Probing porous and granular media with noble gas NMR

We have an active program of experimental investigations of granular media, in close collaboration with Prof. Don Candela of UMass-Amherst.

In recent years, much interest has focused on the physics of granular media: assemblies of macroscopic particles such as sand, gravel, ore, powders, or pharmaceutical pills that interact primarily through contact forces. Granular materials are ubiquitous in a wide range of industrial processes and complex systems. The properties of flowing granular matter are dominated by rapid inelastic collisions between the grains, which quickly dissipate the kinetic energy unless it is replenished by gravity, external vibration, or interstitial gas flow. Granular flows have revealed a host of behaviors that are unexpected from ordinary fluids, such as non-Newtonian boundary layers, propagating density waves, and avalanches. Furthermore, if a mixture of the material is stirred, shaken or rotated, different particle sizes or masses typically segregate into different regions of the confining container, in contrast to the homogenizing effect stirring has in ordinary fluids.

Theories of granular media dynamics have attempted to extend established notions from fluid mechanics, kinetic theory, or nonequilibrium thermodynamics. However, testing these theories is difficult for 3D systems, because the opacity of most granular materials presents a key difficulty to obtaining structural and dynamical information from within the system. Thus an outstanding problem in this field is to develop non-invasive, high-resolution spatial probes of 3D granular media and their interstitial spaces.

To address this problem, we are applying novel technologies such as pulsed-field-gradient (PFG) NMR, MRI, and hyperpolarized noble gas. Our current projects include the use of PFG NMR and MRI (i) to map grain density and displacements in a three-dimensional vibrofluidized granular medium, and (ii) to probe the flow and dynamics of hyperpolarized noble gas in a gas-fluidized granular bed. For example, in the first of these projects, we have been able to determine, for the first time, the "granular temperature" (proportional to the random kinetic energy per motional degree of freedom of the grains) as a function of height in a 3D vibrated sample; and have found reasonable agreement with the predictions of "granular hydrodynamic" theory.

We are also using video observation to investigate the role of interstitial gas in the segregation by mass of a two-component, vertically-vibrated granular medium (bronze and glass spheres). Click here to see examples of the different segregation phenomena that occur in the presence of interstitial gas -- for different vibration frequencies and amplitudes -- and also to see how these segregation phenomena disappear when the interstitial gas is removed (i.e., under vacuum).

We have also developed noble gas nuclear magnetic resonance (NMR) as a non-destructive probe of the structure of porous media. We use pulsed-field-gradient (PFG) NMR and magnetic resonance imaging (MRI) to measure the flow and diffusion of noble gas (³He and ¹²⁹Xe) imbided into the pore space of granular materials, animal lungs, and oil- or water-reservoir rocks. Diagnosing the structure of these materials is relevant to a wide range of scientific and technological problems. For example, knowledge of the fluid transport properties of reservoir rocks is important for the monitoring of contaminant percolation and for oil extraction. Similarly, knowledge of the evolution of the porous structure of materials subjected to large thermal or mechanical stress may help characterize the dynamics of cracking and material failure. Also, non-invasive mapping of lung structure could advance our understanding of normal physiology as well as disease diagnosis.

The advantages of noble gas NMR as a porous media diagnostic arise from three physical effects: (i) gas diffusion coefficients are orders of magnitude larger than those of liquids, which allows long distances to be probed; (ii) noble gas atoms interact weakly with pore surfaces, which enables long NMR lifetimes; and (iii) noble gases are chemically inert and biologically compatible. We employ both laser-polarized and thermally-polarized noble gas in our measurements.

We have shown that noble gas NMR can characterize important porous media parameters such as permeability, effective porosity, tortuosity, and the distribution of pore sizes. Permeability is a measure of the ability of a porous material to transmit fluid. Effective porosity is the volume fraction of a pore space that is fully interconnected and contributes to fluid flow through the material, excluding dead-end or isolated pores that cannot be part of a flow path. Tortuosity is a measure of the long-distance pore connectivity of the medium.

Recent Posters (click to download)

References:

NMR measurements of grain and gas motion in a gas-fluidized granular bed.

Noninvasive measurements of gas exchange in a three-dimensional fluidized bed using hyperpolarized ¹²⁹Xe NMR.

Understanding the breakdown of Fourier’s law in granular fluids.

Study of Gas-Fluidization Dynamics with Laser-Polarized ¹²⁹Xe.

Xenon NMR Measurements of Permeability and Tortuosity in Reservoir Rocks

Simultaneous Measurement of Rock Permeability and Effective Porosity using Laser-Polarized Noble Gas NMR.

NMR Experiments on a Three-DimensionalVibrofluidized Granular Medium.

Applications of Controlled-Flow Laser-Polarized Xenon Gas to Porous and Granular Media Study.

Diffusion NMR Methods Applied to Xenon Gas for Materials Study.

The Narrow Pulse Approximation and Long Length Scale Determination in Xenon Gas Diffusion NMR Studies of
Model Porous Media.
Measurements of Grain Motion in a Dense, Three Dimensional Granular Fluid.

Novel MRI Applications of Laser-Polarized Noble Gases. (Large File - 6.8 MB!)

Measuring surface-area-to-volume ratios in soft porous materials using laser-polarized xenon interphase exchange NMR.

Tortuosity Measurement and the Effects of Finite Pulse Widths on Xenon Gas Diffusion NMR Studies of Porous Media.

Probing porous media with gas diffusion NMR.

Pulsed-field-gradient measurements of time-dependent gas diffusion.