2,158 research outputs found
Renormalisation group determination of the order of the DNA denaturation transition
We report on the nature of the thermal denaturation transition of homogeneous
DNA as determined from a renormalisation group analysis of the
Peyrard-Bishop-Dauxois model. Our approach is based on an analogy with the
phenomenon of critical wetting that goes further than previous qualitative
comparisons, and shows that the transition is continuous for the average
base-pair separation. However, since the range of universal critical behaviour
appears to be very narrow, numerically observed denaturation transitions may
look first-order, as it has been reported in the literature.Comment: 6 pages; no figures; to appear in Europhysics Letter
Tricritical wedge filling transitions with short-ranged forces
We show that the 3D wedge filling transition in the presence of short-ranged
interactions can be first-order or second order depending on the strength of
the line tension associated with to the wedge bottom. This fact implies the
existence of a tricritical point characterized by a short-distance expansion
which differs from the usual continuous filling transition. Our analysis is
based on an effective one-dimensional model for the 3D wedge filling which
arises from the identification of the breather modes as the only relevant
interfacial fluctuations. From such analysis we find a correspondence between
continuous 3D filling at bulk coexistence and 2D wetting transitions with
random-bond disorder.Comment: 7 pages, 3 figures, 6th Liquid Matter Conference Proceedings (to be
published in J. Phys.: Condens. Matter
Orientational transition in nanobridges of nematic liquid crystals in slit pores
In this work, the morphology of nematic capillary nanobridges in slit pores
separated by a vertical distance will be characterised by Monte Carlo
simulations for oblate molecules nematogen modelled by the Gay-Berne potential.
Previous studies on droplets show that the molecules are arranged
homeotropically at the nematic-vapour interface and form spherical droplets
with an annular disclination located in their equatorial plane. In the presence
of pores with attractive substrates that favour homeotropic anchoring, the
formation of nanobridges characterised by anchoring angles that decrease with
increasing particle-substrate interaction intensity is observed. When the
orientational field in the nanobridge is analysed, the formation of an annular
disclination of topological charge located in the plane perpendicular to
the -axis that passes through the centre of mass of the nanobridge is
observed for small . However, when considering higher values of , a
change to a biaxial orientational profile within the nanobridge is observed,
where now the annular disclination is arranged in a plane perpendicular to one
direction of the plane. These results are indicative of the existence of
an orientational phase transition for an intermediate value between a
uniaxial and a biaxial orientational configuration in the capillary nanobridge.Comment: 20 pages, 8 figures. This article has been accepted for publication
in Liquid Crystals, published by Taylor & Franci
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
Fluid adsorption near an apex: Covariance between complete and critical wetting
Critical wetting is an elusive phenomenon for solid-fluid interfaces. Using
interfacial models we show that the diverging length scales, which characterize
complete wetting at an apex, precisely mimic critical wetting with the apex
angle behaving as the contact angle. Transfer matrix, renormalization group
(RG) and mean field analysis (MF) shows this covariance is obeyed in 2D, 3D and
for long and short ranged forces. This connection should be experimentally
accesible and provides a means of checking theoretical predictions for critical
wetting.Comment: 4 pages, 1 figure, submitted to Physical Review Letter
Interfacial structure at a two-dimensional wedge filling transition: Exact results and a renormalization group study
nterfacial structure and correlation functions near a two-dimensional wedge filling transition are studied using effective interfacial Hamiltonian models. An exact solution for short range binding potentials and results for Kratzer binding potentials show that sufficiently close to the filling transition a new length scale emerges and controls the decay of the interfacial profile relative to the substrate and the correlations between interfacial positions above different positions. This new length scale is much larger than the intrinsic interfacial correlation length, and it is related geometrically to the average value of the interfacial position above the wedge midpoint. The interfacial behavior is consistent with a breather mode fluctuation picture, which is shown to emerge from an exact decimation functional renormalization group scheme that keeps the geometry invariant
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
Flue gas adsorption by single-wall carbon nanotubes: A Monte Carlo study
Adsorption of flue gases by single-wall carbon nanotubes (SWCNT) has been studied by means of
Monte Carlo simulations. The flue gas is modeled as a ternary mixture of N2, CO2, and O2, emulating
realistic compositions of the emissions from power plants. The adsorbed flue gas is in equilibrium
with a bulk gas characterized by temperature T, pressure p, and mixture composition.We have considered
different SWCNTs with different chiralities and diameters in a range between 7 and 20 Å. Our
results show that the CO2 adsorption properties depend mainly on the bulk flue gas thermodynamic
conditions and the SWCNT diameter. Narrow SWCNTs with diameter around 7 Å show high CO2
adsorption capacity and selectivity, but they decrease abruptly as the SWCNT diameter is increased.
For wide SWCNT, CO2 adsorption capacity and selectivity, much smaller in value than for the narrow
case, decrease mildly with the SWCNT diameter. In the intermediate range of SWCNT diameters,
the CO2 adsorption properties may show a peculiar behavior, which depend strongly on the bulk flue
gas conditions. Thus, for high bulk CO2 concentrations and low temperatures, the CO2 adsorption
capacity remains high in a wide range of SWCNT diameters, although the corresponding selectivity
is moderate.We correlate these findings with the microscopic structure of the adsorbed gas inside the
SWCNTs.Peer reviewe
Non-locality and short-range wetting phenomena
We propose a non-local interfacial model for 3D short-range wetting at planar
and non-planar walls. The model is characterized by a binding potential
\emph{functional} depending only on the bulk Ornstein-Zernike correlation
function, which arises from different classes of tube-like fluctuations that
connect the interface and the substrate. The theory provides a physical
explanation for the origin of the effective position-dependent stiffness and
binding potential in approximate local theories, and also obeys the necessary
classical wedge covariance relationship between wetting and wedge filling.
Renormalization group and computer simulation studies reveal the strong
non-perturbative influence of non-locality at critical wetting, throwing light
on long-standing theoretical problems regarding the order of the phase
transition.Comment: 4 pages, 2 figures, accepted for publication in Phys. Rev. Let
Surface and capillary transitions in an associating binary mixture model
We investigate the phase diagram of a two-component associating fluid mixture
in the presence of selectively adsorbing substrates. The mixture is
characterized by a bulk phase diagram which displays peculiar features such as
closed loops of immiscibility. The presence of the substrates may interfere the
physical mechanism involved in the appearance of these phase diagrams, leading
to an enhanced tendency to phase separate below the lower critical solution
point. Three different cases are considered: a planar solid surface in contact
with a bulk fluid, while the other two represent two models of porous systems,
namely a slit and an array on infinitely long parallel cylinders. We confirm
that surface transitions, as well as capillary transitions for a large
area/volume ratio, are stabilized in the one-phase region. Applicability of our
results to experiments reported in the literature is discussed.Comment: 12 two-column pages, 12 figures, accepted for publication in Physical
Review E; corrected versio
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