Conservação e restauro de uma urna em vidro do século I d.C., encontrada em Mértola (Portugal)

International audienceThe evolution of capillary forces during evap-oration and the corresponding changes in the geometrical characteristics of liquid (water) bridges between two glass spheres with constant separation are examined experimen-tally. For comparison, the liquid bridges were also tested for mechanical extension (at constant volume). The obtained results reveal substantial differences between the evolution of capillary force due to evaporation and the evolution due to extension of the liquid bridges. During both evaporation and extension, the change of interparticle capillary forces consists in a force decrease to zero either gradually or via rupture of the bridge. At small separations between the grains (short & wide bridges) during evaporation and at large volumes during extension, there is a slight initial increase of force. During evaporation, the capillary force decreases slowly at the begin-ning of the process and quickly at the end of the process; during extension, the capillary force decreases quickly at the beginning and slowly at the end of the process. Rup-ture during evaporation of the bridges occurs most abruptly for bridges with wider separations (tall and thin), sometimes occurring after only 25 % of the water volume was evapo-rated. The evolution (pinning/depinning) of two geometri-cal characteristics of the bridge, the diameter of the three-phase contact line and the "apparent" contact angle at the solid/liquid/gas interface, seem to control the capillary force evolution. The findings are of relevance to the mechanics of unsaturated granular media in the final phase of drying

Evolution and failure of liquid bridges between grains due to evaporation and due to extension

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Laplace pressure evolution and four instabilities in evaporating two-grain liquid bridges

International audienceDynamic variables characterizing evolution during evaporation of capillary bridge between two spheres are analyzed. The variables include: average Laplace pressure, pressure resulting force, surface tension force and total capillary force calculated based on the previously reported geometrical variables using Young-Laplace law [1,2]. This is the first time to our knowledge that Laplace pressure is calculated from the measured bridge curvatures along the process of evaporation and compared to experimental measurement data. A comparison with the experimental data from analogous capillary bridge extension tests is also shown and discussed. The behavior of evaporating liquid bridges is seen as strongly dependent on the grain separation. Initial negative Laplace pressure at small separations is seen to significantly augment during an advanced stage of evaporation, but to turn into positive pressure, after an instability toward the end of the process, and prior to rupture. At larger separations the pressure is positive all the time, changing a little, but rupturing early. Rupture in all cases occurs at positive pressure. However, because of the evolution of the surface area of contact, the resultant total capillary forces are always tensile, and decreasing toward zero in all cases. Comparison between measured total resultant capillary forces and those calculated from the Young-Laplace law is very good, except for some discrepancies at very small separations (below 50 μm). Up to four consecutive instabilities of capillary bridge are seen developing at some sphere separations. They are: re-pinning-induced suction (pressure) instability; Rayleigh nodoid/catenoid/unduloid unstable transition, associated with zero-pressure; Rayleigh unduloid/cylinder unstable transition, associated with the formation of a liquid-wire; and lastly, a pinching instability of the liquid-wire, associated with the bridge rupture. Rupture of the bridges is seen at large separations to occur quite early, at only 1/4-1/3 of the initial water volume evaporated. At smallest separations, rupture occurs in a seemingly unstable way when water evaporates from the bridge thinnest section of the neck

Analysis of capillary bridges using imaging techniques and recent analytical model

National audienceMechanical properties of non-saturated granular materials are directly connected with capillary interactions, resultingfrom the presence of water between grains. In this paper, we use the recent analytical model obtained in [GAG 14]to analyze capillary interactions at the scale of liquid bridge between two spherical grains. Geometrical profiles of capillarybridges are obtained resolving Young-Laplace equation, considered as an inverse problem, with unknown capillary pressure.Data of contact angle, half-filling angle and gorge radius, determined experimentally by image processing, allow to calculateall the data associated with capillary bridge (form of bridge profile, Laplace pressure, capillary force). Results of theoreticalmodeling match very accurately experimental ones at small volumes and/or small separation distances between grains, wheninfluence of gravity is limited. However, for larger liquid volumes and/or larger separation distances between grains the influenceof the gravity is manifested as a distortion of capillary bridge. To avoid the gravity influence, experimental tests wererealized in micro-gravity conditions (parabolic flight). For these tests, theoretical results are in good agreement with experimentalones, independently of liquid volumes and/or separations distances between grains.Les propriétés mécaniques des matériaux granulaires non saturés sont directement liées aux interactions capillairesrésultant de la présence d’eau entre les grains. Dans ce l’article, le modèle analytique récent obtenu dans [GAG 14] est utilisépour analyser les interactions capillaires à l’échelle du pont liquide entre deux grains sphériques. Les profils géométriques desponts capillaires sont obtenus en résolvant l’équation de Young-Laplace considérée comme un problème inverse, la pressioncapillaire étant inconnue. La donnée de l’angle de remplissage, de l’angle de mouillage et du rayon de gorge, qui sont déterminésexpérimentalement par traitement d’image, permet de calculer toutes les données associées au pont capillaire (forme de laméridienne, volume du pont capillaire, force capillaire). Les résultats de la modélisation sont en bon accord avec les résultatsexpérimentaux dans le cas de petits volumes d’eau et/ou de petites distances entre les grains, où l’influence de la gravité estlimitée. En revanche, pour des volumes d’eau plus importants et/ou des distances entre les grains plus grandes, l’influence de lagravité induit une distorsion du pont capillaire. Pour s’affranchir de la gravité, des essais ont été réalisés dans des conditionsde micro-gravité (vol parabolique). Pour ces essais, les résultats théoriques et expérimentaux sont en bon accord, indépendammentdes volumes de liquide et/ou des distances entre grains

Effect of water on granular matter mechanics, local scale: evaporation, extension and rupture of liquid bridges

International audienceThe mechanical strength and cohesion of granular matter depend strongly on water content (i.e. in soil). Water exists in different forms between the solid particles, due to surface tension generates internal stresses in granular material and develops mass exchange with the environment during the evaporation process. During evaporation the internal stress evolves, which may lead to the medium shrinkage, air entry and damage. The most sensitive is the final stage of evaporation, which corresponds to the rupture of capillary bridges (Péron et al., 2010). In this study, the phenomenon of air entry, evolution of intergranular forces and behavior and rupture of capillary bridges are analyzed experimentally at the local scale, on the example of capillary bridges between two and three spherical grains

Modes of capillary bridge evolution during evaporation

The aim of the study is to identify the mechanisms and critical variables that control the evolution of the water body within capillary bridges during evaporation process. The initial series of experiments has been focused on the changes of the liquid/gas interface forming during desiccation between small clusters of two or three grains, constructed of precision glass spheres with diameter of 8 mm, and linked by a single multi-branch liquid bridge. The beads are fixed in a single plane, the space between them is filled with deionized water and exposed to room temperature evaporation in controlled conditions. Different distances between the spheres are considered. The capillary forces and the mass of the evaporating water are measured in time. The evolving configuration of the liquid bridge is recorded with digital cameras. It is hence possible to correlate bridge evolution with changes of capillary forces and to determine precisely geometrical properties of bridge (contact angle, meniscus radii, areas). A significant change in bridge geometry and its rupture occurs in several different modes. To observe the moment of rupture, a highspeed digital camera with 27000 fps was used. For two-spheres configuration the rupture of the liquid bridge resembles breaking of a ductile/brittle rod (for example steel rod) during axial extension tests, when the strength of material is exceeded. The capillary bridge evolves from a form of a thinning anvil with receding lateral meniscus surfaces, into an extremely thin water-wire, which is eventually ruptured. The capillary forces decrease monotonically during the process until the rapture, associated with a force jump to zero. Fig.1 The moment of reconfiguration of the capillary bridge between three spheres (time in milliseconds) For three in-plane spheres two possible rupture modes arise depending on the intergranular distance. In one, water drains from a single intergranular contact area bond, which progressively disappears. In another scenario, two external opposite boundary surfaces converge near a concavity between the grains to evolve into a thin film, which within about 5 miliseconds ruptures by forming a spherical gas bubble or a cylindrical channel (Fig. 1). Then water mass drains into three separate toroidal bonds. A significant jump in capillary forces is seen at the point of rupture

Micro-scale testing of capillary bridge evolution due to evaporation

Capillary bridge evolution between two fixed glass spheres during its natural convective evaporation is examined experimentally. For comparison extension tests were also carried out. The calibrated balance recording and digital image processing allow monitoring of a number of key variables of the process: the resultant capillary force, the water mass loss, radii of the bridge curvature. On that basis evaporating surface area, suction and surface tension force, interparticle force, axial stress vs (relative) volumetric mass loss are calculated. Testing shows a gradual decrease of suction within bridges down to zero and into a positive pressure range before a two step failure including a formation of a water thread according to a traditional Rayleigh instability pattern followed by a simultaneous rupture at two points of the lowest (negative) total (Gauss) curvatures of the bridge surface

Adhesion-force micro-scale study of desiccating granular material

International audienceExperiments on five-, four-, three-and two-wet-hydrophilic-grain clusters were performed to investigate evolution of adhesion of granular media during drying on the micro-scale. The experiments show that the adhesion-force of a cluster initially grows at most to three times the original value before decreasing to zero by the end of evaporation. The adhesion-force is composed of capillary pressure force acting over the liquid/solid contact surface area, and surface tension forces acting over the three-phase contact perimeter length. This is in contrast with most macro-scale phenomenological models, in which the only desaturation process variables affecting strength are suction and saturation. Both the contact surface area and contact perimeter length are reduced to zero upon complete liquid evaporation. The morphology of an evaporating water body evolves through slow flow controlled by evaporation rate, interrupted by various modes of fast air entry, which are non-equilibrium jumps of liquid/gas interfaces (Haines jumps). The instabilities involve large adhesion force discontinuities and substantial water mass reconfiguration with water flow in an extremely short time, which makes the process transient. The reconfigurations can reduce the original multi-grain water clusters to four-, three-and two-grain clusters by way of three different instability modes: of thin-sheet instability, or meniscus snap-through instability, depending on the sign of the Gauss curvature of the liquid surface, or finally, for two-grain bridges only, a liquid wire pinch-off. For larger meso-scale assemblies, however, the global adhesion-force evolution is little affected by the jumps. The air entries are potential sites for drying cracks. The (approximately) calculated capillary pressure for two-and three-grain clusters, in no cases is seen to reach high values, predicted from water retention curves

Micro-scale study of rupture in desiccating granular media

International audienceCapillary bridges between two, three, and multiple fixed glass spheres are examined experimentally during their natural evaporation. The key variables of the process of evolution are measured using a calibrated balance recording and digital image processing with still and high-speed cameras. The calculations of Laplace pressure, as well as suction and surface tension resultant components of the interparticle force are made for two-grain systems. Evolution, properties and failure of evaporating liquid bridge are controlled and induced by decreasing liquid volume. For the two grain configuration, tests show a gradual decrease of suction down to zero and into a positive pressure range before a two step failure occurs, including a formation of a water wire according to a Rayleigh instability pattern followed by a simultaneous rupture at two points of the lowest (negative) total (Gauss) curvature of the bridge surface. For more complex systems, a thin-film pinching instability is shown to result from two-dimensional cavitation of water, leading to a reconfiguration of the water body into separate bridges between individual pairs of grains, which then rupture as described above. Water body instability generated dynamic penetration of air may also provide an imperfection for the granular system potentially leading to cracking