11,730 research outputs found
Wetting and capillary nematization of binary hard-platelet and hard-rod fluids
Density-functional theory is used to investigate the phase behavior of
colloidal binary hard-platelet and hard-rod fluids near a single hard wall or
confined in a slit pore. The Zwanzig model, in which the orientations of the
particles of rectangular shape are restricted to three orthogonal orientations,
is analyzed by numerical minimization of the grand potential functional. The
density and orientational profiles as well as the surface contributions to the
grand potential are determined. The calculations exhibit a wall-induced
continuous surface transition from uniaxial to biaxial symmetry for the
hard-rod fluid. Complete wetting of the wall -- isotropic liquid interface by a
biaxial nematic film for rods and a uniaxial nematic film for platelets is
found. For the fluids confined by two parallel hard walls we determine a
first-order capillary nematization transition for large slit widths, which
terminates in a capillary critical point upon decreasing the slit width.Comment: 11 pages, 11 figure
Critical Casimir forces between planar and crenellated surfaces
We study critical Casimir forces between planar walls and geometrically
structured substrates within mean-field theory. As substrate structures,
crenellated surfaces consisting of periodic arrays of rectangular crenels and
merlons are considered. Within the widely used proximity force approximation,
both the top surfaces of the merlons and the bottom surfaces of the crenels
contribute to the critical Casimir force. However, for such systems the full,
numerically determined critical Casimir forces deviate significantly fromthe
pairwise addition formalismunderlying the proximity force approximation. A
first-order correction to the proximity force approximation is presented in
terms of a step contribution arising from the critical Casimir interaction
between a planar substrate and the right-angled steps of the merlons consisting
of their upper and lower edges as well as their sidewalls.Comment: 9 pages, 6 figure
Bulk and interfacial properties of binary hard-platelet fluids
Interfaces between demixed fluid phases of binary mixtures of hard platelets
are investigated using density-functional theory. The corresponding excess free
energy functional is calculated within a fundamental measure theory adapted to
the Zwanzig model, in which the orientations of the particles of rectangular
shape are restricted to three orthogonal orientations. Density and
orientational order parameter profiles at interfaces between coexisting phases
as well as the interfacial tension are determined. A density inversion,
oscillatory density profiles, and a Fisher-Widom line have been found in a
mixture of large thin and small thick platelets. The lowest interfacial tension
corresponds to the mean bulk orientation of the platelets being parallel to the
interface. For a mixture of large and small thin platelets, complete wetting of
an isotropic-nematic interface by a second nematic phase is found.Comment: 7 pages, 6 figure
Colloidal hard-rod fluids near geometrically structured substrates
Density functional theory is used to study colloidal hard-rod fluids near an
individual right-angled wedge or edge as well as near a hard wall which is
periodically patterned with rectangular barriers. The Zwanzig model, in which
the orientations of the rods are restricted to three orthogonal orientations
but their positions can vary continuously, is analyzed by numerical
minimization of the grand potential. Density and orientational order profiles,
excess adsorptions, as well as surface and line tensions are determined. The
calculations exhibit an enrichment [depletion] of rods lying parallel and close
to the corner of the wedge [edge]. For the fluid near the geometrically
patterned wall, complete wetting of the wall -- isotropic liquid interface by a
nematic film occurs as a two-stage process in which first the nematic phase
fills the space between the barriers until an almost planar isotropic --
nematic liquid interface has formed separating the higher-density nematic fluid
in the space between the barriers from the lower-density isotropic bulk fluid.
In the second stage a nematic film of diverging film thickness develops upon
approaching bulk isotropic -- nematic coexistence.Comment: 9 pages, 9 figure
Critical Casimir effect for colloids close to chemically patterned substrates
Colloids immersed in a critical or near-critical binary liquid mixture and
close to a chemically patterned substrate are subject to normal and lateral
critical Casimir forces of dominating strength. For a single colloid we
calculate these attractive or repulsive forces and the corresponding critical
Casimir potentials within mean-field theory. Within this approach we also
discuss the quality of the Derjaguin approximation and apply it to Monte Carlo
simulation data available for the system under study. We find that the range of
validity of the Derjaguin approximation is rather large and that it fails only
for surface structures which are very small compared to the geometric mean of
the size of the colloid and its distance from the substrate. For certain
chemical structures of the substrate the critical Casimir force acting on the
colloid can change sign as a function of the distance between the particle and
the substrate; this provides a mechanism for stable levitation at a certain
distance which can be strongly tuned by temperature, i.e., with a sensitivity
of more than 200nm/K.Comment: 27 pages, 14 figure
Alignment of cylindrical colloids near chemically patterned substrates induced by critical Casimir torques
Recent experiments have demonstrated a fluctuation-induced lateral trapping
of spherical colloidal particles immersed in a binary liquid mixture near its
critical demixing point and exposed to chemically patterned substrates.
Inspired by these experiments, we study this kind of effective interaction,
known as the critical Casimir effect, for elongated colloids of cylindrical
shape. This adds orientational degrees of freedom. When the colloidal particles
are close to a chemically structured substrate, a critical Casimir torque
acting on the colloids emerges. We calculate this torque on the basis of the
Derjaguin approximation. The range of validity of the latter is assessed via
mean-field theory. This assessment shows that the Derjaguin approximation is
reliable in experimentally relevant regimes, so that we extend it to Janus
particles endowed with opposing adsorption preferences. Our analysis indicates
that critical Casimir interactions are capable of achieving well-defined,
reversible alignments both of chemically homogeneous and of Janus cylinders.Comment: 24 pages, 12 figures; v2: 22 pages, 12 figure
Bulk and wetting phenomena in a colloidal mixture of hard spheres and platelets
Density functional theory is used to study binary colloidal fluids consisting
of hard spheres and thin platelets in their bulk and near a planar hard wall.
This system exhibits liquid-liquid coexistence of a phase that is rich in
spheres (poor in platelets) and a phase that is poor in spheres (rich in
platelets). For the mixture near a planar hard wall, we find that the phase
rich in spheres wets the wall completely upon approaching the liquid demixing
binodal from the sphere-poor phase, provided the concentration of the platelets
is smaller than a threshold value which marks a first-order wetting transition
at coexistence. No layering transitions are found in contrast to recent studies
on binary mixtures of spheres and non-adsorbing polymers or thin hard rods.Comment: 6 pages, 4 figure
Tunability of Critical Casimir Interactions by Boundary Conditions
We experimentally demonstrate that critical Casimir forces in colloidal
systems can be continuously tuned by the choice of boundary conditions. The
interaction potential of a colloidal particle in a mixture of water and
2,6-lutidine has been measured above a substrate with a gradient in its
preferential adsorption properties for the mixture's components. We find that
the interaction potentials at constant temperature but different positions
relative to the gradient continuously change from attraction to repulsion. This
demonstrates that critical Casimir forces respond not only to minute
temperature changes but also to small changes in the surface properties.Comment: 4 figures;
http://www.iop.org/EJ/article/0295-5075/88/2/26001/epl_88_2_26001.htm
Contact line stability of ridges and drops
Within the framework of a semi-microscopic interface displacement model we
analyze the linear stability of sessile ridges and drops of a non-volatile
liquid on a homogeneous, partially wet substrate, for both signs and arbitrary
amplitudes of the three-phase contact line tension. Focusing on perturbations
which correspond to deformations of the three-phase contact line, we find that
drops are generally stable while ridges are subject only to the long-wavelength
Rayleigh-Plateau instability leading to a breakup into droplets, in contrast to
the predictions of capillary models which take line tension into account. We
argue that the short-wavelength instabilities predicted within the framework of
the latter macroscopic capillary theory occur outside its range of validity and
thus are spurious.Comment: 6 pages, 1 figur
Critical adsorption and critical Casimir forces for geometrically structured confinements
We study the behavior of fluids, confined by geometrically structured
substrates, upon approaching a critical point at T = Tc in their bulk phase
diagram. As generic substrate structures periodic arrays of wedges and ridges
are considered. Based on general renormalization group arguments we calculate,
within mean field approximation, the universal scaling functions for order
parameter profiles of a fluid close to a single structured substrate and
discuss the decay of its spatial variation into the bulk. We compare the excess
adsorption at corrugated substrates with the one at planar walls. The
confinement of a critical fluid by two walls generates effective critical
Casimir forces between them. We calculate corresponding universal scaling
functions for the normal critical Casimir force between a flat and a
geometrically structured substrate as well as the lateral critical Casimir
force between two identically patterned substrates.Comment: 25 pages, 21 figure
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