636 research outputs found
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
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
Raman Spectroscopic Analysis of Geological and Biogeological Specimens of Relevance to the ExoMars Mission
H.G.M.E., I.H., and R.I. acknowledge the support of the STFC Research Council in the UK ExoMars programme. J.J. and P.V. acknowledge the support of the Grant Agency of the Czech Republic (210/10/0467) and of the Ministry of Education of the Czech Republic (MSM0021620855).Peer reviewedPublisher PD
Bilayered smectic phase polymorphism in the dipolar Gay-Berne liquid crystal model
We present computer simulations of the Gay–Berne model with a strong terminal dipole. We report
the existence of
different
stable antiferroelectric interdigitated bilayered phases in this model with
diverse in-plane organization. The occurrence of these phases depends crucially on the value of the
molecular elongation
. For
= 3 we find an interdigitated bilayered smectic-A phase
absent when
there is no dipole
and a bilayered smectic-T
or crystal
with positional in-plane tetragonal
ordering, different from the hexatic observed in the absence of the molecular dipole. For
=4,
bilayered smectic-A and in-plane hexatic-ordered smectic-B
or crystal
phases are observe
Phase equilibria and critical behavior of square‐well fluids of variable width by Gibbs ensemble Monte Carlo simulation
The vapor–liquid phase equilibria of square†well systems with hard†sphere diameters σ, well†depths ε, and ranges λ=1.25, 1.375, 1.5, 1.75, and 2 are determined by Monte Carlo simulation. The two bulk phases in coexistence are simulated simultaneously using the Gibbs ensemble technique. Vapor–liquid coexistence curves are obtained for a series of reduced temperatures between about Tr=T/Tc=0.8 and 1, where Tc is the critical temperature. The radial pair distribution functions g(r) of the two phases are calculated during the simulation, and the results extrapolated to give the appropriate contact values g(σ), g(λσ−), and g(λσ+). These are used to calculate the vapor†pressure curves of each system and to test for equality of pressure in the coexisting vapor and liquid phases. The critical points of the square†well fluids are determined by analyzing the density†temperature coexistence data using the first term of a Wegner expansion. The dependence of the reduced critical temperature T* c=kTc/ε, pressure P* c=Pcσ3/ε, number density Ï * c=Ï cσ3, and compressibility factor Z=P/(Ï kT), on the potential range λ, is established. These results are compared with existing data obtained from perturbation theories. The shapes of the coexistence curves and the approach to criticality are described in terms of an apparent critical exponent β. The curves for the square†well systems with λ=1.25, 1.375, 1.5, and 1.75 are very nearly cubic in shape corresponding to near†universal values of β (β≊0.325). This is not the case for the system with a longer potential range; when λ=2, the coexistence curve is closer to quadratic in shape with a near†classical value of β (β≊0.5). These results seem to confirm the view that the departure of β from a mean†field or classical value for temperatures well below critical is unrelated to long†range, near†critical fluctuations
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Physical weathering of carbonate host-rock by precipitation of soluble salts in caves: A case study in El Orón-Arco Cave (Region of Murcia, SE Spain)
The dissolution of carbonate host-rock by freshwater in phreatic or vadose conditions is the most common mechanism for the formation of caves; however, circulation of saline solutions through carbonate materials and precipitation of soluble salts may also play an important role. We studied the stable isotope composition (δ18O and δ34S of sulfate, δ18O and δD of structurally-bound gypsum hydration water and 87Sr/86Sr) and salinity of fluid inclusions in gypsum speleothems found in El Orón-Arco Cave (Cartagena, SE Spain). We suggest that physical weathering of carbonate host-rock was driven by precipitation of soluble sea-salts (mostly gypsum and halite), and this process controlled the recent geomorphological evolution of the cave. The Triassic carbonate host-rock shows clear evidence for salt weathering, including gypsum/halite infillings in cracks of the bedrock, mechanical spalling of the carbonate, and detachment of rock fragments that lead to the formation cave voids and in-situ accumulations of piles of unsorted rubble. Sulfur and oxygen isotopes of gypsum sulfate (3.0‰ < δ18O < 11.6‰ and 16.7‰ < δ34S < 20.7‰) are generally lower than modern seawater sulfate and suggest contributions from a 34S-depleted source (i.e. oxidation of pyrite). The δ18O and δD of gypsum hydration water are relatively low compared to expected values for the evaporation of pure seawater to gypsum saturation, suggesting that gypsum precipitation involved a secondary calcium-sulfate source or recycling of gypsum from previous stages, along with mixing of seawater and meteoric water seepage to the cave. The 87Sr/86Sr in gypsum shows intermediate values between modern seawater and Triassic carbonate values because of interaction between the solution and the bedrock. The salinities of the speleothem-forming solutions are relatively high (13.2 ± 3.2 wt% eq. NaCl) compared to gypsum formed from evaporated brackish solutions (i.e. ~4–8 wt% eq. NaCl) and indicate dissolution of earlier evaporites before secondary gypsum precipitation. This cave-forming mechanism, which is related to saline water circulation and precipitation of evaporitic minerals, may be common in other coastal caves
Computer simulations of nematic drops: Coupling between drop shape and nematic order
We perform Monte Carlo computer simulations of nematic drops in equilibrium with their vapor
using a Gay-Berne interaction between the rod-like molecules. To generate the drops, we initially
perform NPT simulations close to the nematic-vapor coexistence region, allow the system to equilibrate
and subsequently induce a sudden volume expansion, followed with NVT simulations. The
resultant drops coexist with their vapor and are generally not spherical but elongated, have the rodlike
particles tangentially aligned at the surface and an overall nematic orientation along the main
axis of the drop. We find that the drop eccentricity increases with increasing molecular elongation,
κ. For small κ the nematic texture in the drop is bipolar with two surface defects, or boojums, maximizing
their distance along this same axis. For sufficiently high κ, the shape of the drop becomes
singular in the vicinity of the defects, and there is a crossover to an almost homogeneous texture; this
reflects a transition from a spheroidal to a spindle-like dro
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