Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Magnetismus Poster: Do., 13:00–15:30 D-P248
Structural changes in cobalt-based ferrofluids under shear stress observed
by small angle neutron scattering
Loredana Mirela Pop 1 , Stefan Odenbach 1 , Martin Kammel 2 , Albrecht
Wiedenmann 2
1 Technische Universität Dresden, 01062 Dresden, Germany – 2 Hahn Meitner Institute,
Glienicker Str. 100, 14109 Berlin, Germany
Long term stable colloidal suspensions of nanometer-sized magnetic particles in appropriate
carrier liquids, ferrofluids show normal liquid behaviour coupled with superparamagnetic
properties. Due to the possibility to change their physical properties
by means of moderate magnetic fields, they can be used for an expanding number of
applications like, for example, as a cooling medium in loudspeakers or as a sealing for
rotary feedthroughs. Additional application fields concerning the use of ferrofluids in
biomedical applications for cancer therapy are subject of actual research activities.
In the presence of magnetic fields ferrofluids show an increase of their viscosity, the
so called magnetoviscous effect, of several hundred percent compared to the viscosity
without magnetic field. Due to a strong shear thinning in commercial ferrofluids, the
viscosity changes diminish for technical useful shear rates to values not suitable for
applications. Therefore, in the last years, a lot of experimental and theoretical studies
have been done in order to explain the microscopic mechanisms of the magnetoviscous
effect. As a result of these efforts, a model, based on numerical and experimental data,
has been established. Chain formation of magnetic particles with strong particle particle
interaction as well as structure destruction by means of shear influence are the
essential processes for the understanding of the magnetoviscous phenomena.
Since until now experimental data proving such a connection between the structure of
ferrofluids and their viscous properties were missing the study of the microstructure
of ferrofluids under shear for different magnetic field strengths and shear rates and
its consequences on the magnetoviscous effect is in the focus of recent research. Using
a specially designed rheometer, rheological as well as small angle neutron scattering
investigations have been performed in the same experimental environment.
The obtained results show a strong connection between structure formation in ferrofluids
and their rheological behaviour. The evidence for a formation of chains and
their deviation in a shear flow validates therefore the model of chain formation as an
explanation for the magnetoviscous effect. Further information, concerning the local
magnetisation of the sample in a shear flow relative to the direction of the chains,
obtained from experiments using polarised neutrons, will be presented. Additionally,
the possibility of combining X-Ray and neutron experiments will be discussed.
Establishing a connection between the changes of the microstructure of ferrofluids,
under the influence of a magnetic field, and their consequences on the macroscopical
behaviour of these systems could allow an optimisation of the fluids. Based on the
knowledge gained from experimental and theoretical studies, new applications can be
developed, making ferrofluids an attractive medium for potential further research.