Plasticity and aging of folded elastic sheets.

We investigate the dissipative mechanisms exhibited by creased material sheets when subjected to mechanical loading, which comes in the form of plasticity and relaxation phenomena within the creases. After demonstrating that plasticity mostly affects the rest angle of the creases, we devise a mapping between this quantity and the macroscopic state of the system that allows us to track its reference configuration along an arbitrary loading path, resulting in a powerful monitoring and design tool for crease-based metamaterials. Furthermore, we show that complex relaxation phenomena, in particular memory effects, can give rise to a nonmonotonic response at the crease level, possibly relating to the similar behavior reported for crumpled sheets. We describe our observations through a classical double-logarithmic time evolution and obtain a constitutive behavior compatible with that of the underlying material. Thus the lever effect provided by the crease allows magnified access to the material's rheology.

Foldable cones as a framework for nonrigid origami.

The study of origami-based mechanical metamaterials usually focuses on the kinematics of deployable structures made of an assembly of rigid flat plates connected by hinges. When the elastic response of each panel is taken into account, novel behaviors take place, as in the case of foldable cones (f-cones): circular sheets decorated by radial creases around which they can fold. These structures exhibit bistability, in the sense that they can snap through from one metastable configuration to another. In this work, we study the elastic behavior of isometric f-cones for any deflection and crease mechanics, which introduce nonlinear corrections to a linear model studied previously. Furthermore, we test the inextensibility hypothesis by means of a continuous numerical model that includes both the extended nature of the creases, stretching and bending deformations of the panels. The results show that this phase-field-like model could become an efficient numerical tool for the study of realistic origami structures.

Local mechanical description of an elastic fold.

To go beyond the simple model for the fold as two flexible surfaces or faces linked by a crease that behaves as an elastic hinge, we carefully shape and anneal a crease within a polymer sheet and study its mechanical response. First, we carry out an experimental study that involves recording both the shape of the fold in various loading configurations and the associated force needed to deform it. Then, an elastic model of the fold is built upon a continuous description of both the faces and the crease as a thin sheet with a non-flat reference configuration. The comparison between the model and experiments yields the local fold properties and explains the significant differences we observe between tensile and compression regimes. Furthermore, an asymptotic study of the fold deformation enables us to determine the local shape of the crease and identify the origin of its mechanical behaviour.

Potential Drug-Drug Interactions with Antimicrobials in Hospitalized Patients: A Multicenter Point-Prevalence Study.

BACKGROUND Improper use of antimicrobials can cause adverse drug events and high costs. The purpose of this study was to investigate the frequency and potential drug-drug interactions associated with antimicrobials among hospitalized patients. MATERIAL AND METHODS This study was conducted on the same day in 5 different hospitals in Turkey. We included patients aged ³18 years who received at least 1 antimicrobial drug and at least 1 of any other drug. The Micromedex® online drug reference system was used to control and describe the interactions. Drug interactions were classified as contraindicated, major, moderate, and minor. RESULTS Potential drug-drug interactions with antimicrobials were 26.4% of all interactions. Five (42%) of 12 contraindicated interactions and 61 (38%) of 159 major interactions were with antimicrobials. Quinolones, triazoles, metronidazole, linezolid, and clarithromycin accounted for 173 (25.7%) of 673 prescribed antimicrobials, but were responsible for 141 (92.1%) of 153 interactions. In multivariate analysis, number of prescribed antimicrobials (odds ratio: 2.3001, 95% CI: 1.6237-3.2582), number of prescribed drugs (odds ratio: 1.2008, 95% CI: 1.0943-1.3177), and hospitalization in the university hospital (odds ratio: 1.7798, 95% CI: 1.0035-3.1564) were independent risk factors for developing drug interactions. CONCLUSIONS Due to risk of drug interactions, physicians should be more cautious when prescribing antimicrobials, particularly when prescribing quinolones, linezolid, azoles, metronidazole, and macrolides.

Elastic theory of origami-based metamaterials.

Origami offers the possibility for new metamaterials whose overall mechanical properties can be programed by acting locally on each crease. Starting from a thin plate and having knowledge about the properties of the material and the folding procedure, one would like to determine the shape taken by the structure at rest and its mechanical response. In this article, we introduce a vector deformation field acting on the imprinted network of creases that allows us to express the geometrical constraints of rigid origami structures in a simple and systematic way. This formalism is then used to write a general covariant expression of the elastic energy of n-creases meeting at a single vertex. Computations of the equilibrium states are then carried out explicitly in two special cases: the generalized waterbomb base and the Miura-Ori. For the waterbomb, we show a generic bistability for any number of creases. For the Miura folding, however, we uncover a phase transition from monostable to bistable states that explains the efficient deployability of this structure for a given range of geometrical and mechanical parameters. Moreover, the analysis shows that geometric frustration induces residual stresses in origami structures that should be taken into account in determining their mechanical response. This formalism can be extended to a general crease network, ordered or otherwise, and so opens new perspectives for the mechanics and the physics of origami-based metamaterials.

Inverse Leidenfrost Effect: Levitating Drops on Liquid Nitrogen.

We explore the interaction between a liquid drop (initially at room temperature) and a bath of liquid nitrogen. In this scenario, heat transfer occurs through film-boiling: a nitrogen vapor layer develops that may cause the drop to levitate at the bath surface. We report the phenomenology of this inverse Leidenfrost effect, investigating the effect of the drop size and density by using an aqueous solution of a tungsten salt to vary the drop density. We find that (depending on its size and density) a drop either levitates or instantaneously sinks into the bulk nitrogen. We begin by measuring the duration of the levitation as a function of the radius R and density ρd of the liquid drop. We find that the levitation time increases roughly linearly with drop radius but depends weakly on the drop density. However, for sufficiently large drops, R ≥ Rc(ρd), the drop sinks instantaneously; levitation does not occur. This sinking of a (relatively) hot droplet induces film-boiling, releasing a stream of vapor bubbles for a well-defined length of time. We study the duration of this immersed-drop bubbling finding similar scalings (but with different prefactors) to the levitating drop case. With these observations, we study the physical factors limiting the levitation and immersed-film-boiling times, proposing a simple model that explains the scalings observed for the duration of these phenomena, as well as the boundary of (R,ρd) parameter space that separates them.

Generic Bistability in Creased Conical Surfaces.

The emerging field of mechanical metamaterials has sought inspiration in the ancient art of origami as archetypal deployable structures that carry geometric rigidity, exhibit exotic material properties, and are potentially scalable. A promising venue to introduce functionality consists in coupling the elasticity of the sheet and the kinematics of the folds. In this spirit, we introduce a scale-free, analytical description of a very general class of snap-through, bistable patterns of creases naturally occurring at the vertices of real origami that can be used as building blocks to program and actuate the overall shape of the decorated sheet. These switches appear at the simplest possible level of creasing and admit straightforward experimental realizations.

Mechanical response of a creased sheet.

We investigate the mechanics of thin sheets decorated by noninteracting creases. The system considered here consists of parallel folds connected by elastic panels. We show that the mechanical response of the creased structure is twofold, depending both on the bending deformation of the panels and the hingelike intrinsic response of the crease. We show that a characteristic length scale, defined by the ratio of bending to hinge energies, governs whether the structure's response consists in angle opening or panel bending when a small load is applied. The existence of this length scale is a building block for future works on origami mechanics.

Tuning the ordered states of folded rods by isotropic confinement.

The packing of elastic objects is increasingly studied in the framework of out-of-equilibrium statistical mechanics and thus these appear to be similar to glassy systems. Here, we present a two-dimensional experiment whereby a rod is confined by a parabolic potential. The setup enables spanning a wide range of folded configurations of the rod. Measurements of the distributions of length and curvature in the system reveal the importance of a stacking process whereby many layers of the rod are grouped into branches. The geometrical order of patterns increases with the confinement strength. Measurements of the distributions of energies lead to the definition of an energy scale that is correlated with the elastic energy of the stacked parts of the rod. This scale imposes energy partition in the system and might be relevant to the framework of the thermodynamics of disordered systems. Following these observations, we describe the patterns as excited states of a ground state corresponding to the most ordered geometry. Eventually, we provide evidence that the disordered state of a folded rod becomes spontaneously closer to the ground state as confinement is increased.

Stability and roughness of tensile cracks in disordered materials.

We study the stability and roughness of propagating cracks in heterogeneous brittle two-dimensional elastic materials. We begin by deriving an equation of motion describing the dynamics of such a crack in the framework of linear elastic fracture mechanics, based on the Griffith criterion and the principle of local symmetry. This result allows us to extend the stability analysis of Cotterell and Rice [B. Cotterell and J. R. Rice, Int. J. Fract. 16, 155 (1980)] to disordered materials. In the stable regime we find stochastic crack paths. Using tools of statistical physics, we obtain the power spectrum of these paths and their probability distribution function and conclude that they do not exhibit self-affinity. We show that a real-space fractal analysis of these paths can lead to the wrong conclusion that the paths are self-affine. To complete the picture, we unravel a systematic bias in such real-space methods and thus contribute to the general discussion of reliability of self-affine measurements.

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 indicates a new elastocapillary phenomenon that stems from the high bendability of very thin elastic sheets 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 in modeling "drop-on-a-floating-sheet" experiments and enabling a quantitative, calibration-free use of this setup for the metrology of ultrathin films.

Comparative study of crumpling and folding of thin sheets.

Crumpling and folding of paper are at first sight very different ways of confining thin sheets in a small volume: the former one is random and stochastic whereas the latest one is regular and deterministic. Nevertheless, certain similarities exist. Crumpling is surprisingly inefficient: a typical crumpled paper ball in a waste-bin consists of as much as 80% air. Similarly, if one folds a sheet of paper repeatedly in two, the necessary force becomes so large that it is impossible to fold it more than six or seven times. Here we show that the stiffness that builds up in the two processes is of the same nature, and therefore simple folding models allow us to capture also the main features of crumpling. An original geometrical approach shows that crumpling is hierarchical, just as the repeated folding. For both processes the number of layers increases with the degree of compaction. We find that for both processes the crumpling force increases as a power law with the number of folded layers, and that the dimensionality of the compaction process (crumpling or folding) controls the exponent of the scaling law between the force and the compaction ratio.

Fractographic aspects of crack branching instability using a phase-field model.

A phase-field model of a crack front propagating in a three-dimensional brittle material is used to study the fractographic patterns induced by the branching instability. The numerical results of this model give rise to crack surfaces that are similar to those obtained in various experimental situations. Depending on applied loading configurations and initial conditions, we show that the branching instability is either restricted to a portion of the crack front or revealed through quasi-two-dimensional branches. For the former, the crack front leaves on the main broken surface either aligned or disordered parabolic marks. For the latter, fractography reveals the so called échelon cracks showing that branching instability can also induce crack front fragmentation.

Crack front dynamics across a single heterogeneity.

We study the spatiotemporal dynamics of a crack front propagating at the interface between a rigid substrate and an elastomer. We first characterize the kinematics of the front when the substrate is homogeneous and find that the equation of motion is intrinsically nonlinear. We then pattern the substrate with a single defect. Steady profiles of the front are well described by a standard linear theory with nonlocal elasticity, except for large slopes of the front. In contrast, this theory seems to fail in dynamical situations, i.e., when the front relaxes to its steady shape, or when the front pinches off after detachment from a defect. More generally, these results may impact the current understanding of crack fronts in heterogeneous media.

Relaxation mechanisms in the unfolding of thin sheets.

When a thin sheet is crumpled, creases form in which plastic deformations are localized. Here we study experimentally the relaxation process of a single fold in a thin sheet subjected to an external strain. The unfolding process is described by a quick opening at first and then a progressive slow relaxation of the crease. In the latter regime, the necessary force needed to open the folded sheet at a given displacement is found to decrease logarithmically in time, allowing its description through an Arrhenius activation process. We accurately determine the parameters of this law and show its general character by performing experiments on both Mylar and paper sheets.

Finite-distance singularities in the tearing of thin sheets.

We investigate the interaction between two cracks propagating quasistatically in a thin sheet. Two different experimental geometries allow us to tear sheets by imposing an out-of-plane shear loading. A single tear propagates in a straight line independently of its position in the sheet. In contrast, we find that two tears converge along self-similar paths and annihilate each other. These finite-distance singularities display geometry-dependent similarity exponents, which we retrieve using scaling arguments based on a balance between the stretching and the bending of the sheet close to the tips of the cracks.

Bending waves in crumpled sheets.

Crumpled paper has recently emerged as a model for disordered media. Here we use wave propagation to probe aluminum foils crumpled into balls made by hand or into cylinders obtained by confinement in a container. Surprisingly, the raw dispersion relations appear to differ from sample to sample. They correspond to bending waves that follow an effective path that is shorter than the distance between the input and output points. This can be interpreted in terms of two modes of propagation: slow bending waves and a fast mode whose possible origin is discussed. In addition, the effective paths behave differently in spheres and in cylinders. These results enable the characterization of the sample structure and point toward the geometric rigidity of the configurations.

Contribución de la etapa de secado a la maduración de la longaniza crudo-curada fermentada / Contribution of drying stage to the ripening of dry-cured fermented Longaniza

El objetivo fue estudiar el grado de maduración alcanzado por una Longaniza crudo-curada fermentada,elaborada con carne de cerdo en tripa natural y sometida a un secado industrial estándar (12 días a 15ºC/85-65%HR). Se determinaron diversos parámetros (composición, mermas, actividad de agua, pH, acidez total, colorCIELab, proteolisis, lipolisis, TBARS y ácidos grasos) y atributos sensoriales (enrojecimiento, olor y sabor,acidez, fi rmeza) en Longaniza cruda y curada. El tratamiento de secado produjo una merma de peso del 30%por unidad de embutido y permitió alcanzar un adecuado grado de deshidratación, acidifi cación y enrojecimiento,auxiliado por la adición de cultivos iniciadores y de colorante Rojo. Se obtuvo una Longaniza fi rme al corte.Se alcanzaron poblaciones efectivas de micrococáeas y bacterias ácido-lácticas, que proporcionaron seguridadmicrobiológica y una apreciable acidez. Sin embargo, la trasformación de lípidos y proteínas no tuvo magnitudsufi ciente para desarrollar un marcado aroma y sabor a carne crudo-curada, no apreciándose matices aromáticospropios de la oxidación de la grasa, debido en parte a la adición de antioxidantes. La calidad sensorial de Longanizaseco-curada estaría limitada por el lento desarrollo del aroma y sabor durante la etapa de secado(AU)

The objective was to study ripening of dry-cured fermented Longaniza, a small calibre Salami manufacturedwith pork, stuffed into natural casing and dried following a standard industrial process (12 days at 15 º C/85-65% RH). Several parameters (composition, shrinkage, water activity, pH, total acidity, CIELab colour,proteolysis, lipolysis, TBARS and fatty acids) and sensory attributes (redness, odour and fl avour, acidity, fi rmness) were determined in raw and cured samples. The drying process produced weight loss of 30% perunit of sausage and allowed to achieve an adequate degree of dehydration, acidifi cation and reddening, aidedby the addition of starter cultures and artifi cial red colouring. Longaniza presented suffi cient strength to besliced. Effective populations of Micrococaeae and lactic-acid bacteria were achieved, providing signifi cantmicrobiological safety and acidity. However, the transformation of lipids and proteins was not intense enough todevelop a pronounced dry-cured meat aroma and taste, since no fat-oxidation aromatic traits were found. Thismay be partly due to the addition of antioxidants. Sensory quality of Longaniza would be limited by the slowdevelopment of aroma and fl avor during the drying stage(AU)

Solution of the Percus-Yevick equation for hard hyperspheres in even dimensions.

We solve the Percus-Yevick equation in even dimensions by reducing it to a set of simple integrodifferential equations. This work generalizes an approach we developed previously for hard disks. We numerically obtain both the pair correlation function and the virial coefficients for a fluid of hyperspheres in dimensions d = 4, 6, and 8, and find good agreement with the available exact results and Monte Carlo simulations. This paper confirms the alternating character of the virial series for d > or = 6 and provides the first evidence for an alternating character for d = 4. Moreover, we show that this sign alternation is due to the existence of a branch point on the negative real axis. It is this branch point that determines the radius of convergence of the virial series, whose value we determine explicitly for d = 4, 6, 8. Our results complement, and are consistent with, a recent study in odd dimensions [R. D. Rohrmann et al., J. Chem. Phys. 129, 014510 (2008)].

Solution of the Percus-Yevick equation for hard disks.

The authors solve the Percus-Yevick equation in two dimensions by reducing it to a set of simple integral equations. They numerically obtain both the pair correlation function and the equation of state for a hard disk fluid and find good agreement with available Monte Carlo results. The present method of resolution may be generalized to any even dimension.