A New Analytical Approximation for the Flow into a Well in a Finite Reservoir

A new approach is presented for the pressure calculation around a horizontal well or a partially penetrating vertical well in a finite height reservoir. The new method matches the asymptotic solutions close to- and far from the well. To this end, the reservoir is divided into two distinct volumes with ellipsoidal and elliptically cylindrical symmetry, respectively. Comparison with other techniques shows that the new expressions yield excellent results. The method is also applicable to reservoirs containing one elliptical, vertical or horizontal fracture, and can be used with account of a zone of altered permeability around the well or fracture due to formation damage or stimulation.     

Limits on an energy dependence of the speed of light from a flare of the active galaxy PKS 2155-304

In the past few decades, several models have predicted an energy dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this Letter a search for such time lags during the High Energy Stereoscopic System observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.     

Molecular order and disorder in the frictional response of alkanethiol self-assembled monolayers

Molecular processes in the frictional response of an alkanethiol monolayer, self-assembled on a Au(111) surface, are studied by means of high-resolution friction force microscopy in ultrahigh vacuum. With increasing load, three regimes are observed on defect-free domains of the monolayer: smooth sliding with negligible friction, regular molecular stick-slip motion with increasing friction, and the onset of wear in the monolayer. Molecular contrast in the lateral force is found for inequivalent molecules within the unit cell of the c(4 × 2) superstructure. Significant differences in the frictional response are found between defect-free domains and areas including a domain boundary. Friction increases by an order of magnitude on domain boundaries in connection with irregular stick-slip motion. This increased friction at domain boundaries is observed at loads below the onset of wear.     

Search for a dark matter annihilation signal from the galactic center halo with H.E.S.S

A search for a very-high-energy (VHE; ≥100 GeV) γ-ray signal from self-annihilating particle dark matter (DM) is performed towards a region of projected distance r～45-150 pc from the Galactic center. The background-subtracted γ-ray spectrum measured with the High Energy Stereoscopic System (H.E.S.S.) γ-ray instrument in the energy range between 300 GeV and 30 TeV shows no hint of a residual γ-ray flux. Assuming conventional Navarro-Frenk-White and Einasto density profiles, limits are derived on the velocity-weighted annihilation cross section (σv) as a function of the DM particle mass. These are among the best reported so far for this energy range and in particular differ only little between the chosen density profile parametrizations. In particular, for the DM particle mass of ～1 TeV, values for (σv) above 3×10(-25) cm(3)?s(-1) are excluded for the Einasto density profile.     

Squeeze-out from mixed monolayers of dipalmitoylphosphatidylcholine and egg phosphatidylglycerol

Plots of surface pressuer (π) vs surface area (A) are taken from mixed monolayers of dipalmitoylphosphatidylcholine (DPPC) and “egg” phosphatidylglycerol (egg PG) with the concentration of egg PG (X) ranging from 0 to 100%, the temperatures (T) from 37 to 41°C, and the compression rate (dA/dt) from −13.6 to −688 mm²s⁻, and at a relative humidity of over 90%. Between limiting values of X, T, and dA/dt the π-A plots of compression show regions of nearly constant π (plateaus) which start at values of π (π_pb) of 48 ± 2 mN m⁻¹, and which can be followed by an increase in π until collapse occurs at 70 mN m⁻¹. π_pb is independent of X, T, and dA/dt. The plateau is correlated with a loss of molecules from the monolayer, which increases strongly with X and T and decreases with dA/dt. The quantitative results are not in agreement with separate collapse of the components. A model is presented stating that plateaus occur when the transition from the liquid-expanded (LE) to the liquid-condensed phase (LC) is incomplete at π_pb. During plateau formation a mixed LE phase is squeezed out at its collapse pressure. The remaining LC phase can be compressed to 70 mN m⁻¹. Results of calculations based on this model are in agreement with the experimental results and predict slight enrichment of the squeezed-out phase with respect to egg PG.     

Simulation of Three-Phase Displacement Mechanisms Using a 2D Lattice-Boltzmann Model

Using a numerical technique, known as the lattice-Boltzmann method, we study immiscible three-phase flow at the pore scale. An important phenomenon at this scale is the spreading of oil onto the gas–water interface. In this paper, we recognize from first principles how injected gas remobilizes initially trapped oil blobs. The two main flow mechanisms which account for this type of remobilization are simulated. These are the double-drainage mechanism and (countercurrent) film flow of oil. The simulations agree qualitatively with experimental findings in the literature. We also simulate steady-state three-phase flow (fixed and equal saturations) in a small segment of a waterwet porous medium under both spreading and nonspreading conditions. The difference between the two conditions with respect to the coefficients in the generalized law of Darcy (which also includes viscous coupling) is investigated.     

HESS observations of the galactic center region and their possible dark matter interpretation

The detection of gamma rays from the source HESS J1745-290 in the Galactic Center (GC) region with the High Energy Spectroscopic System (HESS) array of Cherenkov telescopes in 2004 is presented. After subtraction of the diffuse gamma-ray emission from the GC ridge, the source is compatible with a point source with spatial extent less than 1.2;{'}(stat) (95% C.L.). The measured energy spectrum above 160 GeV is compatible with a power law with photon index of 2.25+/-0.04(stat)+/-0.10(syst) and no significant flux variation is detected. It is finally found that the bulk of the very high energy emission must have non-dark-matter origin.     

Three-Phase Effective Contact Angle in a Model Pore

We consider a model pore (2D) in which a sharp interface between two fluids contact a third fluid which wets the solid boundary. If the configuration is capillary dominated, the geometry can be determined analytically in terms of the effective contact angle. This angle depends not only on the interfacial tensions, but also on the capillary pressures. However, if the height of the cusp formed by the wetting fluid is much smaller than the pore width, the effective contact angle is a simple function of the interfacial tensions. It turns out to be the same function as in the case of an undeformed wetting layer of molecular thickness. The analytical expression for the effective contact angle has been confirmed by a numerical technique, known as the lattice-Boltzmann method. This method, in turn, has been validated with Neumann's law for the three-phase contact angles.