Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Montag
DY 11 Ferrofluids
Zeit: Montag 10:30–12:00 Raum: H3
DY 11.1 Mo 10:30 H3
Investigation of the microstructure of ferrofluids using small
angle neutron scattering (SANS) — •Loredana Pop 1 , Stefan
Odenbach 1 und Albrecht Wiedenmann 2 — 1 ZARM, University of
Bremen, Am Fallturm 28359 Bremen — 2 Hahn-Meitner-Institut Berlin,
Glienicker Str. 100, 14109 Berlin
Experimetal studies made for different ferrofluid samples under shear
flow have shown that increasing the magnetic field strength yields an
increase of the fluids viscosity, the so called magnetoviscous effect, while
increasing shear rate leads to a decrease of the magnitude of the viscosity
(shear thinning). The change of the viscosity with magnetic field strength
is theoretically explained as an effect of chain-like structure formation in
ferrofluids whereas its magnitude depends on the particle-particle interaction.
Both effects, the shear thinning and the magnetoviscous effect, can
therefore be related to the microstructure and microstructure dynamics
of ferrofluids. Using a specially designed rheometer, ferrofluids having
different magnitude of the magnetoviscous effect were investigated by
SANS. Correlated to the structure formation in the fluid, the scattered
intensity shows a variation with the magnetic field and shear rate only
in the case of the fluids with a high magnetoviscous effect. The results
which will be presented show that there is a strong connection between
the rheological behaviour of ferrofluids and their microstructure.
DY 11.2 Mo 10:45 H3
Structure and Viscosities of Ferrofluids in Stationary Shear
Flow — •Patrick Ilg 1 , Martin Kröger 2 , and Siegfried Hess 1 —
1 Institut für Theoretische Physik, TU Berlin, Hardenbergstr. 36, 10623
Berlin — 2 Institut für Polymere, ETH Zürich, Sonneggstr. 3,8092 Zürich
Thanks to their superparamagnetic properties, the viscosities of ferrofluids
are influenced by external magnetic fields - the so-called magnetoviscous
effect [1]. This effect opens various technical as well as medical
applications. In order to better understand the relationship between microscopic
structure and magnetoviscous properties, we consider ferrofluids
subject to a stationary shear flow. The model systems under consideration
are studied both, with the help of mean-field approximations [2,3],
as well as with nonequilibrium many particle simulations. The dependence
on the magnetic field of the mass diffusion, viscosities and normal
stress differences, cluster sizes and structure factors are presented. It is
found, that mean-field approximations lead to satisfactory results within
their range of validity, but cannot be extrapolated straightforwardly. Additional
comparisons of the simulation results with SANS and rheological
experiments on ferrofluids give valuable information on the importance
of magnetic field and shear flow dependent structure formation for the
strength of the magnetoviscous effect.
[1] M. Kröger, P. Ilg, and S. Hess, J. Phys.: Condens. Matter 15 (2003)
S1403-S1423. [2] P. Ilg, M. Kröger, S. Hess, and A. Yu. Zubarev, Phys.
Rev. E 67 (2003) 061401. [3] P. Ilg and S. Hess, to appear in Z. Naturforsch.
2003.
DY 11.3 Mo 11:00 H3
The Hysteresis of the Normal Field Instability — •Reinhard
Richter — Experimentalphysik 5, Univ. Bayreuth, 95440 Bayreuth
Ferrofluids or magnetic liquids are colloidal suspensions of magnetic
nanoparticles. Their behaviour can be controlled by external magnetic
fields. E.g. a horizontally extended layer of magnetic liquid, subjected
to a normally oriented magnetic field forms liquid spikes when a threshold
of the field amplitude is surpassed. This normal field or Rosensweig
instability can be described by a sub critical bifurcation.
We analyse the profile of the liquid spikes and their field dependence
by means of contact radiography near the onset and in the hysteretic regime,
in order to pave the way for a more precise description by amplitude
equations.
DY 11.4 Mo 11:15 H3
Wave number selection and secondary instabilities of ridge patterns
on magnetic fluids — •Rene Friedrichs — ABB AG, Corporate
Research Center Germany, Wallstadter Str. 59, D-68526 Ladenburg
When the free surface of a magnetizable fluid is subjected to a homogeneous
vertical magnetic field the flat surface becomes unstable above
a certain threshold for the field strength. If, additionally, an appropriate
magnetic field oriented tangential to the fluid interface is applied this so
called “Rosensweig instability“ gives rise to parallel fluid ridges [1].
We perform a nonlinear analysis of the static ridge pattern by means
of an energy variational method [2]. The dependence of the wave number
on the external magnetic field and the detailed shape of the corresponding
ripples are theoretically determined. Using a high order expansion
of the energy we investigate the influence both of the wave number and
the permeability of the fluid on the bifurcations of the periodic patterns.
Secondary instabilities of the quasi-one-dimensional surface and the nonlinear
wave number selection are studied by combining a Floquet Ansatz
with the variational approach.
[1] Y. D. Barkov and V. G. Bashtovoi, Magnetohydrodynamics 13, 497
(1977)
[2] R. Friedrichs, Phys. Rev. E 66, 066215 (2002)
DY 11.5 Mo 11:30 H3
Nonlinear oscillations of a torsional pendulum filled with ferrofluid
— •Michael Zaks 1 and Mark Shliomis 2 — 1 Institut für
Physik, Humboldt-Universität Berlin — 2 University of Beer-Sheva, Israel
We consider torsional motions of a sphere filled with ferrofluid and
suspended on an elastic fiber. In presence of the linearly polarized horizontal
magnetic field microscopic rotational motions of magnetic grains
can sum up into a collective motion of the ferrofluid. Dynamics of the
fluid magnetization and the angle of pendulum deflection is governed by
a non-autonomous differential equation of the 4th order. We perform the
detailed bifurcation analysis of this system.
If the frequency of the magnetic field strongly exceeds the pendulum
eigenfrequency, separation of timescales reduces the problem to the 2nd
order equation of the Van-der-Pol type with supercritical or subcritical
onset of oscillations. If two frequencies are of the same order, bifurcation
scenarios are more complicated. In contrast to the conventional picture
of Arnold tongues, presence of physical symmetries causes a hysteresis
near the resonances: coexistence of periodic states which correspond
to “phase-locked” torsional motions with quasiperiodic oscillations. Our
estimates show that these effects can be observed in experiments with
moderate magnetic fields.
DY 11.6 Mo 11:45 H3
The dispersions relation of surface waves on ferrofluids - an
experimental investigation — •Jan Embs — Universität des Saarlandes,
FR 7.3 Technische Physik
The surface of a ferrofluid undergoes a spontaneous instability [1] in a
static magnetic field oriented perpendicular to the surface. The new configuration
consists of�a hexagonal arrangement of peaks, with the critical
wave number kc =
ρg0/σ (ρ mass density, σ surface tension of the
ferrofluid and g0 ). This behavior may be understood in terms of the dispersion
relation ω 2 0(k) of free surface waves: if the magnetization reaches
a certain value, the dispersion relation becomes non-monotonic with a
local minimum and an anomalous dispersion branch; when the minimum
reaches the ω 2 0 = 0-axis at k = kc , the so-called Rosensweig instability
occurs. In my talk I will show how the anomalous dispersion branch can
be measured by generating surface waves due to the Faraday instability.
Furthermore it is possible to prepare experimentally situations in which
both instabilities occur simultaneously; in these bi-critical situations new
and interesting patterns can be observed.
[1] M. D. Cowley, R. E. Rosensweig, J. Fluid. Mech. 30, 671 (1967)