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

Un estudio de caso de Landnahme: El sector agroexportador en Chile (1973-1990)

El siguiente artículo utiliza el concepto de Landnahme para analizar el desarrollo del sector agroexportador durante la Dictadura Militar (1973-1990). Es así como se plantea que éste se trata de un proceso eminentemente político que implicó la expansión capitalista a través de la comodificación de la tierra, agua y seguridad social así como la desregulación del mercado del trabajo. Al respecto se propone que estos ámbitos se constituyeron como un “otro no capitalista“ que fueron comodificados o recomodificados e incorporados a la producción capitalista, lo que llevó a la precarización tanto del trabajo como de las condiciones de vida de los asalariados del agro

Planificación, políticas públicas y el proceso decisorio en la gestión gubernamental

Siendo la planificación, la determinación de políticas y la forma de toma de decisiones, la base sobre la cual gira el desarrollo del país, es de suma importancia realizar el análisis de este tema para poder efectuar observaciones, obtener conclusiones y presentar recomendaciones. Con el propósito de entender el proceso decisorio, en el sentido de comprender el grado en que se cumplen las acciones previstas por la planificación de carácter nacional, regional y local, es necesario examinar los aspectos fundamentales del método de planificación tradicional, así como de las innovaciones metodológicas que se están incorporando al proceso de planificación, especialmente por parte del CONADE, como en las oficinas de planificación de ministerios y entidades públicas que forman parte del sistema de planificación. La utilidad del estudio será el presentar un análisis sobre la forma en que se ha venido tratando estos temas durante el último periodo democrático del país; servirá de instrumento y guía para que se enmienden errores, se mejore lo que está bien y se establezca una directriz para el futuro

Capillary deformations of bendable films

We address the partial wetting of liquid drops on ultrathin solid sheets resting on a deformable foundation. Considering the membrane limit of sheets that can relax compression through wrinkling at negligible energetic cost, we revisit the classical theory for the contact of liquid drops on solids. Our calculations and experiments show that the liquid-solid-vapor contact angle is modified from the Young angle, even though the elastic bulk modulus (E) of the sheet is so large that the ratio between the surface tension γ and E is of molecular size. This finding establishes a new type of “soft capillarity” that stems from the bendability of thin elastic bodies rather than from material softness. We also show that the size of the wrinkle pattern that emerges in the sheet is fully predictable, thus resolving a puzzle noticed in several previous attempts to model “drop-on-a-floating-sheet” experiments, and enabling a reliable usage of this setup for the metrology of ultrathin films

Measuring Dislocation Density in Aluminum with Resonant Ultrasound Spectroscopy

Dislocations in a material will, when present in enough numbers, change the speed of propagation of elastic waves. Consequently, two material samples, differing only in dislocation density, will have different elastic constants, a quantity that can be measured using Resonant Ultrasound Spectroscopy. Measurements of this effect on aluminum samples are reported. They compare well with the predictions of the theory.Comment: 4 pages, 2 figure

Characterization of Polyphosphoesters by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

FT-ICR mass spectrometry, together with collision-induced dissociation and electron capture dissociation, has been used to characterize the polyphosphoester poly[1,4-bis(hydroxyethyl)terephthalate-alt-ethyloxyphosphate] and its degradation products. Three degradation pathways were elucidated: hydrolysis of the phosphate–[1,4-bis(hydroxyethyl)terephthalate]bonds; hydrolysis of the phosphate–ethoxy bonds; and hydrolysis of the ethyl–terephthalate bonds. The dominant degradation reactions were those that involved the phosphate groups. This work constitutes the first application of mass spectrometry to the characterization of polyphosphoesters and demonstrates the suitability of high mass accuracy FT-ICR mass spectrometry, with CID and ECD, for the structural analysis of polyphosphoesters and their degradation products

Scaling of the buckling transition of ridges in thin sheets

When a thin elastic sheet crumples, the elastic energy condenses into a network of folding lines and point vertices. These folds and vertices have elastic energy densities much greater than the surrounding areas, and most of the work required to crumple the sheet is consumed in breaking the folding lines or ``ridges''. To understand crumpling it is then necessary to understand the strength of ridges. In this work, we consider the buckling of a single ridge under the action of inward forcing applied at its ends. We demonstrate a simple scaling relation for the response of the ridge to the force prior to buckling. We also show that the buckling instability depends only on the ratio of strain along the ridge to curvature across it. Numerically, we find for a wide range of boundary conditions that ridges buckle when our forcing has increased their elastic energy by 20% over their resting state value. We also observe a correlation between neighbor interactions and the location of initial buckling. Analytic arguments and numerical simulations are employed to prove these results. Implications for the strength of ridges as structural elements are discussed.Comment: 42 pages, latex, doctoral dissertation, to be submitted to Phys Rev

Anomalous strength of membranes with elastic ridges

We report on a simulational study of the compression and buckling of elastic ridges formed by joining the boundary of a flat sheet to itself. Such ridges store energy anomalously: their resting energy scales as the linear size of the sheet to the 1/3 power. We find that the energy required to buckle such a ridge is a fixed multiple of the resting energy. Thus thin sheets with elastic ridges such as crumpled sheets are qualitatively stronger than smoothly bent sheets.Comment: 4 pages, REVTEX, 3 figure

Collaboration and Decision Making in Crisis Situations

[EN] Emergencies are critical situations that demand immediate action to avoid adverse consequences to life and property. Recent events around the world highlight the importance of the theme. A key challenge in Emergency Management is decision-making under time pressure, with an overload of unconfirmed, uncertain and conflicting information, including the management of many people, with distinct and possibly fluid roles, in different places. Collaboration in these settings is an interesting element, since emergency response generally involves multiple agencies and the public, which, having different views, protocols and priorities, must act in concert to handle the situation. In addition, an increasing amount of virtual information is necessary to inform and manage volunteers. The goal of this workshop is to identify and map the main challenges of collaboration in crisis situations, review current research methods and approaches to address them and address the lack of formal processes, structures, methodologies and tools.Adriana S Vivacqua is partially supported by CNPq grant 308425/2012-0 and FAPERJ grant E26/103.029/2012. Vaninha Vieira is partially supported by CNPq grant 490084/2013-3Vivacqua, AS.; Garcia, ACB.; Canos Cerda, JH.; Comes, M.; Vieira, V. (2016). Collaboration and Decision Making in Crisis Situations. ACM. 503-508. https://doi.org/10.1145/2818052.2855520S50350