Quantitative stray-field T1 relaxometry with the matrix pencil method.

The matrix pencil method (MPM) is tested as an approach to quantitatively process multiexponential low-field nuclear magnetic resonance T1 relaxometry data. The data is obtained by measuring T1 saturation recovery curves in the highly inhomogeneous magnetic field of a stray-field sensor. 0.9% brine solutions, doped with different concentrations of a Gd3+ containing contrast agent, serve as test liquids. Relaxation-times as a function of contrast-agent concentration along with the T1 relaxation curves for combinations of multiple different test liquids are measured, and the results from processing using MPM as well as inverse Laplace transformation as a benchmark are compared. The relaxation-time resolution limits of both procedures are probed by gradually reducing the difference between the relaxation-times of two liquids measured simultaneously. The sensitivity to quantify the relative contribution of each component to the magnetization build-up curve is explored by changing their volume ratio. Furthermore, the potential to resolve systems with more than two components is tested. For the systems under test, MPM shows superior performance in separating two or three relaxation components, respectively and effectively quantifying the time constants.

The microstructure of Carbopol in water under static and flow conditions and its effect on the yield stress.

In this work, experimental observations of the microstructure of neutralized polyacrylic acid (Carbopol) in water by confocal microscopy under both static and flow conditions are presented. In the former case, a Carbopol-rich phase made by swollen particles dispersed in a water-rich continuous phase is found, so that the system will be henceforth referred to as a suspension, as long as particles are observed. The swollen particles form dendritic-like aggregates, which span the entire solution volume above a critical concentration. In such conditions, a percolated network can be formed, leading to the onset of a yield stress behavior. By separating the dispersed and continuous phase through centrifugation, we provide evidence of a miscibility gap in the phase behavior of Carbopol in water. When the Carbopol suspensions flow in a microfluidic capillary, a particle-concentrated plug core can be distinguished from a less concentrated layer corresponding to a steep velocity decrease. Confocal imaging also shows that the apparent slip found in Carbopol suspensions is due to a particle-concentrated near-wall region, where no flow is observed. Such flow-induced microstructure is responsible for the different nature of the yield stress values measured by classical rheometry and by flow velocimetry. While the yield stress measured by the former can be here related to the presence of a percolated network, the yield stress obtained from the velocity profile is due to the heterogeneous particle distribution along the capillary radius. These results provide a novel insight on the mechanisms governing yield stress in complex fluids.

Fast and robust quantification of liquid inside thin fibrous porous materials with single-sided NMR.

Single-sided NMR with the NMR-MOUSE is employed for the characterization of fluids in fibrous and open foam materials. One of the key aspects of this study is the quantification of the fluid amount. To this end critical information was provided by a relaxation study. 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. To assess reproducibility and instrument precision, calibration experiments were repeated several times and their variation investigated. The results showed that the device is highly precise and robust with a standard deviation for liquid quantification of less than 1%.

Revealing how interfaces in stacked thin fibrous layers affect liquid ingress and transport properties by single-sided NMR.

Offering multifaceted applications, 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 liquid ingress. Thus, the main objective of the study is to substantiate that an interface layer is present during liquid infiltration between stacked thin fibrous layers and that it affects the fluid transport properties. A compact single-sided NMR device with a low static gradient of about 2 T/m perpendicular to the sensor surface and a uniform magnetic field in lateral directions 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 Fourier-transforming the NMR signal and processing the time-dependent 1D profiles with a newly developed mathematical method. The flow characteristics and liquid distribution profiles of a 400-µm thick layer were compared with those of two stacked 200-µm thick layers from the same material but with an interface between them. 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.

Continuum-Scale Modeling of Liquid Redistribution in a Stack of Thin Hydrophilic Fibrous Layers.

Macroscale three-dimensional modeling of fluid flow in a thin porous layer under unsaturated conditions is a challenging task. One major issue is that such layers do not satisfy the representative elementary volume length-scale requirement. Recently, a new approach, called reduced continua model (RCM), has been developed to describe multiphase fluid flow in a stack of thin porous layers. In that approach, flow equations are formulated in terms of thickness-averaged variables and properties. In this work, we have performed a set of experiments, where a wet 260 - µ m -thin porous layer was placed on top of a dry layer of the same material. We measured the change of average saturation with time using a single-sided low-field nuclear magnetic resonance device known as NMR-MOUSE. We have employed both RCM and the traditional Richards equation-based models to simulate our experimental results. We found that the traditional unsaturated flow model cannot simulate experimental results satisfactorily. Very close agreement was obtained by including the dynamic capillary term as postulated by Hassanizadeh and Gray in the traditional equations. The reduced continua model was found to be in good agreement with the experimental result without adding dynamic capillarity term. Moreover, the computational effort needed for RCM simulations was one order of magnitude less than that of traditional models.