Mark W. Hunter

Propagator-resolved 2D exchange in porous media in the inhomogeneous magnetic field

We present a propagator-resolved 2D exchange spectroscopy technique for observing fluid motion in a porous medium. The susceptibility difference between the matrix and the fluid is exploited to produce an inhomogeneous internal magnetic field, causing the Larmor frequency to change as molecules migrate. We test our method using a randomly packed monodisperse 100 microm diameter glass bead matrix saturated with distilled water. Building upon previous 2D exchange spectroscopy work we add a displacement dimension which allows us to obtain 2D exchange spectra that are defined by both mixing time and spatial displacement rather than by mixing time alone. We also simulate our system using a Monte Carlo process in a random nonpenetrating monodisperse bead pack, finding good agreement with experiment. A simple analytic model is used to interpret the NMR data in terms of a characteristic length scale over which molecules must diffuse to sample the inhomogeneous field distribution.

Nuclear magnetic resonance measurement and lattice-Boltzmann simulation of the nonlocal dispersion tensor

The nonlocal dispersion tensor DNL provides a fundamental description of velocity correlations and displacement information in a dispersive system. It is shown that pulsed gradient spin echo nuclear magnetic resonance can be used to measure this tensor, and we present here the first measurement of DNL in a complex flow by this or any other methods. These measurements are complemented by simulations based on a lattice-Boltzmann calculation of the fluid flow. For dispersive flow in a random bead pack of monosized spheres, six nonzero, independent components remain. These components have been measured at three times less than τv, the time to flow one bead diameter. It is shown here that the various elements of DNL provide insights regarding the dispersive flow, which are extremely sensitive to the details of local correlations.

Chapter 4 - Earth's Field Spectroscopy

Abstract The Earths magnetic field provides a convenient B 0 for performing magnetic resonance spectroscopy experiments. Providing certain environmental factors that can be overcome, the natural homogeneity of the Earths field results in NMR signals with very narrow linewidths. In most cases, it is necessary to use a combination of a pre-polarisation scheme, and a more sensitive detection technique. SQUIDs and atomic magnetometers provide a sensitive detection scheme that is independent of the Lamour frequency. Even in the absence of peak splitting due to chemical shift, hetero- and homonuclear couplings, which are independent of field, can clearly be identified. Spin systems in the Earths field are often strongly coupled resulting in complex spectra. As the field is then reduced, the J coupling begins to become the dominant interaction.

PGSE NMR measurement of the non-local dispersion tensor for flow in porous media

The non-local dispersion tensor provides a fundamental description of velocity correlations and displacement information in a pre-asymptotic dispersive system. Here we describe in detail how PGSE NMR may be used to measure this tensor, outlining the pulse sequences needed for signal superposition, as well as the data analysis procedures. The sequence is inherently two-dimensional, the first dimension giving the displacement resolution, the second giving correlation information. The technique is verified against simulated echo attenuation data from a lattice-Boltzmann simulation.

Chapter 6:Real Time PGSE NMR Through Direct Acquisition of Averaged Propagators in the Time Domain Using Pulsed Second Order Magnetic Fields

Brownian motion (diffusion) and coherent flows are fundamental processes in nature. Therefore, their accurate measurement and description is highly desirable in many areas of science, engineering and technology. Here we describe a theoretical and experimental framework which enables one to directly examine the dynamics of fluid matter subject to diffusion and flow through the acquisition of the so-called averaged propagator. This statistical function holds all information on particle mobility due to flow and diffusion averaged over the observed fluid. The method is based on a single instantaneous nuclear magnetic resonance (NMR) measurement event. It also removes the need of data post processing by capturing the averaged propagator directly as the acquired signal which enables the monitoring of diffusion and flow in real time. When the propagator is acquired multiple times within one NMR experiment the real time measurement of surface-to-volume ratios in porous materials become possible. (In parts reprinted with permission from [W. Kittler, M. Hunter and P. Galvosas, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2015, 92, 023016] Copyright (2015) by the American Physical Society and from W. C. Kittler, P. Galvosas and M. W. Hunter, Parallel acquisition of q space using second order magnetic fields for single-shot diffusion measurements, J. Magn.Reson., 244, 46–52, Copyright (2014), with permission from Elsevier.)