74 research outputs found
Domain-walls formation in binary nanoscopic finite systems
Using a simple one-dimensional Frenkel-Kontorowa type model, we have
demonstrated that finite commensurate chains may undergo the
commensurate-incommensurate (C-IC) transition when the chain is contaminated by
isolated impurities attached to the chain ends. Monte Carlo (MC) simulation has
shown that the same phenomenon appears in two-dimensional systems with
impurities located at the peripheries of finite commensurate clusters.Comment: 9 pages, 6 figure
Ordering and order-disorder phase transition in the (1x1) monolayer chemisorbed on the (111) face of an fcc crystal
In this paper we have considered a simple lattice gas model of chemisorbed
monolayer which allows for the harmonic fluctuations of the bond length between
the adsorbate atom and the surface site. The model also involves a short-ranged
attractive potential acting between the adsorbed atoms as well as the surface
periodic corrugation potential. It has been assumed that the adsorbed atoms are
bonded to the uppermost layer of the substrate atoms. In particular, using
Monte Carlo simulation method we have focused on the orderings appearing in the
dense monolayer formed on the (111) face of an fcc solid. Within the lattice
gas limit, the chemisorbed layer forms a (1x1) structure. On the other hand,
when the bonds are allowed to fluctuate, three other different ordered phases
have been found to be stable in the ground state. One of them has been found to
be stable at finite temperatures and to undergo a phase transition to the
disordered state. The remaining two ordered states have been found to be stable
in the ground state only. At finite temperatures, the ordering has been
demonstrated to be destroyed due to large entropic effects.Comment: 15 pages, 15 figure
The mechanism of domain-wall structure formation in Ar-Kr submonolayer films on graphite
Using Monte Carlo simulation method in the canonical ensemble, we have
studied the commensurate-incommensurate transition in two-dimensional finite
mixed clusters of Ar and Kr adsorbed on graphite basal plane at low
temperatures. It has been demonstrated that the transition occurs when the
argon concentration exceeds the value needed to cover the peripheries of the
cluster. The incommensurate phase exhibits a similar domain-wall structure as
observed in pure krypton films at the densities exceeding the density of a
perfect commensurate phase, but the size of
commensurate domains does not change much with the cluster size. When the argon
concentration increases, the composition of domain walls changes while the
commensurate domains are made of pure krypton. We have constructed a simple
one-dimensional Frenkel-Kontorova-like model that yields the results being in a
good qualitative agreement with the Monte Carlo results obtained for
two-dimensional systems.Comment: 14 pages, 9 figure
Pressure-driven flow of oligomeric fluid in nano-channel with complex structure. A dissipative particle dynamics study
We develop a simulational methodology allowing for simulation of the
pressure-driven flow in the pore with flat and polymer-modified walls. Our
approach is based on dissipative particle dynamics and we combine earlier ideas
of fluid-like walls and reverse flow. As a test case we consider the oligomer
flow through the pore with flat walls and demonstrate good thermostatting
qualities of the proposed method. We found the inhomogeneities in both oligomer
shape and alignment across the pore leading to a non-parabolic velocity
profiles. The method is subsequently applied to a nano-channel decorated with a
polymer brush stripes arranged perpendicularly to the flow direction. At
certain threshold value of a flow force we find a pillar-to-lamellar
morphological transition, which leads to the brush enveloping the pore wall by
a relatively smooth layer. At higher flow rates, the flow of oligomer has
similar properties as in the case of flat walls, but for the narrower effective
pore size. We observe stretching and aligning of the polymer molecules along
the flow near the pore walls.Comment: 14 pages, 12 figure
On the phase behavior of mixed Ar-Xe submonolayer films on graphite
Using Monte Carlo simulation methods in the canonical and grand canonical
ensembles, we discuss the melting and the formation of ordered structures of
mixed Ar-Xe submonolayer films on graphite. The calculations have been
performed using two- as well as three-dimensional models of the systems
studied. It is demonstrated that out-of plane motion does not affect the
properties of the adsorbed films as long as the total density is not close to
the monolayer completion. On the other hand, close to the monolayer completion,
the promotion of particles to the second layer considerably affects the
properties of mixed films. It has been shown that the mixture exhibits complete
mixing in the liquid phase and freezes into solid phases of the structure
depending upon the film composition. For submonolayer densities, the melting
temperature exhibits non-monotonous changes with the film composition. In
particular, the melting temperature initially increases when the xenon
concentration increases up to about 20%, then it decreases and reaches minimum
for the xenon concentration of about 40%. For still higher xenon
concentrations, the melting point gradually increases to the temperature
corresponding to pure xenon film. It has been also demonstrated that the
topology of phase diagrams of mixed films is sensitive to the composition of
adsorbed layers.Comment: 16 pages, 16 figure
Non-additive symmetric mixtures at selective walls
The results of Monte Carlo simulation of adsorption and wetting behaviour of
a highly non-additive symmetric mixture at selective walls is discussed. We
have concentrated on the interplay between the surface induced demixing in the
adsorbed films and the properties of the bulk mixture, which exhibits a closed
immiscibility loop. It has been shown that the wetting behaviour depends on the
absolute values of the parameters determining the strengths of interaction
between the mixture components and the surface, as well as on their difference.
In general, an increase of the difference between the adsorption energies of
the components leads to a decrease of the wetting temperature. In the cases
when the wetting of non-selective walls occurs at the temperatures above the
onset of demixing transition in the bulk, an increasing wall selectivity leads
to a gradual decrease of the wetting temperature towards the triple point, in
which the vapour coexists with the mixed and demixed liquid phases. When the
wetting temperature at the non-selective wall is located below the onset of the
demixing transition in the bulk mixture, an increase of the adsorption energy
of the selected component causes the developing adsorbed films to demix and to
show the reentrant mixing upon approaching the bulk coexistence. At the
temperatures above the onset of the demixing transition in the bulk, the
adsorbed films remain demixed up to the bulk coexistence and undergo the
first-order wetting transition. A rather unexpected finding has been the
observation of a gradual increase of the wetting temperature at highly
selective walls.Comment: 15 pages, 12 figure
Changes in the structure of tethered chain molecules as predicted by density functional approach
We use a version of the density functional theory to study the changes in the
height of the tethered layer of chains built of jointed spherical segments with
the change of the length and surface density of chains. For the model in which
the interactions between segments and solvent molecules are the same as between
solvent molecules we have discovered two effects that have not been observed in
previous studies. Under certain conditions and for low surface concentrations
of the chains, the height of the pinned layer may attain a minimum. Moreover,
for some systems we observe that when the temperature increases, the height of
the layer of chains may decrease.Comment: 13 pages, 7 figure
First-order phase transitions in lattice bilayers of Janus-like particles: Monte Carlo simulations
The first-order phase transitions in the lattice model of Janus-like
particles confined in slit-like pores are studied. We assume a cubic lattice
with molecules that can freely change their orientation on a lattice site.
Moreover, the molecules can interact with the pore walls with
orientation-dependent forces. The performed calculations are limited to the
cases of bilayers. Our emphasis is on the competition between the fluid-wall
and fluid-fluid interactions. The oriented structures formed in the systems in
which the fluid-wall interactions acting contrary to the fluid-fluid
interactions differ from those appearing in the systems with neutral walls or
with walls attracting the repulsive parts of fluid molecules.Comment: 12 pages, 11 figure
The liquid-vapor interface of the restricted primitive model of ionic fluids from a density functional approach
We investigate the liquid-vapor interface of the restricted primitive model
for an ionic fluid using a density functional approach. The applied theory
includes the electrostatic contribution to the free energy functional arising
from the bulk energy equation of state and the mean spherical approximation for
a restricted primitive model, as well as the associative contribution, due to
the formation of pairs of ions. We compare the density profiles and the values
of the surface tension with previous theoretical approaches.Comment: 12 pages, 8 figure
The thickness of a liquid layer on the free surface of ice as obtained from computer simulation
Molecular dynamic simulations were performed for ice Ih with a free surface
by using four water models, SPC/E, TIP4P, TIP4P/Ice and TIP4P/2005. The
behavior of the basal plane, the primary prismatic plane and of the secondary
prismatic plane when exposed to vacuum was analyzed. We observe the formation
of a thin liquid layer at the ice surface at temperatures below the melting
point for all models and the three planes considered. For a given plane it was
found that the thickness of a liquid layer was similar for different water
models, when the comparison is made at the same undercooling with respect to
the melting point of the model. The liquid layer thickness is found to increase
with temperature. For a fixed temperature it was found that the thickness of
the liquid layer decreases in the following order: the basal plane, the primary
prismatic plane, and the secondary prismatic plane. For the TIP4P/Ice model, a
model reproducing the experimental value of the melting temperature of ice, the
first clear indication of the formation of a liquid layer appears at about -100
Celsius for the basal plane, at about -80 Celsius for the primary prismatic
plane and at about -70 Celsius for the secondary prismatic plane.Comment: 41 pages and 13 figure
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