Abstract
Ferrofluids consist ofmagnetic nanoparticles dispersed in a carrier liquid. Their strong
thermodiffusive behaviour, characterised by the Soret coefficient, coupled with the
dependency of the fluid's parameters on magnetic fields is dealt with in this work. It
is known from former experimental investigations on the one hand that the Soret coefficient
itself is magnetic field dependent and on the other hand that the accuracy of
the coefficient's experimental determination highly depends on the volume concentration
of the fluid. The thermally driven separation of particles and carrier liquid is
carried out with a concentrated ferrofluid (φ = 0.087) in a horizontal thermodiffusion
cell and is compared to equally detected former measurement data. The temperature
gradient (1 K/mm) is applied perpendicular to the separation layer. The magnetic
field is either applied parallel or perpendicular to the temperature difference. For
three different magnetic field strengths (40 kA/m, 100 kA/m, 320 kA/m) the diffusive
separation is detected. It reveals a sign change of the Soret coefficient with rising
field strength for both field directions which stands for a change in the direction of
motion of the particles. This behaviour contradicts former experimental results with
a dilute magnetic fluid, in which a change in the coefficient's sign could only be detected
for the parallel setup. An anisotropic behaviour in the current data is measured
referring to the intensity of the separation being more intense in the perpendicular
position of the magnetic field: ST∥ =-0.152 K-1 and
ST⊥ =-0.257 K-1 at H = 320
kA/m. The ferrofluiddynamics-theory (FFD-theory) describes the thermodiffusive
processes thermodynamically and a numerical simulation of the fluid's separation
depending on the two transport parameters ξ∥ and ξ⊥ used within the FFD-theory
can be implemented. In the case of a parallel aligned magnetic field, the parameter can
be determined to ξ∥ = {2.8; 9.1; 11.2}×10-11 · D∥ kg/(A2m)
for the different field
strengths and in dependence on the magnetic diffusion coefficient D∥. An adequate
fit in the perpendicular case is not possible, by ξ⊥ = 1×10-17 kg/(Am2) a rather
good agreement between numerical and experimental data can be found for a field
strength of 40 kA/m, a change in the coefficient's sign in the perpendicular setup is not
numerically determinable via this theory. The FFD-theory is only partly applicable
to calculate the concentration profile in concentrated magnetic fluids established due
to a temperature gradient and magnetic field applied.
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