Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Freitag
DY 51.3 Fr 11:15 H3
Nematic-mediated interaction between colloidal particles with
opposing anchoring: a Derjaguin approach — •Maria Inmaculada
Rodriguez-Ponce 1 , Jose Manuel Romero-Enrique 2 , and
Luis F Rull 3 — 1 Lehrstuhl 5 fuer theoretische Physik, Physik Department,
Technische Universitaet Muenchen (Germany) — 2 Imperial
College of Science, Technology and Medecine. Department of Mathematics
(London, U. K.) — 3 Departamento de Fisica Atomica, Molecular y
Nuclear. Area de Fisica Teorica, Universidad de Sevilla (Sp ain)
We will consider the effect of a nematic solvent on the effective interction
between two colloidal particles which favour homeotropic and
planar anchoring, respectively. Neglecting the effect of the nematic director
field distorsion at large distances, the short-distance effective interaction
between colloidal particles can be related to the hybrid slit
pore solvation force via Derjaguin approximation. We present the rich
phenomenology that the hybrid slitpresents as the anchoring potential
strengths are tuned. This study has been performed in the framework of
the Density Functional Theory.
DY 51.4 Fr 11:30 H3
Phase behavior of a 2D colloidal system on a triangular lattice
— •Andreja Sarlah 1 , Thomas Franosch 1 , and Erwin Frey 1,2 —
1 Hahn-Meitner-Institut, Glienicker Straße 100, D-14109 Berlin, Germany
— 2 Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany
We studied the phase behavior in 2D colloidal systems under the influence
of an external periodic potential created by the interference pattern
of laser beams forming a triangular lattice. In the special case, where the
numbers of colloidal particles and of potential minima are commensurate,
an integer number of particles gathers in each potential minimum. The
colloidal particles within a single potential minimum form a composite
object whose effective degrees of freedom are a few discrete orientational
states. Upon increasing the strength of the external potential, first the
composite objects orient, however, above some higher critical value, the
long-range orientational order is lost. In particular, we are interested in
systems with composite objects of 2 or 3 colloidal particles, i.e., dimers
or trimers, respectively. In these systems, the orientational ordering provides
a realization of 2D discrete spin models. The case of trimers can
be mapped to the spin-1/2 Ising model whereas the system of dimers
is mapped to a spin model characterized by several independent control
parameters leading to a richer phase diagram. Our theoretical results are
in good agreement with the recently reported experimental results. We
predict that by varying the strength of the screened interaction among
colloidal particles at least a part of the phase diagram of a spin model
for dimers should be accessible by experimental studies.
DY 51.5 Fr 11:45 H3
Phase diagram of random copolymers — •Christian Wald 1 , Annette
Zippelius 1 und Ben Vollmayr-Lee 2 — 1 Institut für Theoretische
Physik, Universität Göttingen — 2 Department of Physics, Bucknell
University, Lewisburg, PA 17837, USA
The phase diagram of random copolymers is still controversial: Whereas
Flory-Huggins theory predicts a sequence of separations into many homogeneous
phases [1], Landau theory favors microphase separation into
self-organized microstructures [2]. Simulations disagree with both models
[3].
We show that the Flory-Huggins free energy can be obtained by coarse-
graining the same microscopic model which also yields the Ginzburg-
Landau expansion for a second order transition. The phase diagram is
discussed for both pictures. Within Landau theory we find a sequence
of transitions from a disordered phase to two macroscopic phases and
finally to microphases. In contrast to the literature [1] we also find that
the microphase transition is always of first order and characterized by
a finite length scale. Consequently, a coexistence of three phases should
be observable. The treatment of both theories is extended to include the
effects of compressibility.
[1] A. Nesarikar et al., J. Chem. Phys. 98(9), 7385 (1993)
[2] G. H. Fredrickson et al., Macromol. 25(23), 6341 (1992)
[3] J. Houdayer and M. Müller, Europhys. Lett. 58(5), 660 (2002)
DY 51.6 Fr 12:00 H3
Rheology and Microscopic Topology of Entangled Polymeric
Liquids — •Ralf Everaers 1 , Sathish K. Sukumaran 2 , Gary S.
Grest 3 , Carsten Svaneborg 1 , Arvind Sivasubramanian 2 , and
Kurt Kremer 2 — 1 Max-Planck-Institut f¨r Physik komplexer Systeme,
Nöthnitzerstr. 38, 01187 Dresden, Germany — 2 Max-Planck-Institut für
Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany — 3 Sandia
National Laboratories, Albuquerque, New Mexico 87185, USA
The viscoelastic properties of entangled polymeric liquids are dominated
by topological constraints on a molecular level. These entanglements
confine the motion of individual polymers to a curvilinear diffusion
along the coarse grained chain contours, the reptation tube. The relationship
between the reptation tube and the static structure of the entangled
polymers, however, has remained unclear. Here, we present a “primitive
path” analysis of the microscopic topological state of (bead-spring)
model polymer solutions and melts and show that the predicted plateau
moduli, G 0 N, are in quantitative agreement with the experimentally determined
values for all major classes of synthetic polymers. Our approach
opens the way to a systematic study of structure-property relations and a
direct observation of the major relaxation mechanisms for polymers discussed
in the literature: reptation, constraint release and contour length
fluctuations of the primitive path.
DY 51.7 Fr 12:15 H3
Simulation of Polymer Dynamics in a Mesoscopic Solvent —
•Kiaresch Mussawisade, Marisol Ripoll, Roland G. Winkler,
and Gerhard Gompper — Institut Theorie II, Forschungszentrum
Jülich, D-52425 Jülich
We study the dynamics of a polymer chain in a mesoscopic solvent
by combining Multi-Particle-Collision dynamics (MPCD) and molecular
dynamics simulations (MD). In particular, we are interested in hydrodynamic
interactions obtained by this method and their influence on the
dynamics of the polymer chain. One of the advantages of this method,
compared to other simulation methods, is that the viscosity of the solvent
can easily be tuned by changing the system parameters. We observe
an increased diffusion coefficient of the polymer chain in our simulations
which we attribute to the hydrodynamic interactions. On the other hand,
by considering the relaxation modes of the polymer chain, we find that
the strength of the hydrodynamic interactions depends strongly on the
particular choice of the parameter set so that both Rouse- and Zimm-like
dynamics can be obtained. We have studied chains with and without excluded
volume interactions between the polymer beads. Zimm-like scaling
of the relaxation mode amplitudes is found in both cases at large Schmidt
numbers.