Peter J. Eng

Signatures of granular microstructure in dense shear flows

Granular materials and ordinary fluids react differently to shear stresses. Rather than deforming uniformly, materials such as dry sand or cohesionless powders develop shear bands—narrow zones of large relative particle motion, with essentially rigid adjacent regions. Because shear bands mark areas of flow, material failure and energy dissipation, they are important in many industrial, civil engineering and geophysical processes. They are also relevant to lubricating fluids confined to ultrathin molecular layers. However, detailed three-dimensional information on motion within a shear band, including the degree of particle rotation and interparticle slip, is lacking. Similarly, very little is known about how the microstructure of individual grains affects movement in densely packed material. Here we combine magnetic resonance imaging, X-ray tomography and high-speed-video particle tracking to obtain the local steady-state particle velocity, rotation and packing density for shear flow in a three-dimensional Couette geometry. We find that key characteristics of the granular microstructure determine the shape of the velocity profile.

Structure and reactivity of the calcite-water interface

The zetapotential of calcite in contact with aqueous solutions of varying composition is determined for pre-equilibrated suspensions by means of electrophoretic measurements and for non-equilibrium solutions by means of streaming potential measurements. Carbonate and calcium are identified as charge determining ions. Studies of the equilibrium solutions show a shift of isoelectric point with changing CO(2) partial pressure. Changes in pH have only a weak effect in non-equilibrium solutions. The surface structure of (104)-faces of single crystal calcite in contact to solutions corresponding to those of the zetapotential investigations is determined from surface diffraction measurements. The results reveal no direct indication of calcium or carbonate inner-sphere surface species. The surface ions are found to relax only slightly from their bulk positions; the most significant relaxation is a ∼4° tilt of the surface carbonate ions towards the surface. Two ordered layers of water molecules are identified, the first at 2.35±0.05Å above surface calcium ions and the second layer at 3.24±0.06Å above the surface associated with surface carbonate ions. A Basic-Stern surface complexation model is developed to model observed zetapotentials, while only considering outer-sphere complexes of ions other than protons and hydroxide. The Basic-Stern SCM successfully reproduces the zetapotential data and gives reasonable values for the inner Helmholtz capacitance, which are in line with the Stern layer thickness estimated from surface diffraction results.

X-ray Raman scattering study of MgSiO3 glass at high pressure: implication for triclustered MgSiO3 melt in Earth's mantle.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earths interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earths interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earths interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; [3]O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earths lower mantle.

X-Ray-Induced Dissociation of H2O and Formation of an O2-H2 Alloy at High Pressure

When subjected to high pressure and extensive x-radiation, water (H2O) molecules cleaved, forming O–O and H–H bonds. The oxygen (O) and hydrogen (H) framework in ice VII was converted into a molecular alloy of O2 and H2. X-ray diffraction, x-ray Raman scattering, and optical Raman spectroscopy demonstrated that this crystalline solid differs from previously known phases. It remained stable with respect to variations in pressure, temperature, and further x-ray and laser exposure, thus opening new possibilities for studying molecular interactions in the hydrogen-oxygen binary system.

Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa

The partial density of vibrational states has been measured for Fe in compressed FeO (wüstite) using nuclear resonant inelastic x-ray scattering. Substantial changes have been observed in the overall shape of the density of states close to the magnetic transition around 20 GPa from the paramagnetic (low pressure) to the antiferromagnetic (high pressure) state. The results indicate that strong magnetoelastic coupling in FeO is the driving force behind the changes in the phonon spectrum of FeO. The paper presents the first observation of changes in the density of terahertz acoustic phonon states under magnetic transition at high pressure.

GeoCARS microfocusing Kirkpatrick–Baez mirror bender development

We present development of microfocusing Kirkpatrick–Baez mirrors GeoSoilEnviroCARS. The mirror is made of single‐crystal silicon and the desired elliptical shape is formed by elastic bending with a two‐parameter bender. We present data from long trace profiler measurements of the bent shape, which verifies the design principle, and motivates a new bender design. We tested the re‐engineered mirror bender focusing white radiation from a National Synchrotron Light Source bending magnet, achieving a 2.5 μm focus with a gain (using only one mirror) of 91.

Geoscience applications of x-ray computed microtomography

A facility for x-ray computed microtomography (CMT) has been commissioned on the bending magnet beamline at the GeoSoilEnviroCARS sector at the Advanced Photon Source (APS). The APS bending magnet has a critical energy of 20 keV, and thus provides high flux at photon energies up to 100 keV, making it well suited to imaging a wide range of earth materials up to several cm in size. The current apparatus uses a Si (220) channel-cut monochromator covering the energy range from 5 to 35 keV with beam sizes up to 18 mm wide and 4 mm high. The transmitted x-rays are imaged with a single crystal YAG scintillator, a microscope objective and a 1242 X 1152 pixel fast CCD detector. The system spatial resolution is about 3 microns in both the transmission radiographs and the reconstructed slices. Data collection times are approximately 30 minutes. This facility has been used to conduct a number of preliminary studies of earth materials, including inclusion in diamonds, pores in waste repository rocks and fossils. Fluorescence tomography has been conducted on the companion undulator beamline, where we have imaged the internal trace element distribution in interplanetary dust particles.

Facilities for high-pressure research with the diamond anvil cell at GSECARS.

An overview of facilities for high-pressure research with the diamond anvil cell (DAC) at the GeoSoilEnviroCARS (GSECARS) sector at the Advanced Photon Source (Argonne, Illinois) is presented. There are three operational experimental stations (13-ID-C, 13-ID-D and 13-BM-D) where DAC instrumentation is installed for various types of experiments at high pressure and extreme temperature conditions. A fourth station (13-BM-C) is under construction and will be operational in 2006. While most X-ray diffraction experiments have been undertaken with powder samples so far, there is a growing demand for single-crystal diffraction (SCD) at high pressure. As one of the principal components at GSECARS, SCD is currently under rapid development. Other relevant techniques have also been developed for obtaining complementary information from powder or single-crystal samples at high pressure. For example, an on-line Brillouin system is installed and operational at 13-BM-D for acoustic velocity and single-crystal elasticity determinations. In addition, various X-ray spectroscopy techniques (e.g. X-ray emission and X-ray Raman) are employed for measuring electronic and magnetic properties. Future developments are discussed with the DAC program at GSECARS.

Micro-beam X-ray absorption and fluorescence spectroscopies at GSECARS: APS beamline 13ID.

GeoSoilEnviroCARS, sector 13 of the Advanced Photon Source at Argonne National Laboratory, provides a micro-beam facility for XAFS, XANES, XRE and x-ray diffraction studies. A KirkpatrickBaez mirror pair gives a focussed monochromatic undulator beam down to 1 x 1/zm with sufficient intensity for x-ray fluorescence mapping and extended XAFS of dilute systems at energies above 4KeV. Special emphasis for these facilities has been given to environmental and earth science studies, including dilute contaminants in natural sediments. As an example, XRF maps and EXAFS data taken with a 4pm by 7#m spot size are shown for the Pu LIII edge of a natural tuff exposed to a dilute aqueous solution of Pu. In addition, x-ray diffraction and scattering capabilities allow the study of surfaces and surface adsorbates with micron-sized beams using xray reflectivity, x-ray standing-waves, and grazing-incidence XAFS.

Inelastic x-ray scattering of dense solid oxygen: Evidence for intermolecular bonding

The detailing of the intermolecular interactions in dense solid oxygen is essential for an understanding of the rich polymorphism and remarkable properties of this element at high pressure. Synchrotron inelastic x-ray scattering measurements of oxygen K-edge excitations to 38 GPa reveal changes in electronic structure and bonding on compression of the molecular solid. The measurements show that O2 molecules interact predominantly through the half-filled 1πg* orbital <10 GPa. Enhanced intermolecular interactions develop because of increasing overlap of the 1πg* orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O2 molecules at the transition to the ε-phase. The ε-phase, which consists of (O2)4 clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O2)4 clusters develop upon compression of the ε-phase, and provide a potential mechanism for intercluster bonding in still higher-pressure phases.