Oral
The parallel molecular dynamics program
for the simulation of liquid crystalline molecules
with complex topologies
J.Ilnytskyi, M.R.Wilson
Dept. of
Chemistry, University of Durham,
South Road,
Durham, DH1 3LE, United Kingdom
We describe the
parallel molecular dynamics program GBMOLDD.
This program uses the domain decomposition algorithm and is targeted at
large-scale simulations of molecular systems (particularly polymers and liquid
crystals) composed of both spherically-symmetric and nonspherical sites. The
nonspherical sites can be described either by a Gay-Berne potential or by soft
repulsive spherocylinders. The
molecules can be of arbitrary topology and the intramolecular forces are
described via standard force fields. The applications to the computer
simulations of the liquid-crystalline systems with complex topologies are
discussed.
Oral
Investigations on the interplay of colloids and
nematic liquid crystals under confinement
Evelina B. Kim, Roland Faller, Sylvain Grollau, and
Juan J. de Pablo
Department of
Chemical Engineering, University of Wisconsin-Madison, Madison, WI
It has been shown
experimentally that liquid crystals can be used to amplify surface events such
as the specific adsorption of proteins on the substrate of a liquid crystal
cell. These experiments constitute the
basis for the development of optical sensors that rely on liquid crystal
ordering.
To study such
systems by simulation, we compare the effectiveness of three different
techniques for the calculation of chemical potentials. These are the customary
test particle method proposed by Widom, the recently proposed density of states
Monte Carlo, and the expanded canonical ensemble methods. We apply these in a
system of bulk liquid crystal modeled as Gay-Berne mesogens. The results derived from the three methods
agree. The test particle method, however, fails in the nematic regime.
Further, we
perform Monte Carlo simulations to study the effects of suspended proteins on
the structure of a nematic liquid crystal at a molecular level. Liquid crystals modeled as repulsive
Gay-Berne mesogens interact with the proteins represented as soft repulsive
spheres. We consider the case where all
interfaces exhibit homeotropic anchoring conditions. For small proteins (~5nm), we observe that the particles are
surrounded by a Saturn ring disclination line.
We investigate the effect of the proteins size on the defect
structure. Defect structure from the
simulations serves as a starting point for continuum elastic theory. We compare our results with theoretical
calculations performed for this system.
Finally, we
consider a system where a spherical particle is suspended in a confined nematic
liquid crystal. We calculate the potential of mean force between the sphere and
a substrate mediated by the liquid crystal by means of Monte Carlo simulations.
Three methods are used: canonical Monte Carlo, umbrella sampling, and a novel
technique that combines canonical expanded ensemble simulations with the
recently proposed density of states formalism. The latter method offers clear
advantages over the other ones in that it ensures good sampling of phase space
without prior knowledge of the energy landscape of the system.
The resulting
potential of mean force, computed as a function of the normal distance between
a sphere and a surface, suggests that a sphere is attracted to the surface,
even in the absence of attractive molecular interactions.
As a side note, we
investigate the effect of the interaction potential on the defect
structure. We perform Monte Carlo
simulations of a colloid in a nematic environment under conditions of
confinement. The liquid crystal is modeled in two different ways. In the
attractive case, LC molecules interact with themselves, the sphere, and the
surfaces via a generalized Gay-Berne (GB) potential. The LCs anchor homeotropicaly to all interfaces. The surface potential is only dependent on
the distance from the surface and the orientation with respect to the
plane. In separate simulations we
employ again a purely repulsive potential. Here, the interactions with the
surface and the sphere are simplified further to eliminate the dependence on
the orientation of the LC particle. It
has been shown that such potentials result in homeotropic anchoring of LC. It is extremely difficult to find equivalent
conditions for such dissimilar systems.
Our criterion for matching the conditions includes density profiles in
the bulk-like region of the confined systems.
With a sphere fixed at the center between the surfaces, we found that
Saturn Ring defects have very different structures for the two models. In both cases, a solvation shell is formed
close to the sphere. The order within
this shell extends farther away from the colloid for the attractive
system. Also, the order parameter which
is close to unity at the surface of the colloid decreases strongly towards the
defect core for the attractive system; in the repulsive case this is less
pronounced. A repulsive potential for
all interaction is beneficial in several ways: smaller systems may be simulated
due to missing cutoff restrictions as well as shorter times may be required to
attain equilibrium. Additionally, the results corroborate experiments and
theory closer especially in regards to defect structures. The only anchoring
that can be reached by the applied repulsive potentials, however, is
homeotropic. In contrast, any type of
anchoring is possible with the attractive potential and relative strength of
interactions can be fine tuned.
Oral
Lattice
Boltzman nemato-dynamics
with variable order parameter.
S.V.Lishchuk, C.M.Care, I.Halliday
Materials Research
institute, Sheffield Hallam University,
Pond Street,
Sheffield, S1 1WB, United Kingdom
A lattice
Boltzmann (LB) scheme is described which recovers the equations developed by
Qian-Sheng for the hydrodynamics of a nematic liquid crystal with a tensor
order parameter. The standard LB scalar density is enhanced to a tensor
quantity and the macroscopic momentum, density and tensor order parameter are
recovered from appropriate moments of this mesoscopic density. A single lattice
Boltzmann equation is used with a direction dependent BGK collision term.
Additional forcing terms are used to recover the antisymmetric terms in the
stress tensor. A Chapman Enskog
analysis is presented which demonstrates that the Qian-Sheng scheme is
recovered provided a lattice with sixth order isotropy is used. The method is
validated against analytical results for a number of cases including flow
alignment of the order tensor and the Miesowicz viscosities in the presence of
an aligning magnetic field. It is also hoped to present results from a LB model
of a mixture of a nematic liquid crystal and an isotropic fluid.
Oral
Searching for nano-structures of the
liquid crystalline cubic phase
Makoto Yoneya
Japan Science and
Technology Corporation,
ERATO Yokoyama
Nano-structured Liquid Crystal Project,
5-9-9 Tokodai,
300-2635, Tsukuba, Japan.
Nano-structures of
a thermotropic cubic phase forming liquid crystal molecule, 1,2-bis-[4-n-octyloxy-benzoyl]-hydrazine
(BABH8) were studied by molecular dynamics (MD) simulations. The model cubic
phase structures were proposed for BABH8 system which are constructed from a
locally orientationally ordered “bundle” as a building unit and the cell parameter
and the space group (Ia3d) from the recent X-ray results. Stability of the
model structure was studied by multi-nanosecond MD simulations. Destabilization
of the model structure to the columnar phase and stabilization with enhanced
hydrogen-bonding were obtained in these simulations.
Poster
Multiple Glassy States:
boundaries of the attractive glass.
Christophe Laforge
Department of
chemical engineering, University of
Amsterdam,
Nieuwe Achtergracht,
166, 1018 WV, Amsterdam, The
Netherlands
In this poster, we
will show how it is possible to probe experimentaly a crossover between the
attractive and the repulsive glass in a
monatomic glass-former.
Poster
Insertion of a linear homopolymerin a lamellar phase
of surfactants: a numerical study
with a coarse grain model
Claire LOISON
Fakultät für Physik,
Universität Bielefeld
Postfach 100131, D-33501
Bielefeld, Germany
We study single polymer chains confined between
amphiphilic bilayer by numerical simulations of a coarse grained model. The model contains solvent particles,
amphiphilic molecules, and a single linear homopolymer.
First, we study
the binary system (solvent + amphiphiles). At high concentration of amphiphilic
molecules, the amphiphiles self-assemble into a lamellar phase: a stack of
bilayers. The undulations of the bilayer positions agree with the theoretical prediction for thermally
fluctuating fluid membranes with no surface tension.
This lamellar matrix is then used to confine a
linear homopolymer between two bilayers. The polymer is soluble in the solvent,
but not in the inner part of the bilayers.
Poster
Realistic Monte Carlo simulation of 5CB at a surface.
Abdon Pijpelink
University of
Amsterdam, Department of Chemical Engineering,
Nieuwe Achtergracht 166,
1018 WV, Amsterdam, Netherlands
Many technological
applications of liquid crystals, in particular liquid crystal displays, depend
on orientational effects caused by an alignment layer on a surface. Therefore,
the behaviour of liquid crystals at surfaces has been widely studied. Next to
the technological importance, understanding the behaviour of liquid crystals at
a surface is also of fundament interest.
Experimental
techniques like Scanning Tunnelling Microscopy and Molecular Dynamics
simulations of mono- or bilayers of adsorbed liquid crystal molecules at
graphite or polymer surfaces have provided insight into the organisation of
these molecularly thin layers. These studies are only partially relevant for
the case of bulk liquid in contact with a surface since the effect of the
liquid crystal on top of the layer adjacent to the surface is largely
neglected. The complete interfacial region between surface and bulk has also
been simulated, using coarse-grained models, treating a complete liquid crystal
molecule as a single anisotropic particle. Unfortunately, these simulations
lack the atomistic detail that can be of large influence in the interaction
between a surface and a liquid crystal.
We present Monte
Carlo studies of large systems of 4-n-pentyl-4\'-cyanobiphenyl (5CB) at simple
surfaces. The molecules are modelled realistically using a United Atom (UA)
approach, in which the molecular structure of the real molecule is present.
Both bulk 5CB as well as 5CB at a surface have been simulated, looking at the
structural and orientational ordering of the liquid crystal.
Poster
CALCULATION OF ROTATIONAL VISCOSITY
AND FLEXOELECTRIC CO-EFFICIENTS
OF A NEMATIC LIQUID CRYSTAL
BY ATOMISTIC COMPUTER SIMULATION
D.L. Cheung 1,2 , S.J. Clark 2 , and M.R. Wilson1
1 Dept of
Chemistry, University of Durham, South Road, Durham, DH1 3LE, UK
2 Dept. of
Physics, University of Durham, South Road, Duharm, DH1 3LE, UK
Equilibrium molecular dynamics simulations
have been performed for the nematic phase of the mesogen 4-(
trans-4-n-pentylcyclohexyl)benzonitrile (PCH5). The simulation was performed
using a harmonic force field with sites
representing hydrogen atoms explicitly included. Long range electrostatic
interactions were modelled using an Ewald sum. Simulation data has been
analysed to calculate important material properties; the rotational viscosity
co-efficent, g1, and the flexoelectric co-efficents, es and eb .
g1 has been
calculated using several methods: using the angular velocity correlation
function of the nematic director n [1], the mean-squared displacement of n [1]
and statistical mechanical methods based on the rotational diffusion
co-efficient [2]. The flexoelectric co-efficients were calculated using a
linear response formalism [3] where
es and eb are related to the correlation between the polarisation and
orientational stress. We find good agreement between experimental values and
those calculated from simulation.
[1] S. Sarman, and
D. J. Evans, J. Chem. Phys. (1990) 99, 9021.
[2] A. V.
Zahkarov, A. V. Komolkin, and A. Maliniak,
Phys. Rev. E (1999) 59, 6802.
[3] V. B. Nemtsov,
and M. A. Osipov, Sov. Phys.
Crystallogr. (1986) 31, 125.
Poster
COMPUTER SIMULATIONS OF P-POLY-PHENYLS
WITH INTERMOLECULAR POTENTIALS
FROM AB INITIO CALCULATIONS
I.
Cacelli, G. Cinacchi, G. Prampolini, A. Tani.
Dipartimento di Chimica, Università di PISA,
Via Risorgimento 35, 56126 PISA, Italy
When dealing with
large molecules, as those forming mesophases, their complexity rules out the
possibility of employing “on-the-fly”
methods and makes straightforward calculation at an ab initio level
computationally very demanding.
To circumvent the
problem we have developed an approach based on decomposing the molecule into
relevant fragments, whose interaction potentials can be ab initio calculated
and employed to reconstruct the potential energy surface (PES) of a pair of the
whole molecules.
This resulting PES
can be then fitted on known model potential functions at desired level of
realism. Here we have applied the method to study the polyphenyls series
modelled as strings of quadrupolar Gay-Berne discs.
Poster
Ordering in a fluid of hard spherocylinders:
An entropy-based approach
D. Costa +
F. Saija *, and P. V. Giaquinta +
+ Istituto Nazionale per la Fisica delta Materia
(INFM) and Dipartimento di Fisica
Università degli Studi di Messina, Contrada Papardo,
C.P. 50 - 98166 Messina, Italy
* CNR - Lit. per i Processi Chimico-Fisici, sez.
Messina, Via La Farina 237 - 98123 Messina, Italy
Hard
spherocylinders (cylinders of length L and diameter D capped with two
hemispheres at both ends) provide a suitable model to investigate entropy
driven phase transitions in real colloidal fluids, composed of rigid rodlike
molecules. The phase behaviour of such system, and in particular the onset and
relative stability of different liquid-crystalline mesophases, has been
recently characterized over the whole L/D range [1].
We have performed
extensive Monte Carlo simulations in the range 3 < LjD < 5, and up to L/
D = 20, in order to investigate the confignrational entropy of the system in
the context of the multi-particle correlation expansion originally derived by
Nettleton and Green [2]. The onset of nematic and smectic order has been
analyzed in the one-phase scheme formerly proposed by Giaquinta and Giunta [3].
This scheme is based on the analysis of a quantity, the so-called residual
multi-particle entropy (RMPE), that includes the re-summed contributions of all
correlations involving more than two particles [3]. The vanishing of the RMPE
signals the structural changes which take place in the system.
We have found that
the ordering threshold detected using the zero-RMPE condition systematically
correlates with the corresponding transition point, whatever the nature of the
higher-density phase coexisting with the isotropic fluid. We have then resolved
the translational and angular contributions to the configurational entropy of
the system; as expected, it is the orientational contribution that drives the transition
while the translational contribution plays only a minor role [4].
[1] P. Bolhuis and
D. Frenkel, J. Chem. Phys. 106 666 (1997).
[2] R. E.
Nettleton and M. S. Green, J. Chem. Phys. 29 1365 (1958).
[3] P. V.
Giaquinta and G. Giunta, Physica A 187 145 (1992).
[4] D. Costa, F.
Saija, and P. V. Giaquinta, Chem. Phys. Lett. 282 86 (1998).
Poster
Simulations of liquid crystal hydrodynamics.
Enzo Orlandini
Department of
Physics, University of Padova,
via Marzolo 8, 35121, Padova,
Italy
We present a
lattice Boltzmann algorithm for liquid crystal hydrodynamics. The coupling between the tensor order
parameter and the flow is treated consistently allowing investigation of a wide
range of non-Newtonian flow behavior.
We present results for the effect of hydrodynamics on defect
coalescence; of the appearance of the log-rolling and kayaking states in
Poiseuille flow; and for banding and coexistence of isotropic and nematic
phases under shear. Lattice Boltzmann simulations are used to explore the behavior
of liquid crystals subject to Poiseuille flow. In the nematic regime at low
shear rates we find two possible steady state configurations of the director
field. The selected state depends on
both the shear rate and the history of the sample. For both director configurations there is clear evidence of
shear-thinning, a decrease in the viscosity with increasing shear rate.
Moreover, at very high shear rates or when the order parameter is large, the
system transforms to a log-rolling state with boundary layers that may exhibit
oscillatory behavior. We study the kinetics of the nematic-isotropic transition
in a two-dimensional liquid crystal by using a
lattice Boltzmann scheme that couples the tensor order parameter and the
flow consistently. The time dependences of the correlation function, energy
density, and the number of topological defects are shown to obey dynamic
scaling laws with growth exponents that, within the numerical uncertainties,
agree with the value 1/2 expected from simple dimensional analysis. We find
that these values are not altered by the hydrodynamic flow.
Poster
Bilayered Mesophases From Tapered Molecules.
Roberto
Berardi, Matteo Ricci, and Claudio Zannoni
Dipartimento di Chimica Fisica e Inorganica,
Università di Bologna,
Viale Risorgimento 4, 40136 Bologna, Italy.
The understading
of molecular factors that could lead to a specific mesophase organization, is a
goal of great interest. Here we develop
a simple molecular model [1] for non--centrosymmetric molecules. With suitable parametrisations
of both shape and interaction part of this model potential we were able to
obtain an interesting mesophase behaviour, i.e. a fluid bilayer structure.
We present results
of Monte carlo (MC) computer simulation in the isobaric--isothermal ensemble
(NPT) for single component systems of 8196 tapered molecules with three
interaction--type parameters. We show
how the combined effect of shape and interaction anisotropy could lead to the
desired collective behaviour, and how the interaction anisotropy can play a
central role in both the long and short range.
Poster
Molecular Dynamics simulations
of spherocylinders interacting via induced electric
dipoles
M.Rotunno
(1,2), T.Bellini (1), M.Glaser (2), Y.Lansac (2) , F.Mantegazza (3)
(1) Dipartimento di Chimica e Biochimica Medica and
INFM,
Università di Milano, Milano, Italy
(2) Department of
Physics and Ferroelectric Material Research Center,
University of
Colorado, Boulder Colorado, USA
(3) Dipartimento di Medicina Sperimentale and INFM,
Università di Milano Bicocca, Italy
We present a
computer simulation study of the effects of dipolar interactions on phase
formation and ordering of molecular
fluids.
The dipole-dipole
coupling is often neglected among the driving forces governing the stability
and phase transitions of Nevertheless,
in some speci_c cases it has been found that dipole interactions promote the
large scale organization of condensed matter phases. In particular, several
properties of liquid crystals depend on the molecular dipole strength. While
recent computer simulation have studied the effect of permanent dipoles on the
molecular order, much less attention has been given to the case of dipoles
induced by external fields.
We focus on a
system of soft spherocylinders [1] characterized by anisotropic polarizability,
which, under the effect of an external electric field, interact throught
induced dipole-induced dipole (id-id) coupling. Our model is suitable for
molecular dynamics.
The id-id
interactions are computed by mean of an iterative loop that determines the
dipole’s strength of every molecule as deriving both from the external electric
field and from the field generated by the dipoles carried by the other
molecules.
We performed NVT
simulations for a system of 900 spherocylinders. In the case of aspect ratio
L/D = 5, the well known [2] isotropic-nematic transition for uncharged hard
spherocylinders is reproduced for soft spherocylinders. For large enought field
the transition is replaced by a smooth concentration-dependent nematic order.
We find that for L/D = 5 such a dependence is weakly affected by the id-id
interactions. A di_erent behavior is found for L/D = 2. In this case the
crystal-isotropic transition is replaced by a continuous transition which
depends not only on the strength of the coupling of each dipole with the
external field, but also on the strength of the interactions between the
induced dipoles.
In particular we
find the id-id interactions decrease the overall alignment of the particles
with the external field.
1. M.P.Allen, Phys. Rev. E, 62, 6706
(2000)
2. S.C.McGrother
et al:, J. Chem. Phys. 104, 6755 (1996)
Poster
COMPUTER SIMULATIONS OF DENDRITIC
AND POLYMERIC LIQUID CRYSTAL SYSTEM
Lorna M. Stimson, Jaroslav M. Ilnytskyi and Mark R.
Wilson.
Department of
Chemistry, University of Durham, South Road, Durham DH1 3LE, U. K.
This paper
describes molecular dynamics and Monte Carlo simulation studies of a flexible
liquid crystal polymer and a dendritic liquid crystal system.
The liquid crystal
polymer study involves molecular dynamics simulations in the NpT ensemble of a
siloxane side-chain liquid crystalline polymer. A hybrid model is employed
which combines spherical Lennard-Jones sites, to represent the siloxane polymer
backbone and flexible alkyl spacers in the side-chain, and anisotropic
Gay-Berne sites to describe the mesogenic moieties. 64 separate molecules are
used to represent a polymer melt, which is slowly cooled from the high
temperature isotropic phase. Simulations are conducted with and without an
external magnetic field present. Below a critical field strength the
simulations demonstrate microphase separation of the melt into mesogen
rich/backbone rich regions as the system is cooled, with the mesogen rich
regions showing orientational ordering of the Gay-Berne particles. Above this
critical level of external field these domains line up and we observe a smectic
liquid crystal phase. Results are presented for the orientational order,
anisotropic structure and dynamics of the LC polymer.
The studies of
dendritic liquid crystal system model the first four generations of a
carbosilane dendrimer, which have been investigated using a fully atomistic
model, employing Monte Carlo techniques. The simulations have been carried out
on single molecules using a variable strength mean field potential to represent
the influence of surrounding molecules in a nematic phase. The overall
molecular shape has been monitored through the moment of inertia spheroid. We
find that as a function of mean field strength the molecules change shape from
spherical to rod-like.
A simplified model
of the same LC dendrimer has also been studied in the presence of a soft
repulsive spherocylinder solvent. Here we observe a similar change in the
structure of the dendrimer as the solvent is cooled from the isotropic phase
into nematic and smectic A phases.
Poster
Constant Pressure Molecular Dynamic Simulation Method
for Anisotropic Liquids
K. M. AOKI, M. YONEYA AND H. YOKOYAMA
JST ERATO Yokoyama
Nano-structured Liquid Crystal Project,
TRC 5-9-9 Tokodai,
Tsukuba 300-2635 Japan
By introducing an
anisotropic factor in the cell dynamics of constant pressure molecular dynamics
simulations, we dramatically lessen the artifacts related to cell shapes and
overcomes the difficulties of
simulating anisotropic molecules under hydrostatic pressure. The method
is especially effective for anisotropic liquids, such as smectic liquid
crystals and membranes, however can also be used for crystals and isotropic
liquids as well. Crystal-smectic-nematic phase transitions in systems of soft
spherocylinders are observed.
Poster
MOLECULAR DYNAMICS SIMULATIONS
OF LIQUID CRYSTAL MOLECULES
ON AIR-WATER INTERFACE.
M. YONEYA
JST ERATO Yokoyama
Nano-structured Liquid Crystal Project,
TRC 5-9-9 Tokodai,
Tsukuba 300-2635 Japan
Some
non-amphiphilic thermotropic liquid crystal (LC) molecules have been known as
they can form stable Langmuir monolayers on air-water interface. However,
formation of such a Langmuir monolayer are not well understood yet except some
LC molecules, e.g. cyano-biphenyl derivatives, which have amphiphilic-like
characters. We apply a molecular simulation approach to study this subject with
special attentions on the LC - water interactions. Molecular dynamics (MD) simulations
were done for various terminally alkyl and/or alkoxy substituted azobenzen LC
molecules on air-water interface with using the realistic (LC and water)
molecular models. The simulation results were compared with those of
corresponding amphiphilic modification, i.e. terminal omega-carboxyalkoxy
substituted azobenzen. We found that the interaction energetics were not so
different between the non-a amphiphilic LC and its amphiphilic modification.
Although, increased adhesive (to water layer) energy was confirmed for the
amphiphilic modification, cohesive energy between LC molecules were dominant to
the adhesion in the both amphiphilic and non-amphiphilic LCs for broad range of
area/molecule.
Poster
MOLECULAR DYNAMICS SIMULATION STUDY
OF SPONTANEOUS ENANTIOMERIC RESOLUTION
OF RACEMIC beta-Me-TFMHPOBC.
M. YONEYA
JST ERATO Yokoyama
Nano-structured Liquid Crystal Project,
TRC 5-9-9 Tokodai,
Tsukuba 300-2635 Japan
Spontaneous
optical resolution in a fluid smectic phase, rather than in a crystalline phase,
was first observed for a racemic modification of beta-Me-TFMHPOBC. The bent
conformation at the double stereogenic part of the (R,S)-beta-Me-TFMHPOBC has
been preferentially localized for its fragment molecule by ab initio molecular
orbital (MO) calculations and suggested as the origin of its anti ferroelectric
property. This strongly fixed bent conformation was pointed out as the
important factor for the spontaneous resolution. However, actual relationship
between this conformational characteristic and spontaneous resolution has not
been clarified. In this study, we try to clarify this relationship with
molecular simulation approach. By semi-empirical MO calculations, we found that
the molecular bend for the two enantiomers were opposite directions each other
strictly depending on their (R,S) and (S,R). It means that the local mirror
symmetry at the stereogenic part is magnified to the large molecular bent in
this beta-Me-TFMHPOBC. Comparison with the single stereogenic TFMHPOBC and the
double stereogenic beta-Me-MHPOBC were also done.