36 research outputs found
Lateral transport of thermal capillary waves
We demonstrate that collective motion of interfacial fluctuations can occur
at the interface between two coexisting thermodynamic phases. Based on computer
simulation results for driven diffusive Ising and Blume-Capel models, we
conjecture that the thermal capillary waves at a planar interface travel along
the interface if the lateral order parameter current j_op(y) is an odd function
of the distance y from the interface and hence possesses opposite directions in
the two phases. Such motion does not occur if j_op(y) is an even function of y.
A discrete Gaussian interface model with effective dynamics exhibits similiar
transport phenomena but with a simpler dispersion relation. These findings open
up avenues for controlled interfacial transport on the nanoscale.Comment: 4 pages, 6 figure
Casimir interactions in Ising strips with boundary fields: exact results
An exact statistical mechanical derivation is given of the critical Casimir
forces for Ising strips with arbitrary surface fields applied to edges. Our
results show that the strength as well as the sign of the force can be
controled by varying the temperature or the fields. An interpretation of the
results is given in terms of a linked cluster expansion. This suggests a
systematic approach for deriving the critical Casimir force which can be used
in more general models.Comment: 10 pages, 4 figure
Interfaces in driven Ising models: shear enhances confinement
We use a phase-separated driven two-dimensional Ising lattice gas to study
fluid interfaces exposed to shear flow parallel to the interface. The interface
is stabilized by two parallel walls with opposing surface fields and a driving
field parallel to the walls is applied which (i) either acts locally at the
walls or (ii) varies linearly with distance across the strip. Using computer
simulations with Kawasaki dynamics, we find that the system reaches a steady
state in which the magnetisation profile is the same as that in equilibrium,
but with a rescaled length implying a reduction of the interfacial width. An
analogous effect was recently observed in sheared phase-separated colloidal
dispersions. Pair correlation functions along the interface decay more rapidly
with distance under drive than in equilibrium and for cases of weak drive can
be rescaled to the equilibrium result.Comment: 4 pages, 3 figures Text modified, added Fig. 3b. To appear in Phys.
Rev. Letter
Distribution of ions near a charged selective surface in critical binary solvents
Near-critical binary mixtures containing ionic solutes near a charged wall
preferentially adsorbing one component of the solvent are studied. Within the
Landau-Ginzburg approach extended to include electrostatic interactions and the
chemical preference of ions for one component of the solvent, we obtain a
simple form for the leading-order correction to the Debye-Huckel theory result
for the charge density profile. Our result shows that critical adsorption
influences significantly distribution of ions near the wall. This effect may
have important implications for the screening of electrostatic interactions
between charged surfaces immersed in binary near-critical solvents.Comment: 24 pages, 3 figure
On the surface critical behaviour in Ising strips: density-matrix renormalization-group study
Using the density-matrix renormalization-group method we study the surface
critical behaviour of the magnetization in Ising strips in the subcritical
region. Our results support the prediction that the surface magnetization in
the two phases along the pseudo-coexistence curve also behaves as for the
ordinary transition below the wetting temperature for the finite value of the
surface field.Comment: 15 pages, 9 figure
Monte Carlo simulation results for critical Casimir forces
The confinement of critical fluctuations in soft media induces critical
Casimir forces acting on the confining surfaces. The temperature and geometry
dependences of such forces are characterized by universal scaling functions. A
novel approach is presented to determine them for films via Monte Carlo
simulations of lattice models. The method is based on an integration scheme of
free energy differences. Our results for the Ising and the XY universality
class compare favourably with corresponding experimental results for wetting
layers of classical binary liquid mixtures and of 4He, respectively.Comment: 14 pages, 5 figure
Crossover of Critical Casimir forces between different surface universality classes
In confined systems near a continuous phase transition the long-ranged
fluctuations of the corresponding order parameter are subject to boundary
conditions. These constraints result in so-called critical Casimir forces
acting as effective forces on the confining surfaces. For systems belonging to
the Ising bulk universality class corresponding to a scalar order parameter the
critical Casimir force is studied for the film geometry in the crossover regime
characterized by different surface fields at the two surfaces. The scaling
function of the critical Casimir force is calculated within mean field theory.
Within our approach, the scaling functions of the critical Casimir force and of
the order parameter profile for finite surface fields can be mapped by
rescaling, except for a narrow crossover regime, onto the corresponding scaling
function of the so-called normal fixed point of strong surface fields. In the
crossover regime, the critical Casimir force as function of temperature
exhibits more than one extremum and for certain ranges of surface field
strengths it changes sign twice upon varying temperature. Monte Carlo
simulation data obtained for a three-dimensional Ising film show similar
trends. The sign of the critical Casimir force can be inferred from the
comparison of the order parameter profiles in the film and in the semi-infinite
geometry
Critical Casimir Effect in 3He-4He films
Universal aspects of the thermodynamic Casimir effect in wetting films of
3He-4He mixtures near their bulk tricritical point are studied within suitable
models serving as representatives of the corresponding universality class. The
effective forces between the boundaries of such films arising from the
confinement are calculated along isotherms at several fixed concentrations of
3He. Nonsymmetric boundary conditions impose nontrivial concentration profiles
leading to repulsive Casimir forces which exhibit a rich behavior of the
crossover between the tricritical point and the line of critical points. The
theoretical results agree with published experimental data and emphasize the
importance of logarithmic corrections.Comment: 12 pages, 4 figures, submitted to the Phys. Rev. Let
Critical Casimir Effect in superfluid wetting films
Recent experimental data for the complete wetting behavior of pure 4He and of
3He-4He mixtures exposed to solid substrates show that there is a change of the
corresponding film thicknesses L upon approaching thermodynamically the
lambda-transition and the tricritical end point, respectively, which can be
attributed to critical Casimir forces f_C. We calculate the scaling functions
vartheta of f_C within models representing the corresponding universality
classes. For the mixtures our analysis provides an understanding of the rich
behavior of vartheta deduced from the experimental data and predicts the
crossover behavior between the tricritical point and the lambda-transition of
pure 4He which are connected by a line of critical points. The formation of a
'soft-mode' phase within the wetting films gives rise to a pronounced maximum
of f_C below the tricritical point as observed experimentally. Near the
tricritical point we find logarithmic corrections ~L^(-3)(ln L)^(1/2) for the
leading behavior of vartheta dominating the contributions from the background
dispersion forces.Comment: 32 pages, 12 figure
Universal scaling functions of critical Casimir forces obtained by Monte Carlo simulations
Effective Casimir forces induced by thermal fluctuations in the vicinity of
bulk critical points are studied by means of Monte Carlo simulations in
three-dimensional systems for film geometries and within the experimentally
relevant Ising and XY universality classes. Several surface universality
classes of the confining surfaces are considered, some of which are relevant
for recent experiments. A novel approach introduced previously EPL 80, 60009
(2007), based inter alia on an integration scheme of free energy differences,
is utilized to compute the universal scaling functions of the critical Casimir
forces in the critical range of temperatures above and below the bulk critical
temperature. The resulting predictions are compared with corresponding
experimental data for wetting films of fluids and with available theoretical
results.Comment: 21 pages, 17 figure