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@PHDTHESIS{Naumann:154743,
author = {Naumann, Philipp},
title = {{I}nvestigation of the thermodiffusion behavior of complex
fluids and development of new methods},
school = {HHU Düsseldorf},
type = {Dr.},
address = {Düsseldorf},
publisher = {Universitätsbibliothek Düsseldorf},
reportid = {FZJ-2014-04023},
pages = {104 p.},
year = {2014},
note = {HHU Düsseldorf, Diss., 2014},
abstract = {Thermodiffusion or thermophoresis occurs when a fluid
mixture is exposed to a temperature gradient, leading to
partial separation of the different components. It is still
an unresolved non-equilibrium problem in physical chemistry.
Thermodiffusion can be used for large scale separation and
polymer characterization and furthermore it has been related
to the origin of life occurring at deep sea regions with
strong temperature gradients caused by hot vents or other
volcano activities. The basic parameter is referred to as
the Soret coeffcient which is defined as the ratio of the
thermal diffusion coeffcient DT and the mass diffusion
coefficientD . Despite a long time of research there is
still a lack of a microscopic understanding of
thermodiffusion. In order to study the origin of
thermodiffusion, it is required to use well suited model
systems and reliable methods. The model systems need to be
accessible with theoretical concepts and show simplified
geometries such as spheres. Additionally the model system
must allow a systematic variation of properties such as
size, mass or charge. The requirements for experimental
methods are a small sample volume to be able to use rare
materials and a well shaped experimental geometry to be
accessible by theoretical models, for example well
characterized walls, edges and interfaces. Furthermore
measurements need to be contact free in order to minimize
artificial distortions like the sample-removal geometry
through an outlet. This work contains both aspects, the
development of a new experimental method and the systematic
investigation of microemulsion droplets, which can be
regarded as a tunable colloidal model system.A well suited
experimental method employs the classical so-called
thermogravitational columns (TGs), which were one of the
first devices using thermodiffusion for separation, and
which rely on sample extraction and additional measurements
to determine the concentration and thereby the
thermodiffusion properties. One main aspect of this work was
the development of a classical TG combined with an optical
detection method which requires only very small sample
volumes. This project was accomplished in cooperation with
the group of M. M. Bou-Ali at the Mondragon-University. The
thermogravitational micro column (m-TG) was constructed with
a small sample volume of 50 mL for investigation of very
rare or expensive samples such as biological samples. The
dimensions are chosen to achieve a parabolic, laminar flow
field inside the column, which is required for theoretical
modeling. We chose a gap width of only around 500 mm and a
height of 3 cm. The m-TG is operated contact free by using
an interferometrical detection method to determine the
concentration differences at two different heights of the
column. This optical method allows sensitive and time
resolved measurements of the concentration difference.
Although, the analysis of the time dependent concentration
profile was not yet possible with existing theoretical
models which assume infinite short rising times of the
temperature gradient. We used an active phase tracking
procedure using a piezo actuator at one of the mirrors,
which changes themirror position, leading to the phase
difference. This robust method is independent of the
intensity and contrast fluctuations. The m-TG has been
validated by measuring three binary benchmark mixtures and
by investigating the mixture of toluene and n-hexane. The
obtained results agree within $5\%$ with literature results.
Additionally, measurements of a microemulsion system were
performed, which allow a systematicinvestigation of the
thermal diffusion behavior as function of the microemulsion
droplet size and their interfacial tension. The size
dependence of the thermodiffusion is controversially
discussed. Theoretical studies propose the Soret coefficient
a linear, quadratic and power laws of higher order.
Experimentally a linear and quadratic size dependence of the
Soret coefficient for hard and soft colloids has been found.
Two different studies on the same sample system led to a
linear and a quadratic dependence. One reason for this
discrepancy might be the surface properties of the studied
colloidal systems. Although hard spheres with different
sizes can be ynthesized, the grafting density or charge may
differ substantially. A phase transition temperature of
colloidal systems stemming from different batches differ
often by several 10 K. To overcome this drawback, we have
hosen microemulsion droplets as model system, which can be
tuned in size, shape and their interfacial tension over a
wide range. They consist of a polar liquid such as water, a
non-polar liquid such as an oil and a surfactant. By
adjusting the appropriate concentration and temperature, a
microemulsion (mE) forms discrete water/oil or oil/water
aggregates, which are thermodynamically stable and can be
used as a colloidal model system. In this study we avoid
complications due to surface charge effects and use a non
ionic surfactant.We chose a system of water, n-alkanes and
the non-ionic surfactant C12E5 at the water-rich side of the
phase diagram. All measurements have been performed in the
one-phase region between the upper near critical boundary
(ncb) and the lower emulsification failure boundary (efb).
The droplets have been characterized by Dynamic Light
Scattering (DLS) and Small Angle Neutron Scattering (SANS).
The results show that the shape varies strongly with
temperature, from network like (close to the ncb) to
elongated to spherical droplets at lower temperature close
to the efb. The thermal diffusion behavior of the
mE-droplets hasbeen investigated by the Infrared Thermal
Diffusion Forced Rayleigh Scattering technique. With this
method we obtain the mass diffusion coefficient D, the
thermal diffusion coefficientDT and the Soret coefficient
§T. In a first study we used only n-decane as oil and
investigated the behavior along three different paths. We
varied the temperature between the ncb and the efb and could
relate the results to the influence of the shape of the
droplets. Close to the efb the results for low temperatures
and thus spherical droplets are compared for different
volume fractions of droplets and different oil content and
thereby size. We found only a very small influence of the
volume fraction below $10\%$ on the thermodiffusion
coefficients. The comparison of different sizes showed an
increase of the Soret coefficient with radius but the
measurement range was too small to differentiate between a
linear and quadratic behavior. Additionally we compared our
results with a model of Parola and Piazza, who proposed that
the Soret coefficient depends linearly on the radius and on
the temperature derivative of the product of the interfacial
tension and some characteristic length l. This length is a
measure for thickness of the interfacial layer, which is
influenced by the temperature gradient. The interfacial
tension measurements were conducted by the group of Strey at
the University of Cologne. The results of the
thermodiffusion measurements and the calculated values show
fairly good agreement for spherical droplets. The drawback
of this study was a shift in the efb temperature accompanied
by increasing the oil content and thereby the size. To
access a wider parameter space for size and interfacial
tension, we used different n-alkanes, so it was possible to
measure different droplet sizes at the same temperature.
Also, as before the micro emulsions were characterized by
DLS and SANS measurements to obtain thestructure and size
close to the efb. The Soret coefficient increases linearly
with the droplet radius for spherical particles. Furthermore
we were able to extend the investigation of the
thermodiffusion behavior in relation with interfacial
tension. We determined the characteristic length l between 1
and 2 A. Although we used different oils in our studies, we
assumed that the droplets core does not influence the
thermodiffusion behavior but only the shell and the
interface interact with the surrounding. We found that for
our sample system, the interfacial tension dependence is
dominated by the particle size due to a small derivative
ofthe interfacial tension by temperature, which was proposed
in the theory. Thereby we gavea first insight into this
model applied to soft colloidal particles.},
keywords = {Dissertation (GND)},
cin = {ICS-3},
cid = {I:(DE-Juel1)ICS-3-20110106},
pnm = {451 - Soft Matter Composites (POF2-451)},
pid = {G:(DE-HGF)POF2-451},
typ = {PUB:(DE-HGF)11},
url = {https://juser.fz-juelich.de/record/154743},
}
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