Single-sided NMR in the context of experimental and numerical investigation of Newtonian and Non-Newtonian fluids flow in porous media

A major application area of Nuclear
Magnetic Resonance (NMR) is the study of porous materials which started
in the early days of NMR when Torrey investigated the complex diffusion
dynamics of fluids inside porous media. Since then, NMR relaxation has
been explored extensively in studying porous materials such as cement
and subsurface-rock formations. However, fibrous substrates and in
particular thin fibrous materials have received much less attention.
Such materials are employed in a wide range of products such as
fuel-cells, paper, filters, fluid absorbents and barrier materials,
textiles, diapers, and pads. In this thesis single-sided NMR has been
used to study the interaction of Newtonian and non-Newtonian fluids with
the fibrous porous materials. Furthermore, the experimental results are
used to validate the numerical
models that simulate fluid flow in these materials.
In the first step, single-sided NMR with the NMR-MOUSE®
is employed for quantification of the fluid amount. Using 2 mM/l of a
Gd3+ relaxation agent the repetition time could be shortened to 250 ms,
improving the correlation coefficient between liquid amount and signal
amplitude from R2 = 0.893 to R2 = 0.982 for different liquid and porous
materials. To assess reproducibility and instrument precision,
calibration experiments were repeated several times and their variation
was investigated. The results showed that the device is
highly precise and robust with a standard deviation for liquid
quantification of less than 1%. In the second step, the NMR-MOUSE® is
used for dynamic measurements of drainage processes through thin porous
layers. The experimental results from NMR were used to improve the
continuum-scale modeling of liquid flow from the top to the bottom
layer. Thin fibrous porous materials are mostly used in stacks of
layers, each layer having a defined functionality. Since only a few
pores exist across a layer a couple of hundred
microns thick, the interface between layers may
significantly affect the liquid ingress. The Fourier NMR-MOUSE® device
with a low static gradient was used to profile a 2-mm thick slice in one
shot. The liquid ingress into the thin fibrous layers and their
interfaces was visualized by Fouriertransforming the NMR signal and
processing the time-dependent 1D profiles with a newly developed
mathematical method. The results show major differences in distributions
and flow dynamics for the single and dual layer cases, which reveal the
importance of the interface in fluid flow.
Moreover, a new device called Multivariable NMR
Acquisition System (MNAS) has been developed and employed to study the
liquid flow inside multi-layered swelling absorbent articles such as
diapers. The experimental procedure which consist of a dynamic
measurement and profiling measurement, was utilized to investigate the
performance of four different absorbent
products. The information obtained from these
measurements could be used in developing new products. The flow of
non-Newtonian liquids inside fibrous porous media was studied
numerically as well as experimentally. It was shown that the
single-sided NMR is a powerful tool to study the flow and distribution
of non-Newtonian liquids in thin fibrous porous media.