3,838 research outputs found
Two Parameters for Three Dimensional Wetting Transitions
Critical effects at complete and critical wetting in three dimensions are
studied using a coupled effective Hamiltonian H[s(y),\ell]. The model is
constructed via a novel variational principle which ensures that the choice of
collective coordinate s(y) near the wall is optimal. We highlight the
importance of a new wetting parameter \Omega(T) which has a strong influence on
critical properties and allows the status of long-standing Monte-Carlo
simulation controversies to be re-examined.Comment: 4 pages RevTex, 2 encapsulated postscript figures, to appear in
Europhys. Let
Interfacial Structural Changes and Singularities in Non-Planar Geometries
We consider phase coexistence and criticality in a thin-film Ising magnet
with opposing surface fields and non-planar (corrugated) walls. We show that
the loss of translational invariance has a strong and unexpected non-linear
influence on the interface structure and phase diagram. We identify 4
non-thermodynamic singularities where there is a qualitative change in the
interface shape. In addition, we establish that at the finite-size critical
point, the singularity in the interface shape is characterized by two distint
critical exponents in contrast to the planar case (which is characterised by
one). Similar effects should be observed for prewetting at a corrugated
substrate. Analogy is made with the behaviour of a non-linear forced oscillator
showing chaotic dynamics.Comment: 13 pages, 3 figure
Crossover scaling of apparent first-order wetting in two dimensional systems with short-ranged forces
Recent analyses of wetting in the semi-infinite two dimensional Ising model,
extended to include both a surface coupling enhancement and a surface field,
have shown that the wetting transition may be effectively first-order and that
surprisingly the surface susceptibility develops a divergence described by an
anomalous exponent with value . We reproduce
these results using an interfacial Hamiltonian model making connection with
previous studies of two dimensional wetting and show that they follow from the
simple crossover scaling of the singular contribution to the surface
free-energy which describes the change from apparent first-order to continuous
(critical) wetting due to interfacial tunnelling. The crossover scaling
functions are calculated explicitly within both the strong-fluctuation and
intermediate-fluctuation regimes and determine uniquely and more generally the
value of which is non-universal for the latter regime.
The location and the rounding of a line of pseudo pre-wetting transitions
occurring above the wetting temperature and off bulk coexistence, together with
the crossover scaling of the parallel correlation length, is also discussed in
detail
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
Bridging transitions for spheres and cylinders
We study bridging transitions between spherically and cylindrically shaped
particles (colloids) of radius separated by a distance that are
dissolved in a bulk fluid (solvent). Using macroscopics, microscopic density
functional theory and finite-size scaling theory we study the location and
order of the bridging transition and also the stability of the liquid bridges
which determines spinodal lines. The location of the bridging transitions is
similar for cylinders and spheres, so that for example, at bulk coexistence the
distance at which a transition between bridged and unbridged
configurations occurs, is proportional to the colloid radius . However all
other aspects, and, in particular, the stability of liquid bridges, are very
different in the two systems. Thus, for cylinders the bridging transition is
typically strongly first-order, while for spheres it may be first-order,
critical or rounded as determined by a critical radius . The influence of
thick wetting films and fluctuation effects beyond mean-field are also
discussed in depth
The Influence of Intermolecular Forces at Critical Point Wedge Filling
We use microscopic density functional theory to study filling transitions in
systems with long-ranged wall-fluid and short-ranged fluid-fluid forces
occurring in a right-angle wedge. By changing the strength of the wall-fluid
interaction we can induce both wetting and filling transitions over a wide
range of temperatures and study the order of these transitions. At low
temperatures we find that both wetting and filling transitions are first-order
in keeping with predictions of simple local effective Hamiltonian models.
However close to the bulk critical point the filling transition is observed to
be continuous even though the wetting transition remains first-order and the
wetting binding potential still exhibits a small activation barrier. The
critical singularities for adsorption for the continuous filling transitions
depend on whether retarded or non-retarded wall-fluid forces are present and
are in excellent agreement with predictions of effective Hamiltonian theory
even though the change in the order of the transition was not anticipated
Filling transitions in acute and open wedges
We present numerical studies of first-order and continuous filling
transitions, in wedges of arbitrary opening angle , using a microscopic
fundamental measure density functional model with short-ranged fluid-fluid
forces and long-ranged wall-fluid forces. In this system the wetting transition
characteristic of the planar wall-fluid interface is always first-order
regardless of the strength of the wall-fluid potential . In the
wedge geometry however the order of the filling transition depends not only on
but also the opening angle . In particular we show that
even if the wetting transition is strongly first-order the filling transition
is continuous for sufficient acute wedges. We show further that the change in
the order of the transition occurs via a tricritical point as opposed to a
critical-end point. These results extend previous effective Hamiltonian
predictions which were limited only to shallow wedges
Droplet shapes on structured substrates and conformal invariance
We consider the finite-size scaling of equilibrium droplet shapes for fluid
adsorption (at bulk two-phase co-existence) on heterogeneous substrates and
also in wedge geometries in which only a finite domain of the
substrate is completely wet. For three-dimensional systems with short-ranged
forces we use renormalization group ideas to establish that both the shape of
the droplet height and the height-height correlations can be understood from
the conformal invariance of an appropriate operator. This allows us to predict
the explicit scaling form of the droplet height for a number of different
domain shapes. For systems with long-ranged forces, conformal invariance is not
obeyed but the droplet shape is still shown to exhibit strong scaling
behaviour. We argue that droplet formation in heterogeneous wedge geometries
also shows a number of different scaling regimes depending on the range of the
forces. The conformal invariance of the wedge droplet shape for short-ranged
forces is shown explicitly.Comment: 20 pages, 7 figures. (Submitted to J.Phys.:Cond.Mat.
Surface Phase Diagrams for Wetting on Heterogenous Substrates
We propose a simplified description of fluid adsorption on heterogenenous
micropatterned substrates. Using this approach, we are able to rederive results
obtained earlier using effective interfacial Hamiltonian methods and predict a
number of new examples of surface phase behaviour for both singly and
periodically striped substrates. In particular, we show that, for a singly
striped system, the manner in which the locus of surface unbending phase
transitions approaches the pre-wetting line of the infinite pure system, in the
limit of large stripe widths, is non-trivial and sensitive to several
characteristic lengthscales and competing free-energies. For periodic
substrates, we investigate finite-size deviations from Cassie's law for the
wetting temperature of the heterogeneous system when the domain sizes are
mesoscopic.Comment: 12 pages, 13 figure
- …