Electro-hydrodynamic investigations of fluids in complex systems by NMR mapping experiments and computer simulations [Elektronische Ressource] / vorgelegt von Bogdan Buhai

Buhai

Sujets

• Physik

Informations

• Publié par: universitat_ulm
• Publié le: 01 janvier 2006
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 8 Mo

Extrait

Universität Ulm
Sektion Kernresonanzspektroskopie




Electro-hydrodynamic investigations of fluids in complex systems
by NMR mapping experiments and computer simulations



DISSERTATION
zur Erlangung des Doktorgrades Dr. rer. nat.
der Fakultät Naturwissenschaften
der Universität Ulm



vorgelegt von
Bogdan Buhai
aus Dej, Rumänien

Ulm, 2006


















Amtierender Dekan: Prof. Dr. Klaus-Dieter Spindler
Erstgutachter: Prof. Dr. Rainer Kimmich
Zweitgutachter: Prof. Dr. Harmut Jex
Tag der Promotion:

Electro-hydrodynamic investigations of fluids in complex systems
by NMR mapping experiments and computer simulations


Abstract


Nuclear Magnetic Resonance (NMR) techniques offer valuable insights into the
transport phenomena in complex systems like porous media. This cumulative thesis presents a
number of experimental NMR mapping techniques for retrieving and recording valuable
information about transport quantities in simple and complex geometries. NMR investigation
protocols have been developed for velocity mapping, acceleration mapping, ionic current
density mapping and electro-hydrodynamic mapping. Complementary information about the
same objects can be obtained in most situations with the aid of Computational Fluid
Dynamics (CFD) simulations. This procedure allowed us a direct comparison between the
experimental and simulated data. The simple objects presented here are to be seen as test
devices for microfluidics, and the complex percolation structures as model objects for real
porous materials.
The acceleration mapping technique developed here allows the direct measurement of
spatial acceleration distributions in simple and complex test objects. Experimentally recorded
acceleration maps highlighted the tortuous pathway of the fluid in site-correlated percolation
objects.
The correlated site-percolation models presented here try to mimic the high
connectivity of the real porous materials, as they occur in nature. Well established NMR
investigation protocols and CFD simulations permitted us to characterize the test objects only
from the geometrical (percolation threshold, correlation length, fractal dimension) but also
from the dynamic point of view.
Transport phenomena under combined action of pressure and electrical field gradients
are of interest for many microfluidics applications. Electroosmotic flow patterns are strongly
modified by hydrodynamic pressure gradients and ionic current density distributions built up
in the pore system. The appearance of closed loops and vortices was detected. Electro-osmotic
flow and current density mapping reveal dissimilarity in the Helmholtz/Smoluchovski
equation for such complex systems. All NMR experiments mentioned above (velocity,
acceleration, electro-osmosis and current density mapping) can be performed independently of each other but in the same model object offering a broad range of information on the
existing transport phenomena. Contents


1 Introduction ...................................................................................................................... 2
2 Theoretical Background .................................................................................................. 3
2.1 Site-Percolation models............................................................................................ 3
2.2 Pressure driven flow................................................................................................. 5
2.3 Electroosmotic flow.................................................................................................. 6
3 Experimental and Computational Fluid Dynamics methods....................................... 8
3.1 Fabrication of test objects........................................................................................ 8
3.2 Computational Fluid Dynamics (CFD) .................................................................. 8
3.3 Nuclear Magnetic Resonance Mapping................................................................ 12
4 Results ............................................................................................................................. 15
4.1 Mapping of the higher orders of fluid motion – acceleration mapping ............ 15
4.2 Ising-Correlated Percolation Models ................................................................... 17
4.3 Dissimilar behaviour of electro-osmotic flow and ionic current........................ 21
5 Discussion and Conclusions........................................................................................... 24
6 Literature ........................................................................................................................ 27
7 Zusammenfassung.......................................................................................................... 29
8 Original papers for cumulative thesis .......................................................................... 30









1 Introduction

Transport phenomena in microsystem devices are of general interest in many applied
fields like protein separation in chromatographic analysis [1], mixing of fluids in narrow
channels and capillaries [2] or efficient pumping of fluids at large flow rates in micron scale
structures [3]. Other applications can be found extensively described in Ref. [4].
The cumulative thesis presented here deals with the investigation of transport
phenomena of liquids and electrolytes under hydrodynamic pressure and electrical field
gradients in different test objects. The test objects can be seen as microsystem devices as they
are used in many microfluidics applications, where different transport phenomena act alone or
in a combined way. For example, we have used a simple test structure composed of
semicircles connected in series and parallel, to prove that higher order motions like
acceleration, can be directly recorded [I]. On the other hand, simple structures often do not
model complex systems like real porous materials. Therefore we chose site-percolation
models as paradigms for porous materials. These sorts of systems are characterized by a good
mathematical description and reproducibility. As method of investigation in such complex
systems, we performed experiments based exclusively on Nuclear Magnetic Resonance
(NMR) mapping techniques and, wherever possible, simulations consisting of Computational
Fluid Dynamics (CFD) protocols.
The main research directions focus on 3 topics: developing new NMR techniques to
access supplementary flow parameters (acceleration [I]), constructing and investigating Ising
correlated structures as phantoms of real porous materials [II] and comparing combined
electro-hydrodynamic phenomena (electroosmosis, current density, pressure driven flow) in
polar and non-polar matrices [III], [IV].
Under hydrodynamic pressure gradients the distributions of the velocity and
acceleration vectors in simple structures consisting of semicircles connected in series and
parallel were recorded. A new NMR mapping technique was developed for recording such
acceleration distribution maps. Once the technique was established to be reliable, the same
procedures were applied in a site-percolation model highlighting especially the positions were
the fluid is strongly accelerated or decelerated [I].
It was observed that natural porous materials sustain the transport of fluids even at
very low porosities. Purely random site-percolation clusters cannot describe flow at such low
porosities. That is why the Ising correlated percolation cluster mimic better the structure of
real porous material, a result which was confirmed through CFD simulations and NMR
2