Abstract
Applying a temperature gradient to a layer of a binary fluid establishes a diffusive
transport mechanism called thermophoresis or Soret effect which separates the two
fluid's components and is measured by the Soret coefficient. Recent investigations
carried out on concentrated magnetic fluids showed that the intensity of the Soret effect
depends on the concentration of the nanoparticles transported. The present article,
therefore, deals with the concentration-dependence of the Soret coefficient using five
equally composed magnetic fluids only varying in the concentration of the particles
from 2 vol. % to 10 vol. % of magnetic material. The current investigations point
out that the determination of the Soret coefficient and especially its dependence on
the particles' concentration is based on the determination of the thermal and particle
diffusion coefficient. The article, therefore, presents a theoretical approach for the
determination of the thermal diffusion coefficient and adapts a commonly used Ansatz
for the particle diffusion coefficient for the present case of concentrated magnetic
fluids. It is thereby possible to determine a theoretical Soret coefficient in dependence
on an empirical parameter α. The coefficient is compared with the experimental
approaches which have been previously used, these will be referred to as "analytical
approach" throughout the text. A second comparison is achieved with a hybrid
Soret coefficient which fits the experimentally detected separation curves numerically.
Within the investigations, the hydrodynamic concentration of the particles is used,
assuming a surfactant layer's thickness of 2 nm per magnetic particle which leads
to concentrations between approximately 11 vol. % and 47 vol. %. The diffusion
coefficient ranges from 0.6·10-11 m2s to 2.5·10-11 m2/s
depending on the analytical
model used. The theoretical Soret coefficient decreases with increasing particles'
concentration; the experimental values derived from the analytical approach decrease
from 0.06 K-1 to 0.01 K-1 for increasing particles' concentration. The numerically
determined coefficient ranges from 0.11 K-1 to 0.022 K-1. The experimental values
are smaller than former experimental results suggest (0.16 K-1), which is due to the fact
that in former works, only magnetic concentrations had been considered. All three current
investigations prove what could also be partly seen in former experiments that the
higher the particles' concentration the weaker is thermophoresis. The particle diffusion
coefficient has to be known for the determination of the Soret coefficient. It is carried
out a proof of principle in the article showing that the horizontal thermophoresis cell
can also be used to determine the rehomogenisation process which takes place after
separating the fluid by applying a homogeneous temperature to the fluid. The diffusion
coefficients that could be determined experimentally range from 1·10-11 m2/s to
6·10-11 m2/s.
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