142 research outputs found
Complete Wetting of Pits and Grooves
For one-component volatile fluids governed by dispersion forces an effective
interface Hamiltonian, derived from a microscopic density functional theory, is
used to study complete wetting of geometrically structured substrates. Also the
long range of substrate potentials is explicitly taken into account. Four types
of geometrical patterns are considered: (i) one-dimensional periodic arrays of
rectangular or parabolic grooves and (ii) two-dimensional lattices of
cylindrical or parabolic pits. We present numerical evidence that at the
centers of the cavity regions the thicknesses of the adsorbed films obey
precisely the same geometrical covariance relation, which has been recently
reported for complete cone and wedge filling. However, this covariance does not
hold for the laterally averaged wetting film thicknesses. For sufficiently deep
cavities with vertical walls and close to liquid-gas phase coexistence in the
bulk, the film thicknesses exhibit an effective planar scaling regime, which as
function of undersaturation is characterized by a power law with the common
critical exponent -1/3 as for a flat substrate, but with the amplitude
depending on the geometrical features.Comment: 12 page
Wetting of a plane with a narrow solvophobic stripe
We present a numerical study of a simple Density Functional Theory model of fluid adsorption occurring on a planar wall decorated with a narrow deep stripe of a weaker adsorbing (relatively solvophobic) material, where wall-fluid and fluid-fluid intermolecular forces are considered to be dispersive. Both the stripe and outer sub-strate exhibit first-order wetting transitions with the wetting temperature of the stripe lying above that of the outer material. This geometry leads to a rich phase diagram due to the interplay between the pre-wetting transition of the outer sub-strate and an unbending transition corresponding to the local evaporation of liquid near the stripe. Depending on the
width of the stripe the line of unbending transi-tions merges with the pre-wetting line inducing a 2D wetting transition occurring across the substrate. In turn, this leads to the continuous pre-drying of the thick pre-wetting film as the pre-wetting line is approached from above. Interestingly we find that the merging of the unbending and pre-wetting lines occurs even for the widest stripes considered. This contrasts markedly with the scenario where the outer material has the higher wetting temperature, for which the merging of the unbending and pre-wetting lines only occurs for very narrow stripes.We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the U.K. through Grants No. EP/L027186, EP/L020564 and EP/K503733 (EPSRC-Imperial College Pathways to Impact-Impact Acceleration Award), by the European Research Council (ERC) through Advanced Grant No. 247031 and FIS2015-66523-P (MINECO/FEDER, UE)
Classical density functional study of wetting transitions on nanopatterned surfaces
Even simple fluids on simple substrates can exhibit very rich surface phase behaviour. To illustrate this, we consider fluid adsorption on a planar wall chemically patterned with a deep stripe of a different material. In this system, two phase transitions compete: unbending and pre-wetting. Using microscopic density-functional theory, we show that, for thin stripes, the lines of these two phase transitions may merge, leading to a new two-dimensional-like wetting transition occurring along the walls. The influence of intermolecular forces and interfacial fluctuations on this phase transition and at complete pre-wetting are considered in detail.PY is grateful to Dr Miguel A Durán-Olivencia from the Chemical Engineering Department of Imperial College London for numerous stimulating discussions. We acknowledge financial support from the Engineering and Physical Sciences Research Council of the UK through Grants No. EP/L027186 and EP/L020564 and by the European Research Council through Advanced Grant No. 247031. CR also acknowledges the support of the Spanish Ministerio de Economía y Competitividad under grant FIS2015-66523-P
Grafitos sobre Terra Sigillata Hispánica hallados en un vertedero del siglo I en la casa de Hippolytus (Complutum)
Presentamos en este estudio un conjunto de materiales inéditos del yacimiento romano de "hippolytus". Estos grafitos, presentes en la mayoría de los establecimientos romanos, no han recibido, en algunas ocasiones, la debida atención por parte de los investigadores. Su análisis pormenorizado, junto a la evidente ventaja de provenir de un contexto arqueológico cerrado y bien definido, nos acerca al conocimiento de las relaciones sociales, parentesco y origen de los moradores de este nuevo enclave altoimperial complutense
Geometry dominated fluid adsorption on sculptured substrates
Experimental methods allow the shape and chemical composition of solid
surfaces to be controlled at a mesoscopic level. Exposing such structured
substrates to a gas close to coexistence with its liquid can produce quite
distinct adsorption characteristics compared to that occuring for planar
systems, which may well play an important role in developing technologies such
as super-repellent surfaces or micro-fluidics. Recent studies have concentrated
on adsorption of liquids at rough and heterogeneous substrates and the
characterisation of nanoscopic liquid films. However, the fundamental effect of
geometry has hardly been addressed. Here we show that varying the shape of the
substrate can exert a profound influence on the adsorption isotherms allowing
us to smoothly connect wetting and capillary condensation through a number of
novel and distinct examples of fluid interfacial phenomena. This opens the
possibility of tailoring the adsorption properties of solid substrates by
sculpturing their surface shape.Comment: 6 pages, 4 figure
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
Derivation of a Non-Local Interfacial Hamiltonian for Short-Ranged Wetting II: General Diagrammatic Structure
In our first paper, we showed how a non-local effective Hamiltionian for
short-ranged wetting may be derived from an underlying Landau-Ginzburg-Wilson
model. Here, we combine the Green's function method with standard perturbation
theory to determine the general diagrammatic form of the binding potential
functional beyond the double-parabola approximation for the
Landau-Ginzburg-Wilson bulk potential. The main influence of cubic and quartic
interactions is simply to alter the coefficients of the double parabola-like
zig-zag diagrams and also to introduce curvature and tube-interaction
corrections (also represented diagrammatically), which are of minor importance.
Non-locality generates effective long-ranged many-body interfacial interactions
due to the reflection of tube-like fluctuations from the wall. Alternative wall
boundary conditions (with a surface field and enhancement) and the diagrammatic
description of tricritical wetting are also discussed.Comment: (14 pages, 2 figures) Submitted J. Phys. Condens. Matte
Stability of Colloidal Quasicrystals
Freezing of charge-stabilized colloidal suspensions and relative stabilities
of crystals and quasicrystals are studied using thermodynamic perturbation
theory. Macroion interactions are modelled by effective pair potentials
combining electrostatic repulsion with polymer-depletion or van der Waals
attraction. Comparing free energies -- counterion terms included -- for
elementary crystals and rational approximants to icosahedral quasicrystals,
parameters are identified for which one-component quasicrystals are stabilized
by a compromise between packing entropy and cohesive energy.Comment: 6 pages, 4 figure
Phase transitions in nanosystems caused by interface motion: The Ising bi-pyramid with competing surface fields
The phase behavior of a large but finite Ising ferromagnet in the presence of
competing surface magnetic fields +/- H_s is studied by Monte Carlo simulations
and by phenomenological theory. Specifically, the geometry of a double pyramid
of height 2L is considered, such that the surface field is positive on the four
upper triangular surfaces of the bi-pyramid and negative on the lower ones. It
is shown that the total spontaneous magnetization vanishes (for L -> infinity)
at the temperature T_f(H), related to the "filling transition" of a
semi-infinite pyramid, which can be well below the critical temperature of the
bulk. The discontinuous vanishing of the magnetization is accompanied by a
susceptibility that diverges with a Curie-Weiss power law, when the transition
is approached from either side. A Landau theory with size-dependent critical
amplitudes is proposed to explain these observations, and confirmed by finite
size scaling analysis of the simulation results. The extension of these results
to other nanosystems (gas-liquid systems, binary mixtures, etc.) is briefly
discussed
Universality for 2D Wedge Wetting
We study 2D wedge wetting using a continuum interfacial Hamiltonian model
which is solved by transfer-matrix methods. For arbitrary binding potentials,
we are able to exactly calculate the wedge free-energy and interface height
distribution function and, thus, can completely classify all types of critical
behaviour. We show that critical filling is characterized by strongly universal
fluctuation dominated critical exponents, whilst complete filling is determined
by the geometry rather than fluctuation effects. Related phenomena for
interface depinning from defect lines in the bulk are also considered.Comment: 4 pages, 1 figur
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