From liquid to solid bonding in cohesive granular media

We study the transition of a granular packing from liquid to solid bonding in the course of drying. The particles are initially wetted by a liquid brine and the cohesion of the packing is ensured by capillary forces, but the crystallization of the solute transforms the liquid bonds into partially cemented bonds. This transition is evidenced experimentally by measuring the compressive strength of the samples at regular intervals of times. Our experimental data reveal three regimes: 1) Up to a critical degree of saturation, no solid bonds are formed and the cohesion remains practically constant; 2) The onset of cementation occurs at the surface and a front spreads towards the center of the sample with a nonlinear increase of the cohesion; 3) All bonds are partially cemented when the cementation front reaches the center of the sample, but the cohesion increases rapidly due to the consolidation of cemented bonds. We introduce a model based on a parametric cohesion law at the bonds and a bond crystallization parameter. This model predicts correctly the phase transition and the relation between microscopic and macroscopic cohesion.Comment: 20

Liquid clustering and capillary pressure in granular media

International audienceBy means of extensive lattice Boltzmann simulations, we investigate the process of growth and coalescence of liquid clusters in a granular material as the amount of liquid increases. A homogeneous grain-liquid mixture is obtained by means of capillary condensation, thus providing meaningful statistics on the liquid distribution inside the granular material. The tensile stress carried by the grains as a function of the amount of condensed liquid reveals four distinct states, with a peak stress occurring at the transition from a primary coalescence process, where the cohesive strength is carried mostly by the grains, to a secondary process governed by the increase of the liquid cluster volumes. We show that the evolution of capillary states is correctly captured by a simple model accounting for the competing effects of the Laplace pressure and grain-liquid interface

From pixel to vine parcel: A complete methodology for vineyard delineation and characterization using remote-sensing data

International audienceThe increasing availability of Very High Spatial Resolution images enables accurate digital maps production as an aid for management in the agricultural domain. In this study we develop a comprehensive and automatic tool for vineyard detection, delineation and characterizationusing aerial images and without any parcel plan availability. In France, vineyard training methods in rows or grids generate periodic patterns which make frequency analysis a suitable approach. The proposed method computes a Fast Fourier Transform on an aerial image, providing the delineation of vineyards and the accurate evaluation of row orientation and interrow width. These characteristics are then used to extract individual vine rows, with the aim of detecting missing vine plants and characterizing cultural practices. Using the red channel of an aerial image, 90\% of the parcels have been detected; 92\% have been correctly classified according to their rate of missing vine plants and 81\% according to their cultural practice (weed control method). The automatic process developed can be easily integrated into the final user's Geographical Information System and produces useful information for vineyard managemen

"Bacchus" Methodological approach for vineyard inventory and management. Chap.4: Textural and structural analysis

Ce chapitre présente les méthodes qui ont été développées dans le projet Bacchus pour la détection et la caractérisation des parcelles de vigne en imagerie aérienne en se basant sur leur structure. Une analyse texturale est d'abord mise en oeuvre, et complétée par l'introduction de contraintes de régularité des contours pour améliorer la segmentation. Finalement, les parcelles issues de ces premières étapes sont vérifiées et caractérisées au moyen d'une analyse de leur spectre de Fourier. Les résultats obtenus sur diverses zones d'étude du projet Bacchus sont présentés et discutés. / This chapter presents the methodologies that have been developed during the Bacchus project concerning the automatic detection and characterisation of vineyard plots in satellite and aerial images, based on their structural properties. First, a textural analysis has been used. Then shape regularity constraints have been introduced to improve the image segmentation. Finally, the vineyard plots issued from these previous steps are checked and characterised using a Fourier spectrum analysis. Results on various study areas of the Bacchus project are presented and discussed

An automatized frequency analysis for vine plot detection and delineation in remote sensing

La mise à disposition d'un outil automatique pour la détection et la caractérisation des parcelles de vigne est un besoin très important d'un point de vue gestion. Un procédé automatique récursif basé sur l'analyse fréquentielle (utilisation de la Transformée de Fourier et des filtres de Gabor) a été développé pour y répondre. Il permet la détermination des contours de parcelle et une estimation précise de leur inter-rang et de leur orientation. Dans l'optique d'une application à grande échelle, les tests et la validation ont été menés à partir de données standard de télédétection à très haute résolution.. Environ 89% des parcelles sont détectées qui correspondent à plus de 84 % de la surface viticole, et 64% d'entre elles avec des contours corrects. L'orientation des rangs et la largeur d'inter-rang sont obtenus avec une précision de 1 degré et 3,3 cm respectivement. / The availability of an automatic tool for vine plot detection, delineation, and characterization would be very useful for management purposes. An automatic and recursive process using frequency analysis (with Fourier transform and Gabor filters) has been developed to meet this need. This results in the determination of vine plot boundary and accurate estimation of interrow width and row orientation. To foster large-scale applications, tests and validation have been carried out on standard very high spatial resolution remotely sensed data. About 89% of vine plots are detected corresponding to more than 84% of vineyard area, and 64% of them have correct boundaries. Compared with precise on-screen measurements, vine row orientation and interrow width are estimated with an accuracy of 1°and 3.3 cm, respectively

Effect of particle shape non-convexity on the rheology of granular media : 3D contact dynamics simulations

International audienceWe analyze the effect of particle shape non-convexity on the quasi-static behavior of granular materials by means of contact dynamics simulations. The particles are regular aggregates of four overlapping spheres described by a nonconvexity parameter depending on the relative positions of the particles. Several packings are first submitted to isotropic compression without friction. We find that, as in 2D, the solid fraction of isotropic packings increases with non-convexity up to a maximum value and then declines to be nearly equal to that of a packing composed of only spheres. It is also remarkable that the coordination number increases quickly and saturates so that the packings composed of grains with a high level of nonconvexity are looser but more strongly connected. Then, the quasi-static behavior, structural and force anisotropies are analyzed by subjecting each packing to a triaxial compression. We find that the shear strength increases with non-convexity. We show that this increase results from the presence of multiple contacts between trimers leading to enhanced frictional interlocking

Rheology of three-dimensional packings of aggregates: Microstructure and effects of nonconvexity

International audienceWe use 3D contact dynamics simulations to analyze the rheological properties of granular materials composed of rigid aggregates. The aggregates are made from four overlapping spheres and described by a nonconvexity parameter depending on the relative positions of the spheres. The macroscopic and microstructural properties of several sheared packings are analyzed as a function of the degree of nonconvexity of the aggregates. We find that the internal angle of friction increases with nonconvexity. In contrast, the packing fraction increases first to a maximum value but declines as nonconvexity further increases. At high level of nonconvexity, the packings are looser but show a higher shear strength. At the microscopic scale, the fabric and force anisotropy, as well as friction mobilization are enhanced by multiple contacts between aggregates and interlocking, revealing thus the mechanical and geometrical origins of shear strength

Changes in the starch-protein interface depending on common wheat grain hardness revealed using atomic force microscopy

The Atomic Force Microscope tip was used to progressively abrade the surface of non-cutted starch granules embedded in the endosperm protein matrix in grain sections from wheat near-isogenic lines differing in the puroindoline b gene and thus hardness. In the hard near-isogenic wheat lines, starch granules exhibited two distinct profiles corresponding either to abrasion in the surrounding protein layer or the starch granule. An additional profile, only identified in soft lines, revealed a marked stop in the abrasion at the protein-starch transition similar to a lipid interface playing a lubricant role. It was related to the presence of both wild-type puroindolines, already suggested to act at the starch-protein interface through their association with polar lipids.This study revealed, for the first time, in situ differences in the nano-mechanical properties at the starch-protein interface in the endosperm of wheat grains depending on the puroindoline allelic status

Self-stresses and Crack Formation by Particle Swelling in Cohesive Granular Media

We present a molecular dynamics study of force patterns, tensile strength and crack formation in a cohesive granular model where the particles are subjected to swelling or shrinkage gradients. Non-uniform particle size change generates self-equilibrated forces that lead to crack initiation as soon as strongest tensile contacts begin to fail. We find that the coarse-grained stresses are correctly predicted by an elastic model that incorporates particle size change as metric evolution. The tensile strength is found to be well below the theoretical strength as a result of inhomogeneous force transmission in granular media. The cracks propagate either inward from the edge upon shrinkage and outward from the center upon swelling