102 research outputs found
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
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
Critical adsorption on non-spherical colloidal particles
We consider a non-spherical colloidal particle immersed in a fluid close to
its critical point. The temperature dependence of the corresponding order
parameter profile is calculated explicitly. We perform a systematic expansion
of the order parameter profile in powers of the local curvatures of the surface
of the colloidal particle. This curvature expansion reduces to the short
distance expansion of the order parameter profile in the case that the solvent
is at the critical composition.Comment: 9 pages, 7 figure
Interaction of cylindrical polymer brushes in dilute and semi-dilute solution
We present a systematic study of flexible cylindrical brush-shaped macromolecules in a good solvent by small-angle neutron scattering (SANS), static light scattering (SLS), and by dynamic light scattering (DLS) in dilute and semi-dilute solution. The SLS and SANS data extrapolated to infinite dilution lead to the shape of the polymer that can be modeled in terms of a worm-like chain with a contour length of 380 nm and a persistence length of 17.5 nm. SANS data taken at higher polymer concentration were evaluated by using the polymer reference interaction site model (PRISM). We find that the persistence length reduce from 17.5 nm at infinite dilution to 5.3 nm at the highest concentration (volume fraction 0.038). This is comparable with the decrease of the persistence length in semi-dilute concentration predicted theoretically for polyelectrolytes. This finding reveals a softening of stiffness of the polymer brushes caused by their mutual interaction
Softening of the stiffness of bottlebrush polymers by mutual interaction
We study bottlebrush macromolecules in a good solvent by small-angle neutron
scattering (SANS), static light scattering (SLS), and dynamic light scattering
(DLS). These polymers consist of a linear backbone to which long side chains
are chemically grafted. The backbone contains about 1600 monomer units (weight
average) and every second monomer unit carries side-chains with ca. 60 monomer
units. The SLS- and SANS data extrapolated to infinite dilution lead to the
form factor of the polymer that can be described in terms of a worm-like chain
with a contour length of 380 nm and a persistence length of 17.5 nm. An
analysis of the DLS data confirm these model parameters. The scattering
intensities taken at finite concentration can be modeled using the polymer
reference interaction site model. It reveals a softening of the bottlebrush
polymers caused by their mutual interaction. We demonstrate that the
persistence decreases from 17.5 nm down to 5 nm upon increasing the
concentration from dilute solution to the highest concentration 40.59 g/l under
consideration. The observed softening of the chains is comparable to the
theoretically predicted decrease of the electrostatic persistence length of
linear polyelectrolyte chains at finite concentrations.Comment: 4 pages, 4 figure
Structure and equation of state of interaction site models for disc-shaped lamellar colloids
We apply RISM (Reference Interaction Site Model) and PRISM (polymer-RISM)
theories to calculate the site-site pair structure and the osmotic equation of
state of suspensions of circular or hexagonal platelets (lamellar colloids)
over a range of ratios of the particle diameter over thickness. Despite the
neglect of edge effects, the simpler PRISM theory yields results in good
agreement with the more elaborate RISM calculations, provided the correct form
factor, characterizing the intramolecular structure of the platelets, is used.
The RISM equation of state is sensitive to the number of sites used to model
the platelets, but saturates when the hard spheres, associated with the
interaction sites, nearly touch; the limiting equation of state agrees
reasonably well with available simulation data for all densities up to the
isotropic-nematic transition. When properly scaled with the second virial
coefficient, the equations of state of platelets with different aspect ratios
nearly collapse on a single master curve.Comment: 10 Pages, 11 Figures, Typesetted using RevTeX
Fluids of platelike particles near a hard wall
Fluids consisting of hard platelike particles near a hard wall are
investigated using density functional theory. The density and orientational
profiles as well as the surface tension and the excess coverage are determined
and compared with those of a fluid of rodlike particles. Even for low densities
slight orientational packing effects are found for the platelet fluid due to
larger intermolecular interactions between platelets as compared with those
between rods. A net depletion of platelets near the wall is exhibited by the
excess coverage, whereas a change of sign of the excess coverage of hard-rod
fluids is found upon increasing the bulk density.Comment: 6 pages, 9 figure
A fundamental-measure density functional for mixtures of parallel hard cylinders
We obtain a fundamental measure density functional for mixtures of parallel
hard cylinders. To this purpose we first generalize to multicomponent mixtures
the fundamental measure functional proposed by Tarazona and Rosenfeld for a
one-component hard disk fluid, through a method alternative to the cavity
formalism of these authors. We show the equivalence of both methods when
applied to two-dimensional fluids. The density functional so obtained reduces
to the exact density functional for one-dimensional mixtures of hard rods when
applied to one-dimensional profiles. In a second step we apply an idea put
forward some time ago by two of us, based again on a dimensional reduction of
the system, and derive a density functional for mixtures of parallel hard
cylinders. We explore some features of this functional by determining the
fluid-fluid demixing spinodals for a binary mixture of cylinders with the same
volume, and by calculating the direct correlation functions.Comment: 19 pages, 4 figure
Characterization of local dynamics and mobilities in polymer melts - a simulation study
The local dynamical features of a PEO melt studied by MD simulations are
compared to two model chain systems, namely the well-known Rouse model as well
as the semiflexible chain model (SFCM) that additionally incorporates chain
stiffness. Apart from the analysis of rather general quantities such as the
mean square displacement (MSD), we present a new statistical method to extract
the local bead mobility from the simulation data on the basis of the Langevin
equation, thus providing a complementary approach to the classical Rouse-mode
analysis. This allows us to check the validity of the Langevin equation and, as
a consequence, the Rouse model. Moreover, the new method has a broad range of
applications for the analysis of the dynamics of more complex polymeric systems
like comb-branched polymers or polymer blends.Comment: 6 pages, 5 figure
- …