Abstract
Ferrofluids are colloidal suspensions consisting of magnetic nanoparticles dispersed
in a carrier liquid. Their thermodiffusive behaviour is rather strong compared to
molecular binary mixtures, leading to a Soret coefficient (ST) of 0.16 K-1. Former
experiments with dilute magnetic fluids have been done with thermogravitational
columns or horizontal thermodiffusion cells by different research groups.Considering
the horizontal thermodiffusion cell, a former analytical approach has been used
to solve the phenomenological diffusion equation in one dimension assuming a
constant concentration gradient over the cell's height. The current experimental work
is based on the horizontal separation cell and emphasises the comparison of the
concentration development in different concentrated magnetic fluids and at different
temperature gradients. The ferrofluid investigated is the kerosene-based EMG905
(Ferrotec) to be compared with the APG513A (Ferrotec), both containing magnetite
nanoparticles. The experiments prove that the separation process linearly depends
on the temperature gradient and that a constant concentration gradient develops in
the setup due to the separation. Analytical one dimensional and numerical three
dimensional approaches to solve the diffusion equation are derived to be compared
with the solution used so far for dilute fluids to see if formerly made assumptions
also hold for higher concentrated fluids. Both, the analytical and numerical solutions,
either in a phenomenological or a thermodynamic description, are able to reproduce
the separation signal gained from the experiments. The Soret coefficient can then be
determined to 0.184 K-1 in the analytical case and 0.29 K-1 in the numerical case.
Former theoretical approaches for dilute magnetic fluids underestimate the strength
of the separation in the case of a concentrated ferrofluid.
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