Casimir force calculations near the insulator-conductor transition in gold thin films

We present theoretical calculations of the Casimir force for Au thin films near the insulator-conductor transition that has been observed experimentally. The dielectric function of the Au thin films is described by the Drude-Smith model. The parameters needed to model the dielectric function such as the relaxation time, plasma frequency and the backscattering constant depend on the thickness of the film. The Casimir force decreases as the film thickness decreases until it reaches a minimum after which the force increases again. The minimum of the force coincides with the critical film thickness where a percolation conductor-insulator occurs.Comment: 5 figures, 1 tabl

Van der Waals torque induced by external magnetic fields

We present a method for inducing and controlling van der Waals torques between two parallel slabs using a constant magnetic field. The torque is calculated using the Barash theory of dispersive torques. In III-IV semiconductors such as $InSb$, the effect of an external magnetic field is to induce an optical anisotropy, in an otherwise isotropic material, that will in turn induce a torque. The calculations of the torque are done in the Voigt configuration, with the magnetic field parallel to the surface of the slabs. As a case study we consider a slab made of calcite and a second slab made of $InSb$. In the absence of magnetic field there is no torque. As the magnetic field increases, the optical anisotropy of $InSb$ increases and the torque becomes different from zero, increasing with the magnetic field. The resulting torque is of the same order of magnitude as that calculated using permanent anisotropic materials when the magnetic fields is close to 1 T.Comment: to appear in Journal of Applied Physic

Statistics of Velocity from Spectral Data: Modified Velocity Centroids

We address the problem of studying interstellar turbulence using spectral line data. We find a criterion when the velocity centroids may provide trustworthy velocity statistics. To enhance the scope of centroids applications, we construct a measure that we term ``modified velocity centroids'' (MVCs) and derive an analytical solution that relates the 2D spectra of the modified centroids with the underlying 3D velocity spectrum. We test our results using synthetic maps constructed with data obtained through simulations of compressible magnetohydrodynamical (MHD) turbulence. We show that the modified velocity centroids (MVCs) are complementary to the the Velocity Channel Analysis (VCA) technique. Employed together, they make determining of the velocity spectral index more reliable and for wider variety of astrophysical situations.Comment: 4 pages, 1 figure, Accepted for publication in ApJ Letters. minor change

Structural analysis of a quasi-random masonry wall : geometry details, homogenization and application

In many countries, historical buildings were built with masonry walls constituted by random assemblages of blocks and stones of variable dimensions. The analysis of historic masonry structures requires often complex and expensive computational tools that in many cases are difficult to handle, given this condition of large variability of masonry. The present paper addresses a methodology for the characterization of the response of rubble masonry. First, a brief state of the art regarding homogenization is presented. Then, the characterization of the masonry and statistical analysis of the dimensions of the stone units from the walls of Guimarães castle are carried out. This is followed by the homogenized limit analysis of representative volume elements (RVEs) from the Alcaçova wall in the Guimarães castle, in order to obtain its in-plane an out-of-plane failure surfaces at different orientations of a load and increasing compressive loads considering the case of masonry with weak and strong mortar independently. Finally, a safety for seismic loading was carried out in two numerical models of the Alcaçova wall, being the first one built with a heterogeneous material and the second one with a homogeneous material that was obtained by means of homogenized limit analysis of representative volume elements. The purpose is to determinate the reliability of results, in terms of limit load and failure mechanism, from the homogenized model, compared to the heterogeneous model

Nonlocal impedances and the Casimir entropy at low temperatures

The problem with the temperature dependence of the Casimir force is investigated. Specifically, the entropy behavior in the low temperature limit, which caused debates in the literature, is analyzed. It is stressed that the behavior of the relaxation frequency in the $T\to0$ limit does not play a physical role since the anomalous skin effect dominates in this range. In contrast with the previous works, where the approximate Leontovich impedance was used for analysis of nonlocal effects, we give description of the problem in terms of exact nonlocal impedances. It is found that the Casimir entropy is going to zero at $T\to0$ only in the case when $s$ polarization does not contribute to the classical part of the Casimir force. However, the entropy approaching zero from the negative side that, in our opinion, cannot be considered as thermodynamically satisfactory. The resolution of the negative entropy problem proposed in the literature is analyzed and it is shown that it cannot be considered as complete. The crisis with the thermal Casimir effect is stressed.Comment: Accepted in Phys. Rev.

Limit analysis of loaded out-of-plane rubble masonry : a case study in Portugal

An integrated methodology for the characterization of the response of rubble masonry is presented. The behaviour at collapse of a wall belonging to the Guimarães castle (Portugal) is investigated through a rigid-plastic homogenization procedure, accounting both for the actual disposition of the blocks constituting the walls and texture irregularity, given by the variability of block dimensions. A detailed survey is conducted by means of a photogrammetric technique, allowing for a precise characterization of blocks dimensions and disposition. After a geometric simplification assuming mortar joints reduced to interfaces, homogenized masonry in- and out-of-plane strength domains are evaluated on a number of different representing elements of volume (RVEs) having different sizes and sampled on the walls of the castle. Strength domains are obtained using a finite element (FE) limit analysis with a heterogeneous discretization by triangles and interfaces. Finally, a series of limit analyses are carried out on the façade for the safety assessment under seismic loads by means of two numerical models, the first one being a heterogeneous model and the second a homogenized approach. The reliability of the results, in terms of limit load and failure mechanism, provided by the homogenized model, when compared to the heterogeneous one is satisfactory.(undefined

Characterization of the response of quasi-periodic masonry : geometrical investigation, homogenization and application to the Guimarães castle, Portugal

In many countries, historical buildings were built with masonry walls constituted by random assemblages of stones of variable dimensions and shapes. The analysis of historic masonry structures requires often complex and expensive computational tools that in many cases are difficult to handle, given this large variability of masonry. The present paper validates a methodology for the characterization of the ultimate response of quasi periodic masonry. For this purpose, the behaviour at collapse of a wall at the Guimarães castle in Portugal is investigated by means of a rigid-plastic homogenization procedure, accounting for the actual disposition of the blocks constituting the walls and the texture irregularity given by the variability of dimensions in the blocks. A detailed geometric survey is conducted by means of the laser scanning technique, allowing for a precise characterization of dimensions and disposition of the blocks. After a simplification of the geometry and assuming mortar joints reduced to interfaces, homogenized masonry in- and out-of-plane strength domains are evaluated on a number of different Representing Elements of Volume (RVEs) having different sizes and sampled on the walls of the castle. Strength domains are obtained using a Finite Element (FE) limit analysis approach with a heterogeneous discretization of the RVEs with triangular elements representing units and interfaces (mortar joints), at different orientations of the principal actions with respect to the horizontal direction. The role played by vertical compression is also investigated, considering the case of masonry with weak and strong mortar. Finally, a series of limit analyses are carried out at structural level, using two different FE numerical models of the so-called Alcaçova wall, a representative perimeter wall of the caste. The first model is built with a heterogeneous material and the second model is built with a homogeneous material obtained through the homogenization procedure performed previously. The purpose is to determinate the reliability of results, in terms of limit load and failure mechanism, for the homogenized model and to compare these results to the ones obtained with the heterogeneous model

High-multipolar effects on the Casimir force: the non-retarded limit

We calculate exactly the Casimir force or dispersive force, in the non-retarded limit, between a spherical nanoparticle and a substrate beyond the London's or dipolar approximation. We find that the force is a non-monotonic function of the distance between the sphere and the substrate, such that, it is enhanced by several orders of magnitude as the sphere approaches the substrate. Our results do not agree with previous predictions like the Proximity theorem approach.Comment: 7 pages including 2 figures. Submitted to Europjysics Letter

Archaea and Bacteria Acclimate to High Total Ammonia in a Methanogenic Reactor Treating Swine Waste

Citation: Esquivel-Elizondo, S., Parameswaran, P., Delgado, A. G., Maldonado, J., Rittmann, B. E., & Krajmalnik-Brown, R. (2016). Archaea and Bacteria Acclimate to High Total Ammonia in a Methanogenic Reactor Treating Swine Waste. Archaea-an International Microbiological Journal, 10. doi:10.1155/2016/4089684Inhibition by ammonium at concentrations above 1000mgN/L is known to harm the methanogenesis phase of anaerobic digestion. We anaerobically digested swine waste and achieved steady state COD-removal efficiency of around 52% with no fatty-acid or H-2 accumulation. As the anaerobic microbial community adapted to the gradual increase of total ammonia-N (NH3 -N) from 890 +/- 295 to 2040 +/- 30 mg/L, the Bacterial and Archaeal communities became less diverse. Phylotypes most closely related to hydrogenotrophic Methanoculleus (36.4%) and Methanobrevibacter (11.6%), along with acetoclastic Methanosaeta (29.3%), became the most abundant Archaeal sequences during acclimation. This was accompanied by a sharp increase in the relative abundances of phylotypes most closely related to acetogens and fatty-acid producers (Clostridium, Coprococcus, and Sphaerochaeta) and syntrophic fatty-acid Bacteria (Syntrophomonas, Clostridium, Clostridiaceae species, and Cloacamonaceae species) that have metabolic capabilities for butyrate and propionate fermentation, as well as for reverse acetogenesis. Our results provide evidence countering a prevailing theory that acetoclastic methanogens are selectively inhibited when the total ammonia-N concentration is greater than similar to 1000 mgN/L. Instead, acetoclastic and hydrogenotrophic methanogens coexisted in the presence of total ammonia-N of similar to 2000 mgN/L by establishing syntrophic relationships with fatty-acid fermenters, as well as homoacetogens able to carry out forward and reverse acetogenesis