A prototypical model for tensional wrinkling in thin sheets

The buckling and wrinkling of thin films has recently seen a surge of interest among physicists, biologists, mathematicians and engineers. This has been triggered by the growing interest in developing technologies at ever decreasing scales and the resulting necessity to control the mechanics of tiny structures, as well as by the realization that morphogenetic processes, such as the tissue-shaping instabilities occurring in animal epithelia or plant leaves, often emerge from mechanical instabilities of cell sheets. While the most basic buckling instability of uniaxially compressed plates was understood by Euler more than 200 years ago, recent experiments on nanometrically thin (ultrathin) films have shown significant deviations from predictions of standard buckling theory. Motivated by this puzzle, we introduce here a theoretical model that allows for a systematic analysis of wrinkling in sheets far from their instability threshold. We focus on the simplest extension of Euler buckling that exhibits wrinkles of finite length - a sheet under axisymmetric tensile loads. This geometry, whose first study is attributed to Lam´e, allows us to construct\ud a phase diagram that demonstrates the dramatic variation of wrinkling patterns from near-threshold to far-from-threshold conditions. Theoretical arguments and comparison to experiments show that for thin sheets the far-from-threshold regime is expected to emerge under extremely small compressive loads, emphasizing the relevance of our analysis for nanomechanics applications

Optimal Taxation With Heterogeneous Firms

We study steady state optimal taxation in a context where firms differ in productivity and they decide whether to produce or not after comparing after-tax profits vis-à-vis an outside alternative option. The government taxes capital income, firms’ profits and labor income, but does not tax the alternative outside option. In this context, taxation might distort the firms’ decisions to participate in production (extensive margin) as well as their factor allocations once they decide to produce (intensive margin). We find that the government has incentives to subsidize costs to induce firms into production. The optimal capital income tax is negative while the corporate tax rate is positive and the sign of labor income tax is ambiguous.

Urban policy in Barcelona: what they make us believe, but we do not believe.

Urban policies undoubtedly play a key role in addressing the current problems afflicting the world's population, considering that it is heavily concentrated in cities. Placing look at the reality of Barcelona and its major urban reforms, it is suggested here a reflection on the impacts of urban model in the lives of its inhabitants, leaving raised some questions such as what is the real city that extends around Barcelona offered internationally as a paradigm of growth and organization, is it sustainable growth model promoted by the administration, how its people actually live and how it has affected this model in your daily life? In the following paragraphs, this reflection will be developed considering three interlinked issues: the preeminence of a commodified view of the city, understood as an economic space, the imposition of urban solutions in line with this logic and the discursive use of concepts such as sustainability or participation to legitimize urban projects of great social and environmental impact

Comunicación Bilateral de los Nervios Músculocutáneo y Mediano. Aspectos Morfológicos e Importancia Clínica

Cerda, A (Cerda, Aladino). Univ Talca, Talca, ChileCommunication of the musculocutaneous and median nerves of the brachial plexus is common and is important in the evaluation of trauma and surgical procedures in axillary and brachial regions. This paper presents a case in which during a routine dissection of upper limb the presence of bilateral communication between musculocutaneous and medium nerves was observed. Trajectory of communicating branches was described, relation and morphometry was discussed with regard to prevalence and clinical significance of these communications

Magneto-elastic oscillations of neutron stars: exploring different magnetic field configurations

We study magneto-elastic oscillations of highly magnetized neutron stars (magnetars) which have been proposed as an explanation for the quasi-periodic oscillations (QPOs) appearing in the decaying tail of the giant flares of soft gamma-ray repeaters (SGRs). We extend previous studies by investigating various magnetic field configurations, computing the Alfv\'en spectrum in each case and performing magneto-elastic simulations for a selected number of models. By identifying the observed frequencies of 28 Hz (SGR 1900+14) and 30 Hz (SGR 1806-20) with the fundamental Alfv\'en QPOs, we estimate the required surface magnetic field strength. For the magnetic field configurations investigated (dipole-like poloidal, mixed toroidal-poloidal with a dipole-like poloidal component and a toroidal field confined to the region of field lines closing inside the star, and for poloidal fields with an additional quadrupole-like component) the estimated dipole spin-down magnetic fields are between 8x10^14 G and 4x10^15 G, in broad agreement with spin-down estimates for the SGR sources producing giant flares. A number of these models exhibit a rich Alfv\'en continuum revealing new turning points which can produce QPOs. This allows one to explain most of the observed QPO frequencies as associated with magneto-elastic QPOs. In particular, we construct a possible configuration with two turning points in the spectrum which can explain all observed QPOs of SGR 1900+14. Finally, we find that magnetic field configurations which are entirely confined in the crust (if the core is assumed to be a type I superconductor) are not favoured, due to difficulties in explaining the lowest observed QPO frequencies (f<30 Hz).Comment: 21 pages, 16 figures, 6 tables, matched to version accepted by MNRAS with extended comparison/discussion to previous wor

Tomato plant growth as affected by horizontally unequal osmotic concentrations in rock-wool.

Tomato plants, cv. Moneydor, were grown in rockwool in a split-root system with equal or different osmotic concentrations. Fruit yield was negatively correlated with the mean electrical conductivity (EC) of both parts of the system. In treatments with two different EC values in the root zone, root development was better in the part with the low EC, and water uptake was higher. Nutrient concentrations showed an increase in the part with the low EC when differences in EC between both parts were 4 mS/cm [4 mmho/cm] or more. A possible explanation is that solutes move through the roots from the part with high to the part with the low osmotic concentration. (Abstract retrieved from CAB Abstracts by CABI’s permission

The optimal P3M algorithm for computing electrostatic energies in periodic systems

We optimize Hockney and Eastwood's Particle-Particle Particle-Mesh (P3M) algorithm to achieve maximal accuracy in the electrostatic energies (instead of forces) in 3D periodic charged systems. To this end we construct an optimal influence function that minimizes the RMS errors in the energies. As a by-product we derive a new real-space cut-off correction term, give a transparent derivation of the systematic errors in terms of Madelung energies, and provide an accurate analytical estimate for the RMS error of the energies. This error estimate is a useful indicator of the accuracy of the computed energies, and allows an easy and precise determination of the optimal values of the various parameters in the algorithm (Ewald splitting parameter, mesh size and charge assignment order).Comment: 31 pages, 3 figure