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By: Ph.D. Mohebbi Dr Behzad (Author)
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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.
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