** **

** **

**Liquid-crystal
phase transitions **

**in
suspensions of plate-like particles**

** **

G.J.
Vroege and H. Lekkerkerker

*Van 't
Hoff Laboratory for Physical and Colloid Chemistry*

*Debye
Research Institute, Utrecht University*

*H.R.
Kruytgebouw, N-704, Padualaan 8, 3584 CH Utrecht*

*The
Netherlands*

We study the
liquid-crystalline phase behavior of suspensions of hard colloidal platelets,
in particular, the relationship between their size polydispersity and the stability of a nematic, columnar and smectic
phase. A first-order nematic-to-columnar transition is observed for suspensions
up to 25% polydispersity in platelet diameter.

The observed
tolerance of polydispersity in the columnar phase, being a two-dimensional
crystal, seems remarkable in light of current predictions for the terminal polydispersity for hard-sphere
crystallization and hard-rod smectic ordering.

** **

** **

**Simulation
studies of solvent diffusion**

**and the
glass transition in polymers**

** **

Philip L.
Taylor

*Dept. of
Physics, Case Western Reserve University, *

* Cleveland
OH 44106-7079 (USA)*

The glass
transition and non-Fickian diffusion are two related problems in polymer
physics that can be studied through atomistic molecular dynamics
simulations. The mean squared
deviations of atoms, monomers, and molecules from their initial positions can
be analyzed by means of a technique that separates the effects of diffusive motion from the
underlying vibrational motion.

One can interpret
diffusive motion in syndiotactic poly(methyl methacrylate) to find a novel
power-law variation with time, with an
exponent that varies continuously from 0.5 below the glass
transition temperature to unity at high
temperatures. The diffusion of methanol into PMMA is
interesting in that nonlinear effects
dominate those of Fickian diffusion.
Entry of the solvent into the
PMMA is accompanied by a swelling of the polymer matrix.

** **

** **

**Hybrid
Molecular Dynamics / Lattice Boltzmann Approach**

**to Polymer
Solution Dynamics**

** **

Burkhard
Duenweg

*Max-Planck-Institut für Polymerforschung*

*Ackermannweg
10, D-55128 Mainz, Germany*

We establish a new
efficient method for simulating polymer-solvent systems which combines a
lattice Boltzmann approach for the fluid with a continuum molecular dynamics
(MD) model for the polymer chains. The two parts are coupled by a simple
dissipative point-particle force, and the system is driven by Langevin
stochastic forces added to both the fluid and the polymers. Extensive tests of
the new method for the case of a single chain in good solvent are performed. The
dynamic and static properties predicted by simple scaling arguments (which are
explained in the talk) are validated. In this context, the influence of the
finite size of the simulation box is discussed. Usually the finite size
corrections scale as 1/L, where L denotes the linear dimension of the box. This
is a result of the Coulomb-like long-range nature of hydrodynamic interactions.
However, the decay rate of internal polymer relaxation modes is only subject to
an 1/L^3 finite size effect, corresponding to an effective dipolar interaction.
Furthermore, the mapping to an existing MD simulation of the same system is
done so that all physical input values for the new method can be derived from
pure MD simulation. Both methods can thus be compared quantitatively. The main
advantage of the new approach compared to a particle method is the fact that
the solvent is completely structureless, such that the polymer system can be
relaxed into thermal equilibrium without surrounding solvent. Moreover, it
turns out that the lattice spacing provides a convenient lower length scale
cutoff to regularize the hydrodynamics, thus naturally accounting for the fact
that in nature there are no point particles with finite friction. The method is
then applied to a semidilute system of chains of length N = 1000, which would
not be accessible to a pure particle method. We observe the crossover from Zimm
dynamics at short length and time scales to Rouse dynamics at long length and
time scales. The dynamic crossover length is proportional to the static
screening length, as predicted by de Gennes. At short times, the dynamics turns
out to be Zimm-like even for the long-wavelength modes. This suggests the
simple picture of Zimm blobs which can move freely up to their own size, while
later they feel constraints and offer frictional resistance to the flow. This
finding shows that the screening of hydrodynamic interactions is an
intrinsically time-dependent dynamic phenomenon, and therefore invalidates any
theoretical description which is based on a screened hydrodynamic interaction
depending only on distance.

References:

1. P. Ahlrichs and
B. Duenweg, Journal of Chemical Physics 111, 8225 (1999).

2. P. Ahlrichs, R.
Everaers, and B. Duenweg, Physical Review E 64, 040501

(R) (2001).

** **

** **

**COMPUTER
SIMULATION OF **

**LIQUID
CRYSTAL INTERFACES**

** **

Luis F.
Rull

*Departamento
de Fìsica Atòmica, Molecular y Nuclear, *

*Area de
Fìsica Teòrica, Universidad de Sevilla, *

*Aptdo.
1065, Sevilla 41080, Spain.*

A computer
simulation study of a solid-vapour interface of the Gay-Berne fluid will be
presented. The anisotropic parameter k’ has
been chosen such that the bulk system presents a
vapour-isotropic-nematic triple point
(k’ = 1.5, k= 3). Two isotherms, above and below this triple point, are evaluated in the Monte Carlo simulations.

The sytem was
simulated in a box with periodic boundary conditions in x and y directions
while in the z direction, one of the side of the simulation box was specified
as a planar Yukawa potential and, in order to avoid cappillary condensation
effects, a single hard wall was used in the another one. Starting from the
subcritical vapour and increasing the chemical potential and/or the
pressure, the density and order
parameter profiles are obtained during the simulations.

** **

** **

**Simulations
and modeling **

**of defect
dynamics in nematics**

** **

S. Žumer

*Physics
Department, University of Ljubljana, Jadranska 19,*

*SI-1000
Ljubljana, Slovenia*

Advances in
modelling and simulations of defect annihilation in nematic liquid crystals are
presented. On the phenomenological side the effect of the backflow is taken
into account. Generalized nematogenic hydrodynamics for the tensor order
parameter is used. In particular annihilation of the pair of two line defects
is analyzed. On the microscopic side
the Brownian dynamics is used to follow coarsening of a nematic after an
isotropic-nematic quench of a nematogenic material. Particular attention is
focused on the scaling behavior.

** **

** **

**Phase
diagram of discotic liquid crystals**

**in bulk
and in confined geometry.**

** **

L.
Bellier-Castella+*, D. Caprion+ and J.-P. Ryckaert+

*(+)
Physique des polymères, Université Libre de Bruxelles*

*(*)
Laboratoire de physique des matériaux, Université de Lyon.*

The columnar phase
in discotic liquid crystals is of high technological importance. It is
therefore useful to know the appropriate range of intermolecular parameters
which lead to the formation of phases with well defined parallel columns (2D
order) while avoiding at the same time a full crystallisation of the system.

Another question
is to which extend a flat surface can promote a suitable orientation of these
columns and perhaps enhance their stability. This largely depends upon the
nature of the wall-discotic molecule interaction which favours the orientation
of the discs either parallel or perpendicular to the surface.

New NPT Monte
Carlo simulations of discotic liquid crystals in bulk and in confined geometry
will be presented and discussed in this talk along the lines summarized above.

** **

** **

**Computer
Simulations of **

** Real and Virtual Liquid Crystals**

** **

Claudio
Zannoni

*Università
di Bologna, Dipartimento di Chimica Fisica ed Inorganica,*

*Viale
Risorgimento 4, 40136 Bologna, ITALY.*

While atomistic
Molecular Dynamics (MD) simulations are widely and successfully used in many
fields of chemistry and biochemistry, there are not many examples of their
reliability when applied to liquid crystals problems, in particular for the
prediction of nematic-isotropic transition temperatures [1]. Here we present a computer simulation study
of the odd-even effect in liquid crystals [2].
We have investigated the temperature dependence of the orientational
order of the first three homologues of the series of
phenylalkyl-4-(4'-cyanobenzylidene) aminocinnamates, modelled with full
atomistic details with a suitably modified AMBER-like force field description,
using MD simulations [3]. It is well known [4] that those molecules present a
large "odd-even" effect, i.e. a large alternation in properties and
in mesophase transition temperatures when varying the parity of the number of
their alkyl groups.

This effect is
well reproduced in our simulation results, and the transition temperatures are
in relatively good agreement with the experiment. We have a performed
conformational analysis, trying to explain the physical reasons of the
odd-even effect for these molecules. We present here detailed results and a
comparison between the homologues and more generally we try to assess the
potentialities of atomistic simulations in the field at this point in time.

Beside this
simulation of real liquid crystals, we discuss simpler, molecular resolution
models useful to try and facilitate the search for good candidates for yielding
mesophases with specific properties of interest (such as ferroelectricity). We
present a brief summary of recent developments for systems of particles
interacting with model potentials based on the Gay-Berne (GB) molecular level
interactions (see [5] for a review), and discuss, e.g, formation of smectic C
phases from dipolar systems [6] and the modelling of non-centrosymmetric molecules
for the simulation non chiral ferroelectric nematics [7].

[1] see, e.g.
contributions by M. Wilson, M. Glaser, P. Procacci in P. Pasini and C. Zannoni,
eds., Advances in the computer simulations of liquid crystals, Kluwer,
Dordrecht, 2000.

[2] R. Berardi, L.
Muccioli and C. Zannoni, to be published (2002)

[3] P. Procacci,
E. Paci, T. Darden and M. Marchi, J.Comp.Chem., 18, 1848 (1997)

[4] G.W.Gray in
The Molecular Physics of Liquid Crystals, G.R. Luckhurst and G.W. Gray editors,
Academic Press (1979).

[5] C.
Zannoni, J. Mater. Chem., 11, 2637
(2001

[6] R. Berardi, S.
Orlandi and C. Zannoni, to be published
(2002)

[7] R. Berardi, M.
Ricci and C. Zannoni, Chem. Phys. Chem., 2 (2001), 443

** **

** **

**Molecular
Simulations **

**of the
Liquid Crystalline Phases **

**of
Fan-Shaped Molecules**

** **

A.G.
Vanakaras, D.J. Photinos

*Department
of Materials Science, University of Patras, Patras 26500, Greece.*

Recent computer
simulation studies [1] have shown that idealised fan-shaped molecules can form
layered fluid phases exhibiting the symmetry characteristics of the usual
smectic-A phase but with strongly correlated rotations about the fan axes of
neighboring molecules. The physical properties distinguishing these novel
phases from the known mesophases are investigated in the present work using
molecular theory and NPT Monte Carlo simulations.

The relevant order
parameters are defined and evaluated for model systems. The molecular structure
requirements for the stability of the ordered fluid phases are identified for
representative architectures of fan, propeller and cage molecules. Shape
non-convexity, allowing interdigitation-driven orientational and rotational
correlations, is the key feature of these architectures.

[1] A.G.
Vanakaras, D.J. Photinos, Chem. Phys. Lett., 341 (2001) 129.

** **

** **

**Atomistic
simulation of smectics**

** **

Matt
Glaser

*Department
of Physics and*

*Ferroelectric
Liquid Crystal Materials Research Center*

*University
of Colorado*

*Boulder,
CO 80304*

Smectic liquid
crystals constitute an extraordinarily rich and diverse family of low-symmetry
fluids, comprising tilted and untilted 2d fluid and hexatic phases,
ferroelectric, antiferroelectric, and ferrielectric phases, twist grain
boundary phases, and a bewildering variety of ‘banana’ phases, including
spontaneously chiral phases of nonchiral molecules.

Atomistic
simulation is a powerful (albeit under-utilized) tool for confronting the
‘self-assembly problem’ in the context of smectics, i.e., for understanding how
chemical structure encodes macroscopic properties and phase behavior. We
describe atomistic simulation studies aimed at exploring the molecular origins
of tilt and clinicity in tilted smectics and explaining the unusual temperature
dependence of layer spacing and tilt in partially fluorinated ‘de Vries’
smectic materials.

** **

** **

**Simulation
of LC polymers and dendrimers**

** **

Mark R.
Wilson

*Department
of Chemistry, University of Durham South Road,*

* Durham DH1 3LE, U. K.*

This paper
describes results from the study of a number of simplified models for liquid
crystalline systems in which molecular flexibility has been taken into account.
These include a liquid crystal dimer molecule, a low-molecular weight mesogen
with two flexible tails, two liquid crystal polymers and a LC dendrimer in a
liquid crystalline solvent.

In the majority of
studies a hybrid model is employed, which combines spherical Lennard-Jones sites, to represent flexible chains
(e.g. alkyl tails, polymer backbones, flexible spacers in polymers and
dendrimers) and anisotropic Gay-Berne sites to describe the mesogenic moieties.
Parallel molecular dynamics simulations are used to study the phase behaviour
of the models, and to study the structure and dynamics of the molecules in the
mesophases that form. In the case of the liquid crystalline dendrimer system,
we also consider a fully atomistic Monte Carlo study for a single dendrimer
molecule, and use this to derive a coarse-grained model that can be used for
the simulation of bulk phases formed by LC dendrimers.

** **

** **

**Liquid-Crystalline
Phase Behavior **

**of a
Colloidal Rod-Plate Mixture**

** **

G.J.
Vroege and H. Lekkerkerker

*Van 't
Hoff Laboratory for Physical and Colloid Chemistry*

*Debye
Research Institute, Utrecht University*

*H.R.
Kruytgebouw, N-704, Padualaan 8, 3584 CH Utrecht*

*The Netherlands*

The phase behavior
of rod-plate mixtures was investigated using model systems containing
unambiguously rod-and plate-shaped colloids. We find that the theoretically disputed biaxial nematic phase
is unstable with respect to demixing into
an isotropic and two uniaxial nematic phases.

The phase behavior
at very high densities is exceptionally rich and includes the coexistence of up
to four different liquid crystalline phases, which stem from the coupling
between the employed particle shapes
and polydispersity.

** **

** **

**LIQUID-VAPOR
AND LIQUID-SOLID COEXISTENCE**

**IN A DLVO
POTENTIAL MODELIZATION OF**

**GLOBULAR
PROTEIN SOLUTIONS**

** **

C.Caccamo
, M.C.Abramo, D.Costa and G.Pellicane,

*Istituto
Nazionale FIsica della Materia and
Dipartimento di Fisica,*

*University
of Messina, Messina, Italy.*

We report Gibbs
Ensemble Monte Carlo calculations of liquid-vapor coexistence lines, and
perturbation theory calculations of the liquid-solid coexistence lines, for a
DLVO model with potential parameters appropriate to describe globular protein
solutions of ionic strength ranging from 0.5 to 1.2 molar concentration of the
added salt. Similar calculations are also reported for a very narrow square-well potential suited to mimic the protein-protein
interaction in the investigated solutions.

The different
model predictions are compared with the available experimental data for the
phase diagram of lysozime in a H2O+NaCl
solution. Preliminar results indicate that the DLVO modelization is at least qualitatively accurate in reproducing
the phase diagram of the system under study.

** **

** **

**Coarse
graining polymers as soft colloids**

** **

A.A. Louis

* Dept of Theoretical Chemistry, Cambridge University *

*Lensfield
Rd, Cambridge CB2 1EW, United Kingdom *

Coarse-graining
methods are crucial to deriving tractable statistical mechanical treatments of
soft matter systems, where a large number of different length and time-scales
may coexist. In this talk I discuss an
explicit example, where polymers are coarse-grained to single “soft colloids”,
interacting with a pair potential. The
resulting large speedups in simulation times allow us to calculate the
phase-diagrams of polymer-colloid mixtures, where we find good agreement with
recent experiments. This
coarse-graining technique also leads to new insights, including a novel
“mean-field fluid” paradigm, and highlights some of the subtleties arising when
density dependent pair potentials are used to model materials.

references

Beware of density
dependent pair potentials, http://www.arxiv.org/abs/cond-mat/0205110

Coarse-graining
polymers as soft colloids, Physica A 306, 251 (2002)

** **

** **

**From basic
interactions to phase diagrams:**

** solutions of rod-like biomolecules**

** **

Hartmut
Löwen

*Institut
für Theoretische Physik II, *

*Heinrich-Heine-Universität
Düsseldorf*

*Universitätsstrasse
1, D-40225 Düsseldorf, Germany*

One fundamental
problem of statistical mechanics is to predict the phase behaviour on the basis
of the microscopic interactions. This is a formidable task, in particular for
many-body systems of rod-like particles which exhibit a variety of liquid
crystalline phases.

Two cases of
charged rod-like biological macromolecules are discussed in detail which lead
to a rich phase behaviour, namely columnar aggregates of DNA and suspensions of
tobacco mosaic virus particles.

** **

** **

**Phase
behavior and structure of model liquid crystals **

**and of
fluids of Janus particles**

** **

Siegfried
Hess

*Institut
f. Theoretische Physik, Techn. Univ. Berlin, PN 7-1,*

*Hardenbergstr.
36, D - 10623 Berlin*

*(S.Hess@physik.tu-berlin.de)*

In the first part
of the lecture, results of anlytical [1] and of Monte Carlo [2] calculations
are presented for the pressure and the phase behavior of a model liquid
crystal. Isotropic, nematic and smectic
phases are studied, structural properties are investigated for the bulk system
and for the fluid in restricted geometries, in particular between flat
orienting walls.

The second part of
the lecture deals with the thermo-physical properties of a model fluid composed
of Janus particles which have two different faces where like ones (different
ones) have an extra attractive (repulsive) interaction. Data from Monte Carlo
simulations for the pressure and the phase behavior are compared with
analytical calculations [3].

1 S. Hess and Bin
Su, Z. Naturforsch. 54a (1999) 559;

2 H. Steuer, M.
Schoen, and S. Hess, to be published;

3 T. Erdmann, M.
Kroeger, and S. Hess, to be published.

** **

** **

**Simulation
and theory **

**of
inhomogeneous liquid crystals**

** **

Mike Allen

*Centre for
Scientific Computing & Department of Physics*

*University
of Warwick, Coventry CV4 7AL, UK*

There are various
theoretical models of liquid crystals: some assume that the free energy simply
depends on gradients of the director field, some incorporate additional
variation of the degree of ordering, and others attempt to write the free
energy as a functional of the single-particle density.

Near defects and
surfaces, significant spatial variations of the degree of ordering occur and
some or all of the assumptions in these theories may break down.

Molecular
simulations are capable of giving detailed structural information in these
situations. This talk will focus on some specific examples: the
nematic-isotropic interface, suspensions of spherical and non-spherical colloidal
particles, and disclination defects in confined geometry.

** **

** **

**Theory and
Simulation Studies of the**

**Elusive
Polar and Biaxial Phases**

** **

G. Jackson

*Department
of Chemical Engineering and Chemical Technology*

*Imperial
College of Science, Technology and Medicine*

*Prince
Consort Road, London SW7 2BY United Kingdom *

A fine balance
between anisotropic repulsive and dispersive forces, and polar interactions is
crucial in determining the phase behaviour of liquid crystal materials. Two areas have attracted a great deal of
interest and controversy in recent years: 1) the existence of polar nematic
(ferroelectric phases) in thermotropic mesogens; and 2) the possibility of
biaxial nematic phases in mixtures of prolate and oblate molecules. In our
theoretical studies [1] of polar fluid phases formed by model systems, the
anisotropic interactions can be treated using the well known Gay-Berne
potential, which exhibits nematic, and smectic liquid crystalline phases, as
well as the more common vapour and isotropic liquid phases. In the first part of the presentation, the effect of
including a central longitudinal point dipole on the phase behavior of a
Gay-Berne model is studied using a simple density functional theory. The
Gay-Berne potential is divided into repulsive and attractive terms according to
the scheme of Weeks, Chandler, and Andersen (WCA) perturbation theory. The
contribution of the repulsive interactions to the free energy is approximated
by the Parsons-Lee theory of the hard Gaussian overlap potential. The attractive term is taken as a
perturbation; in order to keep the expressions as manageable as possible, a
simple but ad. hoc. mean-field approximation is used, which incorporates the
main properties of the first-order perturbation theory. The dipolar part of the
potential is treated at the level of a second-order virial theory. The
depolarization effects at the boundaries of the sample, which arise due to the
long-range nature of the dipolar forces, are avoided by performing the
calculations in an ellipsoidal sample of infinite aspect ratio using the
technique of Groh and Dietrich. The phase coexistence between orientational
disordered and ordered fluid phases is considered in detail. We focus on the
possibility of finding a stable ferroelectric fluid phase which such an
approach. The stability of the different phases is examined by changing the
molecular elongation, the anisotropy parameter of the attractive interaction,
and the strength of the dipole interaction. We compare our results to existing
Monte Carlo simulation data where available. The theory is found to provide a
good quantitative description of the dipolar Gay-Berne model for small values
of the dipole moment. The second part of the presentation will focus on the
stability or otherwise of the biaxial state for simple models of prolate and
oblate molecules. The phase diagram of binary mixture of hard rod and plate
molecules has been investigated by numerical minimization of a free energy
functional of Onsager and Parsons. Both the rods and plates are represented by
cylinders. The effect of various approximations are discussed: the use of the
second Legendre representation of the excluded volume (L2 model); and the
incorporation of the lower order terms of the excluded volume (end effects).
The subtle competition between the orientational entropy, translational
(packing entropy) and the entropy of mixing is very sensitive to the
approximation that is used. At first the mixture is chosen to be symmetric at
the level of the second virial theory (so that the phase behavior of the two
pure components is identical), and the particles are examined in the Onsager
limit (infinite aspect ratios) [2]. The effect of the unlike rod-plate excluded
volumes on the phase behaviour is examined in detail. Different conclusions are
reached about the stability of the biaxial phase depending on the approximation
that is used. In the case of the full solution (Parsons free energy functional
incorporating end effects) [3] we show that
the biaxial nematic is unstable
relative to demixing even for the symmetric mixture of the very long rod and
very flat plates, where the Parsons-Lee theory becomes identical with the
Onsager theory. Only two types of phase transitions are observed:
isotropic-nematic phase coexistence and demixing transition involving either
two isotropic or two nematic phases. The stability of the nematic region is
found to be very sensitive to the aspect ratios: for moderate aspect ratios of
the rods, the destabilization of the nematic phase is observed over the
isotropic phase, while for large aspect ratios the tendency is the opposite.
Our results are in good agreement with the results of simulation studies [4].
Finally, mixtures in which the volume of the plate is orders of magnitude
larger that the volume of the rod are considered, so that an equivalence can be
made where the plates are colloidal particles while the rods play the role of a
depleting agent. A combined analysis of the isotropic-nematic bifurcation
transition and spinodal demixing is carried out to determine the geometrical
requirements for the stabilization of a demixing transition involving two
isotropic phases. Global phase diagrams are presented in which the boundaries
of isotropic phase demixing are indicated as functions of the molecular parameters
[5]. The stability analysis indicates that for certain aspect ratios, the
isotropic-nematic phase equilibria always preempts the demixing of the
isotropic phase, irrespective of the diameters of the particles. When
isotropic-isotropic demixing is found, there is an upper bound at large size
ratios (Asakura and Oosawa limit), and a lower bound at small size ratios
(Onsager limit) beyond which the system exhibits a miscible isotropic phase. It
is very gratifying to find both of these limits within a single theoretical
framework. We draw comparisons between the predicted regions of stability for
isotropic demixing and recent experimental observations.

[1] S. Varga, I.
Szalai, J. Liszi, and G. Jackson, J. Chem. Phys., 116, 9107 (2002).

[2] S. Varga, A.
Galindo, and G. Jackson, Phys. Rev. E, 66, 0117XX (2002).

[3] S. Varga, A.
Galindo, and G. Jackson, in preparation (2002).

[4] A. Galindo, G.
Jackson, and D. J. Photinos, Chem. Phys. Lett., 325, 631 (2000).

[5] S. Varga, A.
Galindo, and G. Jackson, J. Chem. Phys., submitted (2002)

** **

** **

**A Monte
Carlo study of the chiral columnar**

**organisations
of chiral discotic mesogens**

** **

Roberto
Berardi, Marco Cecchini, and Claudio Zannoni,

*Dipartimento
di Chimica Fisica e Inorganica, *

*Università
di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy.*

Chiral columnar
mesophases1-4 have been of particular interest at least since tilted chiral
phases were found to be ferroelectric and switchable.1,2 From the theory and
modelling point of view the relation between a molecular structure with a
certain chirality5 and the type of mesophases formed is certainly not
clear. This is partly due to the
complexity of the molecular structures that can give chiral columns and to the
difficulty in identifying a precise relation between molecular features and
liquid crystal properties. Here we
examine the different types of chiral columns that can be obtained by self
assembly of chiral discotic molecules.

We introduce to
this end a simple two-site chiral model molecule formed by two suitably
oriented interpenetrating Gay–Berne squashed ellipsoids similar to those that
have proved very useful in the modelling of various liquid crystals.6 We then
perform Monte Carlo computer simulations of various sets of these particles for
different chiralities. At low
temperatures we find discotic mesophases formed by overall chiral columns and
we analyse the results in terms of suitably defined observables and chiral
correlation functions.

We observe a
coupling between molecular tilt and twist between pair of molecules within the
same columnar structure. For our model
system, the column chirality is not originating from a regular chiral
arrangement of particles but it seems to be mainly due to one–particle
high–chirality defects separated by low chirality domains.

[1] H. Bock and W.
Helfrich, Liq. Cryst., 1992, 12, 697;
Liq. Cryst., 1995, 18, 707.

[2] G. Sherowsky
and X.H. Chen, J. Mat. Chem., 1995, 5,
417; Liq. Cryst., 1998, 24, 157.

[3] K. Praefcke,
A. Eckert and D. Blunk, Liq. Cryst.,
1997, 2, 113.

[4] J. Barberá, A.
Elduque, R. Giménez, F.J. Lahoz, J.A. López, L.A. Oro and J.L. Serrano,
Inorg. Chem., 1998, 37, 2960.

[5] A. Ferrarini
and P.L. Nordio, J. Chem. Soc. Perkin Trans. 2, 1998, 455.

[6] C. Zannoni, J.
Mater. Chem., 2001, 11, 2637.

** **

** **

**Wetting of
polymer solutions: **

** Monte Carlo Simulations **

**and
Self-Consistent Field theory**

** **

Marcus
Müller

*Institut
für Physik,WA331,Jo.Gutenberg
Universität*

*55099
Mainz, Germany*

We investigate
surface and interface properties of a coarse grained bead-spring polymer model
by grandcanonical Monte Carlo simulations and self-consistent field theory.
Both approaches are compared qualitatively: we find good agreement for the
structure of the polymers at the surface/interface, but only qualitative
agreement for the packing effects of the density at surfaces.

The surface and
interface free energies are measured in the simulations and the wetting
transition is located via the Young equation. The wetting transition is of
strong first order, the drying transition is a weak first order or second order
transition. Alternative methods for locating the wetting transition in the
Monte Carlo simulations are discussed.

The wetting
properties can be tuned by grafting a brush of identical chains on the surface.
The complex wetting behavior as a function of the grafting density and the
attractive strength (Hamaker constant) of the wall are explored by
self-consistent field calculations. The system exhibits first order wetting
transitions at low and high grafting densities and critical wetting transitions
at intermediate overlap of the grafted chains.

** **

** **

**Potential
of mean force in dimeric proteins: **

**an MD
simulation with constraints**

** **

M.
Ferrario

*Dipartimento
di Fisica, Universita' degli Studi di Modena e Reggio Emilia,*

*Via G.
Campi 213/A, 41100 MODENA, Italy.*

Using holonomic
constraints the potential of mean force of a dimeric protein is calculated as a
function of monomer separation. Some aspects of the application of the method of constraints will be
reviewed in the contexts of extended (non-hamiltonian) dynamics and parametric
free energy calculations.

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**Liquid
Crystals at Surfaces**

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F. Schmid

*Fakultät
für Physik, Universität Bielefeld*

*Universitätsstraße
25, D-33615 Bielefeld*

*Germany*

Fluids of soft
ellipsoids are studied by computer simulations in the bulk and in the vicinity
of surfaces. Our work aims at a deeper understanding of the relationship
between the local structure of liquid crystals and mesoscopically or
macroscopically relevant material properties such as elasticity and surface
anchoring.

The talk shall be
divided into two parts.

First, we discuss
the local liquid structure of the bulk and present results for one of the
central characteristic quantities, the direct correlation function (DCF).
Inserting this information into an appropriate density functional, one can
calculate the elastic constants in the nematic phase, and the density and order
parameter profiles at surfaces. Predictions of the theory are compared with
simulation results.

In the second
part, we present computer simulations of a more complex situation: Surface
anchoring on grafted liquid crystalline polymer brushes. The system is designed
such that the orienting force of the substrate competes with that of stretched
chains. We show that the grafting density can be used to tune the anchoring
angle, and discuss the microscopic mechanisms which contribute to the
anchoring.