Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Donnerstag
Translation und Rotation von Clustern in Abhängigkeit von Größe, Temperatur
und Substratwechselwirkung, sowie (iii) Veränderung der Partikelgröße
Infolge von Diffusionsprozessen beobachten. Es werden einige
experimentelle Ergebnisse dargestellt sowie Monte-Carlo-Simulationen.
Die Proben wurden durch Beschuss von mit amorpher C-Folie bespannten
Cu-Netzen mit Au-Clustern präpariert, die in einer Laser-
Ablationsapparatur erzeugt wurden.
DY 46.73 Do 16:00 Poster D
Simulation of biaxial compression of two-dimensional granular
materials — •Hans-Georg Matuttis and Shinichi Morita —
The University of Electrocommunications, Department of Mechanical
and Control Engineering, Chofu Chofugaoka 1-5-1,Tokyo 182-8585,Japan
For experimental triaxial compression of granular materials, the maximum
of the yield curve corresponds to the maximum of the bulk density.
After the failure sets in, the granular compressibility decreases, as does
the bulk density.
In this work, we simulate a two-dimensional granular assembly of particles
of various sizes and shapes (round, polygonal, elliptic, with various
elongations) via molecular dynamics, hence the process corresponding to
experimental tri-axial compression becomes bi-axial compression. For the
given boundary conditions and size dispersions, only elongated particles
with friction show the maximum of the yield curve, and a maximum of
the bulk density. For round particles, we find neither a maximum in the
yield curve nor in the bulk density.
DY 46.74 Do 16:00 Poster D
Particle imaging velocimetry study of a horizontally shaken
granular bed — •Olaf Br”okmann and Achim Kittel — Energy
and Semiconductor Research Group, Department of Physics, University
of Oldenburg
The study of shaken granular matter has become very popular in recent
years. Surprisingly most of these studies only investigate the behavior of
vertical shaken particles. Thinking about transportation of grain e.g. horizontal
shaking has a great practical importance and therefore has to be
more investigated. Using particle image velocimetry (PIV) we studied
the velocity fields of horizontal shaken granular matter. The experiments
include a variation of the shaking frequency, shaking amplitude and the
size of the investigated particles.
DY 46.75 Do 16:00 Poster D
THE MAGNETIC SORET EFFECT — •Thomas Völker und
Stefan Odenbach — ZARM, University of Bremen, Am Fallturm
28359 Bremen
Investigations were made to determine the Soret coefficient of magnetic
particles in a ferrofluid under the influence of a magnetic field. This
so called magnetic Soret effect was theoretically determined to be two
to three orders of magnitude smaller than the conventional Soret effect.
In contrast former experiments have shown that the magnetic Soret effect
is much higher than the theoretical predictions. However in those
experiments the influence of buoyancy and magnetic driven convection
could not be excluded. So the question how large the magnetic Soret
effect can be is still open. Thus an experimental setup was developed to
minimize parasitic effects and thus to simplify the analysis of the experimental
results. These results show that the magnetic Soret effect can
even be higher than the conventional one and confirm therewith former
experimental results.
DY 46.76 Do 16:00 Poster D
Examination of the magnetisation dynamics of rotating ferrofluids
— •Sonia May 1,2,3 , Jan Embs 1,2 , Christian Wagner 1 , Klaus
Knorr 1 , and Manfred Lücke 2 — 1 Technische Physik, Universität
des Saarlandes — 2 Theoretische Physik, Universität des Saarlandes —
3 Department of Physics, University of Hull
Ferrofluids are colloidal suspensions of nanometre sized magnetic particles
in a magnetically inert liquid which, being coated by polymeric
surfactants do not agglomerate. External magnetic fields exert forces and
torques on the particles that are transmitted via viscous friction on the
ferrofluid as a whole. In equilibrium ferrofluids show (super) paramagnetic
behaviour in a magnetic field H with magnetisation M and field H
being parallel to each other.
Under non-equilibrium conditions where the ferrofluid rotates M and H
are no longer parallel to each other. For such situations various theoretical
models for the dynamic magnetisation equation have been proposed
that are not yet properly tested; in particular in regimes where the (lo-
cal) vorticity Ω is comparable to or larger than the Brownian rotational
relaxation rate 1/τ.
Experiments have been performed on ferrofluids rotating with constant
frequency Ω. The magnetisation component normal to the applied field
is measured using a Hall sensor as a function of applied fields of up to 60
kA/m and as a function of rotational frequencies of up to 2,500 rad/s.
Experimental results for the non-equilibrium situation are compared
with theoretical models.
DY 46.77 Do 16:00 Poster D
Bewegung von Domänenwänden — •Matthias Müller und Ingo
Rehberg — Physikalisches Institut, Universität Bayreuth
Bei Elektrokonvektionsexperimenten in nematischen planar orientierten
Flüssigkristallen treten unter anderem Domänen mit abnormalen
Rollen auf. Diese lassen sich mittels Ellipsometrie sichtbar machen. Wir
untersuchen die Bewegung der Domänenwände, die die zwei Sorten der
abnormalen Rollen trennen.
Theoretische Berechnungen [1] sagen einen linearen Zusammenhang
zwischen der Geschwindigkeit der Domänenwände und der treibenden
Wechselspannung voraus, was für Bewegungsinstabilitäten ein ungewöhnliches
Verhalten darstellt. Das Ziel unserer Arbeit ist es diesen
Zusammenhang experimentell zu untersuchen.
[1] H. Zhao und L. Kramer, Phys. Rev. E 62, 5092-5100 (2000)
DY 46.78 Do 16:00 Poster D
Growth rate of amplitudes of surface instabilities in ferrofluids
— •Holger Knieling, Reinhard Richter, and Ingo Rehberg —
Universität Bayreuth, Lehrstuhl Experimentalphysik V, 95440 Bayreuth
The surface of magnetic fluids subjected to a normal magnetic field is
becoming unstable when a certain threshold of the magnetic induction
is surpassed and the initially flat surface exhibits a stationary array of
peaks (Rosensweig or normal field instability). The more general case is
a field, which is tilted towards the normal of the surface of the magnetic
liquid. In that case you can observe at first liquid ridges instead of the
peaks. Increasing further the magnetic induction a stretched hexagonal
pattern of crests is emerging on top of the liquid ridges.
The amplitudes of all these instabilities and their relaxation can be
measured with the help of a linear array of 32 Hall sensors which
are placed directly under the ferrofluid. Sensors situated under a ridge
(trough) detect higher (lower) values of the local magnetic induction.
With a time resolution of the sensors of about a half millisecond the
maximal growth rate of the modes of the Rosensweig instability can be
measured and compared to a theoretical prediction [1].
[1] A. Lange, Europhys. Lett., 55 (3), 327-333 (2001)
DY 46.79 Do 16:00 Poster D
Differences of the magnetic susceptibility in alternating and rotating
fields — •Robert Krauß, Reinhard Richter, and Ingo
Rehberg — Experimentalphysik V, 95440 Bayreuth
Considering a layer of ferrofluid set in motion by a rotating magnetic
field raises the question how the velocity depends on the frequency of the
field. Investigations and calculations on this effect (referred to as magnetic
pumping) yield that the velocity is a function of the real and imaginary
parts of the complex magnetic susceptibility. In a first approach
the susceptibility was measured in a toroidal channel with a linear alternating
field. We found substantial differences between the measured
velocity and the one calculated from the complex susceptibility. However
taking into account the complex susceptibility measured in a rotating
field, leads to a better understanding of the fluid motion. The ferrofluid
applied shows a strong coupling parameter of λ ≈ 3. This fact indicates
that aggregates in the fluid exist and are responsible for the non-linear
behaviour obtained.
DY 46.80 Do 16:00 Poster D
A phenomenological model for ferrofluid droplet dynamics in
a shear flow — •Nana Sadowsky, Patrick Ilg, and Siegfried
Hess — Institut für Theoretische Physik, Technische Universität Berlin,
Hardenbergstr. 36, D-10623 Berlin
Hydrostatics and hydrodynamics of ferrofluid droplets give valuable
information on magnetic and dynamical properties of the magnetizable
fluid [1].
While the equilibrium deformation of the ferrofluid droplet as a function
of the applied magnetic field is known experimentally [1], the nonequilibrium
dynamics remain rather unknown so far. Here, we propose
a phenomenological model for the dynamics of ferrofluid droplets in the