RESUMEN
Pulsed field gradient nuclear magnetic resonance provides a powerful tool for the measurement of particle diffusion and mobility. When these particles are contained in a porous medium, the diffusive process is influenced by the pore boundaries, and their effect on diffusion measurements provides information about the pore space. The acquisition of the apparent diffusion coefficient and its dependence on time, in the short time limit, reveals the surface to volume ratio of the porous medium, and in the long time limit, its tortuosity. With conventional pulsed field gradient techniques, processes where pore boundaries are evolving on the sub-second time scale cannot be resolved. Using pulsed second order magnetic fields in conjunction with one-dimensional imaging and the pulse sequence Difftrain, this paper presents a proof of concept for the first ever real time single-shot surface to volume NMR measurement.
RESUMEN
A proof of concept is presented for the parallel acquisition of q-space under diffusion using a second order magnetic field. The second order field produces a gradient strength which varies in space, allowing a range of gradients to be applied in a single pulse, and q-space encoded into real space. With the use of a read gradient, the spatial information is regained from the NMR signal, and real space mapped onto q-space for a thin slice excitation volume. As the diffusion encoded image for a thin slice can be mapped onto q-space, and the average propagator is the inverse Fourier transform of the q-space data, it follows that the acquisition of the echo is a direct measurement of the average propagator. In the absence of a thin slice selection, the real space to q-space mapping is lost, but the ability to measure the diffusion coefficient retained with an increase in signal to noise.
RESUMEN
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.
Asunto(s)
Algoritmos , Imagen por Resonancia Magnética/métodos , Modelos Químicos , Reología/métodos , Simulación por Computador , Difusión , PorosidadRESUMEN
The flow and diffusion driven separation of initially adjacent liquid molecules is known as dispersion. The primary physical quantity describing this process, the nonlocal dispersion tensor, provides insight regarding both the spatial and temporal correlations of molecular velocity fluctuations in complex flows. We here propose and demonstrate a nuclear magnetic resonance method for the measurement of this tensor, validating its implementation for the case of cylindrical Couette flow, and demonstrating its application to the study of fluid dispersion in a random bead pack.
Asunto(s)
Espectroscopía de Resonancia Magnética/métodos , Modelos Teóricos , Simulación por Computador , Cómputos Matemáticos , Método de Montecarlo , Resonancia Magnética Nuclear Biomolecular/métodosRESUMEN
A new portable NMR sensor with a novel one-sided access magnet design, termed NMR-MOLE (MObile Lateral Explorer), has been characterised in terms of sensitivity and depth penetration. The magnet has been designed to be portable and create a volume with a relatively homogeneous magnetic field, 15,000 ppm over a region from 4 to 16 mm away from the probe, with maximum sensitivity at a depth of 10 mm. The proton NMR frequency is 3.3 MHz. We have demonstrated that with this approach a highly sensitive, portable, unilateral NMR sensor can be built. Such a design is especially suited for the characterisation of liquids in situations where unilateral or portable access is required.