88 research outputs found
Influence of Homeotropic Anchoring Walls upon Nematic and Smectic Phases
McMillan liquid crystal model sandwiched between strong homeotropic anchoring
walls is studied. Phase transitions between isotropic, nematic, and smectic A
phases are investigated for wide ranges of an interaction parameter and of the
system thickness. It is confirmed that the anchoring walls induce an increase
in transition temperatures, dissappearance of phase transitions, and an
appearance of non-spontaneous nematic phase. The similarity between influence
of anchoring walls and that of external fields is discussed.Comment: 5 pages, 6 figure
Numerical study of surface-induced reorientation and smectic layering in a nematic liquid crystal
Surface-induced profiles of both nematic and smectic order parameters in a
nematic liquid crystal, ranging from an orienting substrate to "infinity", were
evaluated numerically on base of an extended Landau theory. In order to obtain
a smooth behavior of the solutions at "infinity" a boundary energy functional
was derived by linearizing the Landau energy around its equilibrium solutions.
We find that the intrinsic wave number of the smectic structure, which plays
the role of a coupling between nematic and smectic order, strongly influences
the director reorientation. Whereas the smectic order is rapidly decaying when
moving away from the surface, the uniaxial nematic order parameter shows an
oscillatory behavior close to the substrate, accompanied by a non-zero local
biaxiality.Comment: LaTeX, 17 pages, with 4 postscript figure
Orientational transitions in a nematic confined by competing surfaces
The effect of confinement on the orientational structure of a nematic liquid
crystal model has been investigated by using a version of density-functional
theory (DFT). We have focused on the case of a nematic confined by opposing
flat surfaces, in slab geometry (slit pore), which favor planar molecular
alignment (parallel to the surface) and homeotropic alignment (perpendicular to
the surface), respectively. The spatial dependence of the tilt angle of the
director with respect to the surface normal has been studied, as well as the
tensorial order parameter describing the molecular order around the director.
For a pore of given width, we find that, for weak surface fields, the alignment
of the nematic director is perpendicular to the surface in a region next to the
surface favoring homeotropic alignment, and parallel along the rest of the
pore, with a interface separating these regions (S phase). For strong surface
fields, the director is distorted uniformly, the tilt angle exhibiting a linear
dependence with the distance normal to the surface (L phase). Our calculations
reveal the existence of a first-order transition between the two director
configurations, which is driven by changes in the surface field strength, and
also by changes in the pore width. In the latter case the transition occurs,
for a given surface field, between the S phase for narrow pores and the L phase
for wider pores. A link between the L-S transition and the anchoring transition
observed for the semi-infinite case is proposed. We also provide calculations
with a phenomenological approach that yields the same main result that DFT in
the scale length where this is valid.Comment: submitted to PR
Membranes in rod solutions: a system with spontaneously broken symmetry
We consider a dilute solution of infinitely rigid rods near a curved,
perfectly repulsive surface and study the contribution of the rod depletion
layer to the bending elastic constants of membranes. We find that a spontaneous
curvature state can be induced by exposure of BOTH sides of the membrane to a
rod solution. A similar result applies for rigid disks with a diameter equal to
the rod's length. We also study the confinement of rods in spherical and
cylindrical repulsive shells. This helps elucidate a recent discussion on
curvature effects in confined quantum mechanical and polymer systems.Comment: 10 pages, 2 figures, 1 table; submitted to PR
Theory and simulation of the nematic zenithal anchoring coefficient
Combining molecular simulation, Onsager theory and the elastic description of
nematic liquid crystals, we study the dependence of the nematic liquid crystal
elastic constants and the zenithal surface anchoring coefficient on the value
of the bulk order parameter
Elastic constants of nematic liquid crystals of uniaxial symmetry
We study in detail the influence of molecular interactions on the Frank
elastic constants of uniaxial nematic liquid crystals composed of molecules of
cylindrical symmetry. A brief summary of the status of theoretical development
for the elastic constants of nematics is presented. Considering a pair
potential having both repulsive and attractive parts numerical calculations are
reported for three systems MBBA, PAA and 8OCB. For these systems the
length-to-width ratio is estimated from the experimentally proposed
structure of the molecules. The repulsive interaction is represented by a
repulsion between hard ellipsoids of revolution (HER) and the attractive
potential is represented by the quadrupole and dispersion interactions. From
the numerical results we observe that in the density range of nematics the
contribution of the quadrupole and dispersion interactions are small as
compared to the repulsive HER interaction. The inclusion of attractive
interaction reduces the values of elastic constants ratios. The temperature
variation of elastic constants ratios are reported and compared with the
experimental values. A reasonably good agreement between theory and experiment
is observed
Liquid crystal director fluctuations and surface anchoring by molecular simulation
We propose a simple and reliable method to measure the liquid crystal surface
anchoring strength by molecular simulation. The method is based on the
measurement of the long-range fluctuation modes of the director in confined
geometry. As an example, molecular simulations of a liquid crystal in slab
geometry between parallel walls with homeotropic anchoring have been carried
out using the Monte Carlo technique. By studying different slab thicknesses, we
are able to calculate separately the position of the elastic boundary
condition, and the extrapolation length
Phase ordering in bulk uniaxial nematic liquid crystals
The phase-ordering kinetics of a bulk uniaxial nematic liquid crystal is
addressed using techniques that have been successfully applied to describe
ordering in the O(n) model. The method involves constructing an appropriate
mapping between the order-parameter tensor and a Gaussian auxiliary field. The
mapping accounts both for the geometry of the director about the dominant
charge 1/2 string defects and biaxiality near the string cores. At late-times t
following a quench, there exists a scaling regime where the bulk nematic liquid
crystal and the three-dimensional O(2) model are found to be isomorphic, within
the Gaussian approximation. As a consequence, the scaling function for
order-parameter correlations in the nematic liquid crystal is exactly that of
the O(2) model, and the length characteristic of the strings grows as
. These results are in accord with experiment and simulation. Related
models dealing with thin films and monopole defects in the bulk are presented
and discussed.Comment: 21 pages, 3 figures, REVTeX, submitted to Phys. Rev.
Hard-Sphere Fluids in Contact with Curved Substrates
The properties of a hard-sphere fluid in contact with hard spherical and
cylindrical walls are studied. Rosenfeld's density functional theory (DFT) is
applied to determine the density profile and surface tension for wide
ranges of radii of the curved walls and densities of the hard-sphere fluid.
Particular attention is paid to investigate the curvature dependence and the
possible existence of a contribution to that is proportional to the
logarithm of the radius of curvature. Moreover, by treating the curved wall as
a second component at infinite dilution we provide an analytical expression for
the surface tension of a hard-sphere fluid close to arbitrary hard convex
walls. The agreement between the analytical expression and DFT is good. Our
results show no signs for the existence of a logarithmic term in the curvature
dependence of .Comment: 15 pages, 6 figure
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