Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Donnerstag
DY 46.48 Do 16:00 Poster D
Monte-Carlo Simulations of Structural and Elastic Properties
in a two dimensional Model Colloidal Crystal — •Kerstin
Franzrahe and Peter Nielaba — Lehrstuhl für Theoretische Physik,
Fachbereich Physik, Universität Konstanz, D-78457 Konstanz
With the help of Monte-Carlo Simulations the elastic and structural
properties of a two dimensional hard disk system are studied. Simulations
in the NPT ensemble are done in order to analyse lattice formation
in binary mixtures, while simulations in the NVT ensemble enable us to
calculate the elastic constants of the system from microscopic strain fluctuations
[1]. This method was used to study the influence of impurities
on the elastic properties of a two dimensional hard disk system.
[1] S. Sengupta, P. Nielaba, M. Rao, K. Binder Phys. Rev. E 61,1072
(1999)
DY 46.49 Do 16:00 Poster D
Computer simulation studies of structures and elastic properties
of model colloids — •Peter Henseler and Peter Nielaba
— Lehrstuhl für Theoretische Physik, Fachbereich Physik, Universität
Konstanz, D-78457 Konstanz
We perform classical Monte-Carlo simulations to calculate isothermal
elastic constants of crystal phases of mono-disperse hard-sphere colloids.
The elastic constants are determined from microscopic fluctuations of the
instantaneous local Lagrangian strain ǫij(r,t), measured with respect to
a static reference lattice, and the use of a finite-size scaling theory. This
approach is a generalization of the method of Sengupta et al. [1] to three
dimensions.
We furthermore investigate structures of three dimensional colloidal
mixtures with different composition and diameter ratios.
[1] S. Sengupta, P. Nielaba, M. Rao, K. Binder, Phys. Rev. E 61, 1072
(2000)
DY 46.50 Do 16:00 Poster D
Laserinduziertes Gefrieren und Schmelzen in 3D — •Wolfram
Quester, Wolfram Strepp und Peter Nielaba — Fachbereich
Physik, Universität Konstanz
Seit den achtziger Jahren des vorigen Jahrhunderts werden die
Phänomene des laserinduzierten Gefrierens und Schmelzens in 2D
untersucht. Man versteht darunter zwei Phänomene, bei denen sich ein
flüssiges System von Kolloiden in seiner kristallinen Phase anordnet,
wenn es einem periodischen äußeren Potential ausgesetzt wird. Wird die
Amplitude des äußeren Potentials vergrößert, so geht das System in eine
modulierte Flüssigkeit über.
Es wurde das Auftreten dieser Phänomene in 3D für zwei Wechselwirkungspotentiale
untersucht. Einmal handelt es sich um harte Kugeln,
einmal um Lennard-Jones-Teilchen. Als Indikatoren für den Phasenübergang
dienten hauptsächlich der Strukturfaktor S( � k) und die Paar-
Korrelationsfunktion. Im Lennard-Jones-Fall konnte sehr gut das Gefrieren
beobachtet werden. Allerdings ergeben sich keine Anzeichen für ein
Schmelzen des Systems bei Erhöhung des externen Potentials. Bei den
harten Kugen verläuft das Gefrieren fast identisch wie im Lennard-Jones-
System, allerdings erreicht der Ordnungsparameter ein Maximum, um
dann mit zunehmendem Potential wieder leicht abzufallen. Diese Abnahme
des Ordnungsparameters reicht allerdings nicht aus, um von einem
Schmelzen zu sprechen.
DY 46.51 Do 16:00 Poster D
Structure formation in ultrathin films of soft matter on chemically
(in)homogeneous reactive and non-reactive substrates
— •Uwe Thiele 1 , Michael Bestehorn 2 , Karin John 1 , and
Markus Bär 1 — 1 Max-Planck-Institut für Physik komplexer Systeme,
Nöthnitzer Str.38, 01187 Dresden — 2 Lehrstuhl für Statistische Physik
und Nichtlineare Dynamik, BTU-Cottbus, Erich-Weinert-Strasse 1,
03046 Cottbus
A continuos model for an ultrathin partially wetting liquid film on a
solid substrate is employed to study the structure formation of such a
film on a heterogeneous substrate. We describe (1) film profiles on periodically
patterned substrates, (2) sliding drops on substrates with an
externally imposed wettability gradient and (3) moving droplets on a
reactive substrate.
DY 46.52 Do 16:00 Poster D
Construction of an optical line tweezer for colloidal particles
— •Raquel Chulia Jordan, Hans Joachim Schöpe, and Thomas
Palberg — Johannes Gutenberg-Universität Mainz, Institut für Physik,
Staudingerweg 7, D-55099 Mainz
Line tweezers are a valuable tool for investigations of pair interactions
in colloidal suspensions. We report on the construction of such a device
implementing a beam positioning by an acousto-optic modulator, a set of
counter-propagating beams with λ=532nm and an imaging optics with
depolarized back scattered light. Construction principle, calibration and
performance are discussed in detail.
DY 46.53 Do 16:00 Poster D
Swelling Behaviour of Hydrogels — •Bernward A. Mann 1 ,
Ralf Everaers 2 , Christian Holm 1 , and Kurt Kremer 1 —
1 Max-Planck-Institute für Polymerforschung; Ackermannweg 10;
55128 Mainz; Germany — 2 Max-Planck-Institut für Physik komplexer
Systeme; Nöthnitzerstr. 38; 01187 Dresden; Germany
The equilibrium swelling behaviour of a charged polymer network and
the dependence of its properties on the system parameters is studied in
numerical experiments. The results of these computer simulations is related
to current theoretical models and the scaling laws predict therein.
DY 46.54 Do 16:00 Poster D
The influence of confining disordered materials on the equilibrium
behavior of Stockmayer fluids — •Carsten Spöler and
Sabine H.L. Klapp — Stranski-Laboratorium für Physikalische und
Theoretische Chemie, Sekr. TC 7, Fakultät II, Technische Universität
Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany
The phase behavior of fluids is very sensitive to the nature of intermolecular
interactions as well as to the presence of confining material
on a nanometer scale. In the present work, which is based on integral
equation theory (Reference Hypernetted Chain Approximation) in combination
with the replica method we consider dipolar Stockmayer fluids
exposed to disordered porous materials. The latter is modeled by a) a
hard sphere matrix, b) a hard sphere matrix with additional Lennard-
Jones fluid-matrix attraction c) a dipolar hard sphere matrix. As shown
on this poster, we find for all systems a remarkable shift of the boundaries
of the homogeneous, isotropic high temperature phase relatively
to the bulk[1,2]. Our previous studies, however, require a cumbersome
treatment of the orientational dependence of the dipolar potential. It is
known for bulk Stockmayer fluids that angle-averaged potentials like the
Keesom potential lead to remarkable well predictions of the condensation
transition (as long as the dipole moment remains relatively small).
Here we test the performance of the angle-averaged potential for the confined
systems [e.g. model a)], considering particularly the behavior of the
isothermal compressibility and adsorption isotherms.
[1] C. Spöler and S.H.L. Klapp, J. Chem. Phys. 118, 3628 (2003)
[2] C. Spöler and S.H.L. Klapp, submitted
DY 46.55 Do 16:00 Poster D
Crystallization in one and two component charged colloidal systems
— •Patrick Wette, Hans Joachim Schöpe, and Thomas
Palberg — Institut für Physik der Universität Mainz
We investigated the nucleation kinetics of aqueous suspensions of
Polystyrene spheres in the deionized state using static light scattering
and Avrami theory. Phase behaviour and growth kinetics were determined
in parallel from microscopy. For a mixture of 68nm and 100nm
sized particles the phase diagram shows a pronounced minimum at 20%
68nm particles which is reflected in both crystallization kinetics and elasticity.
DY 46.56 Do 16:00 Poster D
Crystallization in one and two component charged colloidal systems
— •Patrick Wette, Hans Joachim Schöpe, and Thomas
Palberg — Institut für Physik der Universität Mainz
We investigated the nucleation kinetics of aqueous suspensions of
Polystyrene spheres in the deionized state using static light scattering
and Avrami theory. Phase behaviour and growth kinetics were determined
in parallel from microscopy. For a mixture of 68nm and 100nm
sized particles the phase diagram shows a pronounced minimum at 20%
68nm particles which is reflected in both crystallization kinetics and elasticity.