269 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
The structure of fluids with impurities
The influence of dilute impurities on the structure of a fluid solvent is
investigated theoretically. General arguments, which do not rely on particular
models, are used to derive an extension of the Ornstein-Zernike form for the
solvent structure factor at small scattering vectors. It is shown that dilute
impurities can influence the solvent structure only if they are composed of
ions with significantly different sizes. Non-ionic impurities or ions of
similar size are shown to not alter the solvent structure qualitatively. This
picture is compatible with available experimental data. The derived form of the
solvent structure factor is expected to be useful to infer information on the
impurity-solvent interactions form measured scattering intensities
Free Isotropic-Nematic Interfaces in Fluids of Charged Platelike Colloids
Bulk properties and free interfaces of mixtures of charged platelike colloids
and salt are studied within density-functional theory. The particles are
modeled by hard cuboids with their edges constrained to be parallel to the
artesian axes corresponding to the Zwanzig model. The charges of the particles
are concentrated in their center. The density functional is derived by
functional integration of an extension of the Debye-H\"uckel pair distribution
function with respect to the interaction potential. For sufficiently small
macroion charges, the bulk phase diagrams exhibit one isotropic and one nematic
phase separated by a first-order phase transition. With increasing platelet
charge, the isotropic and nematic binodals are shifted to higher densities. The
Donnan potential between the coexisting isotropic and nematic phases is
inferred from bulk structure calculations. Non-monotonic density and nematic
order parameter profiles are found at a free interface interpolating between
the coexisting isotropic and nematic bulk phases. Moreover, electrically
charged layers form at the free interface leading to monotonically varying
electrostatic potential profiles. Both the widths of the free interfaces and
the bulk correlation lengths are approximately given by the Debye length. For
fixed salt density, the interfacial tension decreases upon increasing the
macroion charge.Comment: 11 pages, submitted to J. Chem. Phy
Phase behavior of a nematic liquid crystal in contact with a chemically and geometrically structured substrate
A nematic liquid crystal in contact with a grating surface possessing an
alternating stripe pattern of locally homeotropic and planar anchoring is
studied within the Frank--Oseen model. The combination of both chemical and
geometrical surface pattern leads to rich phase diagrams, involving a
homeotropic, a planar, and a tilted nematic texture. The effect of the groove
depth and the anchoring strengths on the location and the order of phase
transitions between different nematic textures is studied. A zenithally
bistable nematic device is investigated by confining a nematic liquid crystal
between the patterned grating surface and a flat substrate with strong
homeotropic anchoring.Comment: 7 pages, 7 figure
Effective free energy method for nematic liquid crystals in contact with structured substrates
We study the phase behavior of a nematic liquid crystal confined between a
flat substrate with strong anchoring and a patterned substrate whose structure
and local anchoring strength we vary. By first evaluating an effective surface
free energy function characterizing the patterned substrate we derive an
expression for the effective free energy of the confined nematic liquid
crystal. Then we determine phase diagrams involving a homogeneous state in
which the nematic director is almost uniform and a hybrid aligned nematic state
in which the orientation of the director varies through the cell. Direct
minimization of the free energy functional were performed in order to test the
predictions of the effective free energy method. We find remarkably good
agreement between the phase boundaries calculated from the two approaches. In
addition the effective energy method allows one to determine the energy
barriers between two states in a bistable nematic device.Comment: 10 pages, 7 figures, submitte
Surface properties of fluids of charged platelike colloids
Surface properties of mixtures of charged platelike colloids and salt in
contact with a charged planar wall are studied within density functional
theory. The particles are modeled by hard cuboids with their edges constrained
to be parallel to the Cartesian axes corresponding to the Zwanzig model and the
charges of the particles are concentrated in their centers. The density
functional applied is an extension of a recently introduced functional for
charged platelike colloids. Analytically and numerically calculated bulk and
surface phase diagrams exhibit first-order wetting for sufficiently small
macroion charges and isotropic bulk order as well as first-order drying for
sufficiently large macroion charges and nematic bulk order. The asymptotic
wetting and drying behavior is investigated by means of effective interface
potentials which turn out to be asymptotically the same as for a suitable
neutral system governed by isotropic nonretarded dispersion forces. Wetting and
drying points as well as predrying lines and the corresponding critical points
have been located numerically. A crossover from monotonic to non-monotonic
electrostatic potential profiles upon varying the surface charge density has
been observed. Due to the presence of both the Coulomb interactions and the
hard-core repulsions, the surface potential and the surface charge do not
vanish simultaneously, i.e., the point of zero charge and the isoelectric point
of the surface do not coincide.Comment: 14 pages, submitte
Phase behavior of ionic liquid crystals
Bulk properties of ionic liquid crystals are investigated using density
functional theory. The liquid crystal molecules are represented by ellipsoidal
particles with charges located in their center or at their tails. Attractive
interactions are taken into account in terms of the Gay-Berne pair potential.
Rich phase diagrams involving vapor, isotropic and nematic liquid, as well as
smectic phases are found. The dependence of the phase behavior on various
parameters such as the length of the particles and the location of charges on
the particles is studied
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
Thermodynamics and phase behavior of the lamellar Zwanzig model
Binary mixtures of lamellar colloids represented by hard platelets are
studied within a generalization of the Zwanzig model for rods, whereby the
square cuboids can take only three orientations along the , or axes.
The free energy is calculated within Rosenfeld's ''Fundamental Measure Theory''
(FMT) adapted to the present model. In the one-component limit, the model
exhibits the expected isotropic to nematic phase transition, which narrows as
the aspect ratio ( is the width and the thickness of the
platelets) increases. In the binary case the competition between nematic
ordering and depletion-induced segregation leads to rich phase behaviour.Comment: 9 pages, 6 figure
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