The electrorheology of suspensions consisting of Na-Fluorohectorite synthetic clay particles in silicon oil

Under application of an electric field greater than a triggering electric field $E_c \sim 0.4$ kV/mm, suspensions obtained by dispersing particles of the synthetic clay fluoro-hectorite in a silicon oil, aggregate into chain- and/or column-like structures parallel to the applied electric field. This micro-structuring results in a transition in the suspensions' rheological behavior, from a Newtonian-like behavior to a shear-thinning rheology with a significant yield stress. This behavior is studied as a function of particle volume fraction and strength of the applied electric field, $E$. The steady shear flow curves are observed to scale onto a master curve with respect to $E$, in a manner similar to what was recently found for suspensions of laponite clay [42]. In the case of Na-fluorohectorite, the corresponding dynamic yield stress is demonstrated to scale with respect to $E$ as a power law with an exponent $\alpha \sim 1.93$, while the static yield stress inferred from constant shear stress tests exhibits a similar behavior with $\alpha \sim 1.58$. The suspensions are also studied in the framework of thixotropic fluids: the bifurcation in the rheology behavior when letting the system flow and evolve under a constant applied shear stress is characterized, and a bifurcation yield stress, estimated as the applied shear stress at which viscosity bifurcation occurs, is measured to scale as $E^\alpha$ with $\alpha \sim 0.5$ to 0.6. All measured yield stresses increase with the particle fraction $\Phi$ of the suspension. For the static yield stress, a scaling law $\Phi^\beta$, with $\beta = 0.54$, is found. The results are found to be reasonably consistent with each other. Their similarities with-, and discrepancies to- results obtained on laponite-oil suspensions are discussed

Salt Mechanics Primer for Near-Salt and Sub-Salt Deepwater Gulf of Mexico Field Developments

The Gulf of Mexico (GoM) is the most active deepwater region in the world and provides some of the greatest challenges in scope and opportunity for the oil and gas industry. The complex geologic settings and significant water and reservoir depths necessitate high development costs, in addition to requiring innovating technology. The investment costs are substantial: because of the extreme water depths (up to 8000 feet) and considerable reservoir depths (to 30,000 feet below mudline), the cost of drilling a single well can be upwards of 50 to 100 million dollars. Central, therefore, to successful economic exploitation are developments with a minimum number of wells combined with a well service lifetime of twenty to thirty years. Many of the wells that are planned for the most significant developments will penetrate thick salt formations, and the combined drilling costs for these fields are estimated in the tens of billions of dollars. In May 2001, Sandia National Laboratories initiated a Joint Industry Project focused on the identification, quantification, and mitigation of potential well integrity issues associated with sub-salt and near-salt deepwater GoM reservoirs. The project is jointly funded by the DOE (Natural Gas and Oil Technology Partnership) and nine oil companies (BHP Billiton Petroleum, BP, ChevronTexaco, Conoco, ExxonMobil, Halliburton, Kerr-McGee, Phillips Petroleum, and Shell). This report provides an assessment of the state of the art of salt mechanics, and identifies potential well integrity issues relevant to deepwater GoM field developments. Salt deformation is discussed and a deformation mechanism map is provided for salt. A bounding steady-state strain rate contour map is constructed for deepwater GoM field developments, and the critical issue of constraint in the subsurface, and resultant necessity for numerical analyses is discussed

Collisional dissipation of Alfvén waves in a partially ionised solar chromosphere

Certain regions of the solar atmosphere are at sufficiently low temperatures to be only partially ionised. The lower chromosphere contains neutral atoms, the existence of which greatly increases the efficiency of the damping of waves due to collisional friction momentum transfer. More specifically the Cowling conductivity can be up to 12 orders of magnitude smaller than the Spitzer value, so that the main damping mechanism in this region is due to the collisions between neutrals and positive ions (Khodachenko et al. 2004, A&A, 422, 1073). Using values for the gas density and temperature as functions of height taken from the VAL C model of the quiet Sun (Vernazza et al. 1981, ApJS, 45, 635), an estimate is made for the dependance of the Cowling conductivity on height and strength of magnetic field. Using both analytic and numerical approaches the passage of Alfvén waves over a wide spectrum through this partially ionised region is investigated. Estimates of the efficiency of this region in the damping of Alfvén waves are made and compared for both approaches. We find that Alfvén waves with frequencies above 0.6 Hz are completely damped and frequencies below 0.01 Hz unaffected

Intercalation-enhanced electric polarization and chain formation of nano-layered particles

Microscopy observations show that suspensions of synthetic and natural nano-layered smectite clay particles submitted to a strong external electric field undergo a fast and extended structuring. This structuring results from the interaction between induced electric dipoles, and is only possible for particles with suitable polarization properties. Smectite clay colloids are observed to be particularly suitable, in contrast to similar suspensions of a non-swelling clay. Synchrotron X-ray scattering experiments provide the orientation distributions for the particles. These distributions are understood in terms of competing (i) homogenizing entropy and (ii) interaction between the particles and the local electric field; they show that clay particles polarize along their silica sheet. Furthermore, a change in the platelet separation inside nano-layered particles occurs under application of the electric field, indicating that intercalated ions and water molecules play a role in their electric polarization. The resulting induced dipole is structurally attached to the particle, and this causes particles to reorient and interact, resulting in the observed macroscopic structuring. The macroscopic properties of these electro-rheological smectite suspensions may be tuned by controlling the nature and quantity of the intercalated species, at the nanoscale.Comment: 7 pages, 5 figure

Magneto-acoustic wave energy from numerical simulations of an observed sunspot umbra

We aim at reproducing the height dependence of sunspot wave signatures obtained from spectropolarimetric observations through 3D MHD numerical simulations. A magneto-static sunspot model based on the properties of the observed sunspot is constructed and perturbed at the photosphere introducing the fluctuations measured with the \SiI\ $\lambda$ 10827 \AA\ line. The results of the simulations are compared with the oscillations observed simultaneously at different heights from the \HeI\ $\lambda$ 10830 \AA\ line, the \CaIIH\ core and the \FeI\ blends in the wings of the \CaIIH\ line. The simulations show a remarkable agreement with the observations. They reproduce the velocity maps and power spectra at the formation heights of the observed lines, as well as the phase and amplification spectra between several pair of lines. We find that the stronger shocks at the chromosphere are accompanied with a delay between the observed signal and the simulated one at the corresponding height, indicating that shocks shift the formation height of the chromospheric lines to higher layers. Since the simulated wave propagation matches very well the properties of the observed one, we are able to use the numerical calculations to quantify the energy contribution of the magneto-acoustic waves to the chromospheric heating in sunspots. Our findings indicate that the energy supplied by these waves is too low to balance the chromospheric radiative losses. The energy contained at the formation height of the lowermost \SiI\ $\lambda$ 10827 \AA\ line in the form of slow magneto-acoustic waves is already insufficient to heat the higher layers, and the acoustic energy which reaches the chromosphere is around 3-9 times lower than the required amount of energy. The contribution of the magnetic energy is even lower.Comment: Accepted for publication in The Astrophysical Journa

Twisting Flux Tubes as a cause of Micro-Flaring Activity

High-cadence optical observations of an H-alpha blue-wing bright point near solar AR NOAA 10794 are presented. The data were obtained with the Dunn Solar Telescope at the National Solar Observatory/Sacramento Peak using a newly developed camera system, the Rapid Dual Imager. Wavelet analysis is undertaken to search for intensity-related oscillatory signatures, and periodicities ranging from 15 to 370 s are found with significance levels exceeding 95%. During two separate microflaring events, oscillation sites surrounding the bright point are observed to twist. We relate the twisting of the oscillation sites to the twisting of physical flux tubes, thus giving rise to reconnection phenomena. We derive an average twist velocity of 8.1 km/s and detect a peak in the emitted flux between twist angles of 180 and 230 degrees.Comment: 8 pages, 10 figure

Scattering statistics of rock outcrops: Model-data comparisons and Bayesian inference using mixture distributions

The probability density function of the acoustic field amplitude scattered by the seafloor was measured in a rocky environment off the coast of Norway using a synthetic aperture sonar system, and is reported here in terms of the probability of false alarm. Interpretation of the measurements focused on finding appropriate class of statistical models (single versus two-component mixture models), and on appropriate models within these two classes. It was found that two-component mixture models performed better than single models. The two mixture models that performed the best (and had a basis in the physics of scattering) were a mixture between two K distributions, and a mixture between a Rayleigh and generalized Pareto distribution. Bayes' theorem was used to estimate the probability density function of the mixture model parameters. It was found that the K-K mixture exhibits significant correlation between its parameters. The mixture between the Rayleigh and generalized Pareto distributions also had significant parameter correlation, but also contained multiple modes. We conclude that the mixture between two K distributions is the most applicable to this dataset.Comment: 15 pages, 7 figures, Accepted to the Journal of the Acoustical Society of Americ

Two-Dimensional Helioseismic Power, Phase, and Coherence Spectra of {\it Solar Dynamics Observatory} Photospheric and Chromospheric Observables

While the {\it Helioseismic and Magnetic Imager} (HMI) onboard the {\it Solar Dynamics Observatory} (SDO) provides Doppler velocity [$V$], continuum intensity [$I_C$], and line-depth [$Ld$] observations, each of which is sensitive to the five-minute acoustic spectrum, the {\it Atmospheric Imaging Array} (AIA) also observes at wavelengths -- specifically the 1600 and 1700 Angstrom bands -- that are partly formed in the upper photosphere and have good sensitivity to acoustic modes. In this article we consider the characteristics of the spatio--temporal Fourier spectra in AIA and HMI observables for a 15-degree region around NOAA Active Region 11072. We map the spatio--temporal-power distribution for the different observables and the HMI Line Core [$I_L$], or Continuum minus Line Depth, and the phase and coherence functions for selected observable pairs, as a function of position and frequency. Five-minute oscillation power in all observables is suppressed in the sunspot and also in plage areas. Above the acoustic cut-off frequency, the behaviour is more complicated: power in HMI $I_C$ is still suppressed in the presence of surface magnetic fields, while power in HMI $I_L$ and the AIA bands is suppressed in areas of surface field but enhanced in an extended area around the active region, and power in HMI $V$ is enhanced in a narrow zone around strong-field concentrations and suppressed in a wider surrounding area. The relative phase of the observables, and their cross-coherence functions, are also altered around the active region. These effects may help us to understand the interaction of waves and magnetic fields in the different layers of the photosphere, and will need to be taken into account in multi-wavelength local helioseismic analysis of active regions.Comment: 18 pages, 15 figures, to be published in Solar Physic