formation
(microheterogeneity), mostly due to depletion forces away from the demixing
region. Effective site-site potentials extracted from the pair correlation
functions using an inverse Monte Carlo approach and an integral equation
inversion procedure exhibit the features characteristic of a short-range
attractive and long-range repulsive potential. When charges are incorporated
into the model, this becomes a coarse grained representation of a room
temperature ionic liquid, and as expected, intermediate range order becomes
more pronounced and stable
Phase behavior of the Confined Lebwohl-Lasher Model
The phase behavior of confined nematogens is studied using the Lebwohl-Lasher
model. For three dimensional systems the model is known to exhibit a
discontinuous nematic-isotropic phase transition, whereas the corresponding two
dimensional systems apparently show a continuous
Berezinskii-Kosterlitz-Thouless like transition. In this paper we study the
phase transitions of the Lebwohl-Lasher model when confined between planar
slits of different widths in order to establish the behavior of intermediate
situations between the pure planar model and the three-dimensional system, and
compare with previous estimates for the critical thickness, i.e. the slit width
at which the transition switches from continuous to discontinuous.Comment: Submitted to Physical Review
Leukocyte adhesion deficiency-I: A comprehensive review of all published cases
Leukocyte adhesion deficiency-I (LAD-I) is a rare, serious disorder with severity determined by defective CD18 expression. LAD-I is characterized by umbilical complications, granulocytosis, and diverse infections. Severe LAD-I is frequently fatal before the age of 2 years without allogeneic transplant. We identified 323 cases published between 1975 and 2017
Low-frequency excitations in glassy selenium: A comparison of neutron-scattering and molecular-dynamics results
The microscopic low-frequency dynamics of glassy selenium is investigated by means of the concurrent use of neutron inelastic scattering and computer simulations. A separation of the dynamic response in terms of intra- and interchain processes is achieved from the analysis of the simulation results. The S(Q,E) dynamic structure factors are analyzed in terms of the frequency moments or from a model scattering law, and the wave-vector dependence of the relevant quantities is established. Finally, the anomalous behavior of the heat capacity at moderately low temperatures is shown to be originated by mostly interchain interactions.Dirección General de Investigación Científica y Técnica PB89-0037-C
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