Elastic behavior in Contact Dynamics of rigid particles

The systematic errors due to the practical implementation of the Contact Dynamics method for simulation of dense granular media are examined. It is shown that, using the usual iterative solver to simulate a chain of rigid particles, effective elasticity and sound propagation with a finite velocity occur. The characteristics of these phenomena are investigated analytically and numerically in order to assess the limits of applicability of this simulation method and to compare it with soft particle molecular dynamics.Comment: submitted to PRE, 7 pages, 6 figure

Particle shape dependence in 2D granular media

Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter $\eta$ describing the degree of distortion from a perfectly spherical shape. Encompassing most specific shape characteristics such as elongation, angularity and nonconvexity, $\eta$ is a low-order but generic parameter that we used in a numerical benchmark test for a systematic investigation of shape-dependence in sheared granular packings composed of particles of different shapes. We find that the shear strength is an increasing function of $\eta$ with nearly the same trend for all shapes, the differences appearing thus to be of second order compared to $\eta$. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with $\eta$ from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as $\eta$ is further increased. These findings suggest that a low-order description of particle shape accounts for the principal trends of packing fraction and shear strength. Hence, the effect of second-order shape parameters may be investigated by considering different shapes at the same level of $\eta$.Comment: 5 pages, 8 figure

Sunglint images of current gradients at high resolution: Critical angle and directional observing strategy

High-resolution satellite images of the ocean surface in and around the sunglint often provide unique observations of sub-mesoscale upper ocean surface processes. Local anomalies of wind, waves, currents or surfactants appear on the images as local anomalies of brightness. A quantitative interpretation of those brightness anomalies must relate them to slope properties of the wave field, which are to the lowest order described by the mean square slope (mss). The prevailing paradigm for such interpretation is that of the critical zenith angle. It states that, for subcritical zenith view angle, brightness and mss anomalies have opposite signs, and this defines the so-called inversion region. This prevailing paradigm implicitly builds on the assumption that the mss decomposition between upwind and crosswind components is conserved. The mss anomalies are then isotropic and can be reduced to a scalar (i.e. one-dimensional) quantity. In such a case, one single sunglint image would be sufficient to retrieve the mss anomaly. This isotropic case likely applies for surface wave changes induced by varying wind speed or by surfactants. Yet, satellite and airborne observations at multiple view angles recently revealed anisotropic mss anomalies, e.g. with mss increase in the upwind direction and decrease in the crosswind direction. This anisotropic behavior likely characterizes wave modulations by anisotropic surface current gradients. This paper details the expected properties of such anisotropic mss modulations. It is shown that: 1) The classical concept of critical angle does not systematically hold, neither for frontal current shear nor for internal wave divergence. 2) At least two sunglint images at different zenith and azimuth angles are needed to retrieve the mss anomalies, and a single observation is not sufficient. 3) A satellite with radiometers looking at multiple zenith angles is capable of providing a geometry favorable to retrieve mss anomalies. An illustration is discussed with internal waves observed by the Multi-angle Imaging SpectroRadiometer (MISR), where the upwind and crosswind components of the retrieved mss anomalies are anisotropic. Those results provide guidelines to interpret available observations and to help refine strategy for future satellite missions

PlantGL: a Python-based geometric library for 3D plant modelling at different scales

In this paper, we present PlantGL, an open-source graphic toolkit for the creation, simulation and analysis of 3D virtual plants. This C++ geometric library is embedded in the Python language which makes it a powerful user-interactive platform for plant modeling in various biological application domains. PlantGL makes it possible to build and manipulate geometric models of plants or plant parts, ranging from tissues and organs to plant populations. Based on a scene graph augmented with primitives dedicated to plant representation, several methods are provided to create plant architectures from either field measurements or procedural algorithms. Because they are particularly useful in plant design and analysis, special attention has been paid to the definition and use of branching system envelopes. Several examples from different modelling applications illustrate how PlantGL can be used to construct, analyse or manipulate geometric models at different scales ranging from tissues to plant communities

Human dentin characteristics of patients with osteogenesis imperfecta: insights into collagen-based biomaterials

International audienceOsteogenesis imperfecta (OI), also known as "brittle bone disease", is a rare genetic disorder of the skeleton, whose most benign form I corresponds to autosomal dominant mutations in the genes encoding type I collagen (COLA1, COLA2). Several associated skeletal manifestations are often observed but, surprisingly, while dentin defects often reflect genetic bone disorders, about half of OI patients have no obvious oral manifestations. Here, we investigated the collagen, mineral and mechanical properties of dentin from deciduous teeth collected from patients with mild form of OI and displaying no obvious clinical signs of dentinogenesis imperfecta. For the first time, an increase in the hardness of OI dentin associated with an increase in mineral content compared to healthy patients was reported. In addition, OI altered the tissue characteristics of the dentin-enamel junction but the interfacial gradient was preserved. The impact of changes in molecular structure due to mutations in OI was assessed by Raman microspectroscopy. Our results highlighted a change in the hydroxyproline-proline ratio in direct association with collagen mineralization. Our findings suggest that the evaluation of teeth could be an important aid for mild types of OI that are often difficult to diagnose clinically and provide experimental evidence that hydroxyproline content should be considered in future studies on collagen-based biomaterials

Microbial carbon and nitrogen production under experimental conditions combining warming with increased ultraviolet-B radiation in Mediterranean coastal waters

The effects of warming and increased ultraviolet-B radiation (OVER, 280-320 nm) have been rarely studied at food web scale and very few studies have considered the effect of combining these two climatic stressors. Microbial carbon and nitrogen dynamics were studied under the single and combined conditions of +3 degrees C warming and +20% UVBR above the natural levels (control) during a 10-day mesocosm experiment in coastal Mediterranean waters. The effect of increased UVBR on primary production (PP) and bacterial production (BP) rates was rarely significant during the experiment. Warming alone or combined with increased UVBR significantly reduced BP by about 30% but also significantly increased PP by an average of 90%. No accumulation of particulate organic matter was observed during the experiment but, in the warmed mesocosms, the cumulative carbon and nitrogen losses were greater (ca. +40%). The main short-term consequence of warming was, therefore, a shift of the food web dynamics leading to higher C and N losses. This suggests a more efficient transfer of the newly produced microbial production to the upper trophic levels and a greater exportation into deeper waters through settlement under warmer conditions in Mediterranean coastal waters in the future