71 research outputs found
Condensation and evaporation transitions in deep capillary grooves
We study the order of capillary condensation and evaporation transitions of a
simple fluid adsorbed in a deep capillary groove using a fundamental measure
density functional theory (DFT). The walls of the capillary interact with the
fluid particles via long-ranged, dispersion, forces while the fluid-fluid
interaction is modelled as a truncated Lennard-Jones-like potential. We find
that below the wetting temperature condensation is first-order and
evaporation is continuous with the metastability of the condensation being well
described by the complementary Kelvin equation. In contrast above both
phase transitions are continuous and their critical singularities are
determined. In addition we show that for the evaporation transition above
there is an elegant mapping, or covariance, with the complete wetting
transition occurring at a planar wall. Our numerical DFT studies are
complemented by analytical slab model calculations which explain how the
asymmetry between condensation and evaporation arises out of the combination of
long-ranged forces and substrate geometry
Crystallisation of soft matter under confinement at interfaces and in wedges
The surface freezing and surface melting transitions exhibited by a model
two-dimensional soft matter system is studied. The behaviour when confined
within a wedge is also considered. The system consists of particles interacting
via a soft purely repulsive pair potential. Density functional theory (DFT) is
used to calculate density profiles and thermodynamic quantities. The external
potential due to the confining walls is modelled via a hard-wall with an
additional repulsive Yukawa potential. The surface phase behaviour depends on
the range and strength of this repulsion: When the repulsion strength is weak,
the wall promotes freezing at the surface of the wall. The thickness of this
frozen layer grows logarithmically as the bulk liquid-solid phase coexistence
is approached. Our mean-field DFT predicts that this crystalline layer at the
wall must be nucleated (i.e. there is a free energy barrier) and its formation
is necessarily a first-order transition, referred to as `prefreezing', by
analogy with the prewetting transition. However, in contrast to the latter,
prefreezing cannot terminate in a critical point, since the phase transition
involves a change in symmetry. If the wall-fluid interaction is sufficiently
long ranged and the repulsion is strong enough, surface melting can instead
occur. Then the interface between the wall and the bulk crystalline solid
becomes wet by the liquid phase as the chemical potential is decreased towards
the value at liquid-solid coexistence. It is observed that the finite thickness
fluid film at the wall has a broken translational symmetry due to its proximity
to the bulk crystal and so the nucleation of the wetting film can be either
first-order or continuous. Our mean-field theory predicts that for certain wall
potentials there is a premelting critical point analogous to the surface
critical point for the prewetting transition. In a wedge...Comment: 11 pages, 12 figure
Low-temperature and high-temperature approximations for penetrable-sphere fluids. Comparison with Monte Carlo simulations and integral equation theories
The two-body interaction in dilute solutions of polymer chains in good
solvents can be modeled by means of effective bounded potentials, the simplest
of which being that of penetrable spheres (PSs). In this paper we construct two
simple analytical theories for the structural properties of PS fluids: a
low-temperature (LT) approximation, that can be seen as an extension to PSs of
the well-known solution of the Percus-Yevick (PY) equation for hard spheres,
and a high-temperature (HT) approximation based on the exact asymptotic
behavior in the limit of infinite temperature. Monte Carlo simulations for a
wide range of temperatures and densities are performed to assess the validity
of both theories. It is found that, despite their simplicity, the HT and LT
approximations exhibit a fair agreement with the simulation data within their
respective domains of applicability, so that they complement each other. A
comparison with numerical solutions of the PY and the hypernetted-chain
approximations is also carried out, the latter showing a very good performance,
except inside the core at low temperatures.Comment: 14 pages, 8 figures; v2: some figures redone; small change
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