Rheo-acoustic gels: Tuning mechanical and flow properties of colloidal gels with ultrasonic vibrations

Colloidal gels, where nanoscale particles aggregate into an elastic yet fragile network, are at the heart of materials that combine specific optical, electrical and mechanical properties. Tailoring the viscoelastic features of colloidal gels in real-time thanks to an external stimulus currently appears as a major challenge in the design of "smart" soft materials. Here we introduce "rheo-acoustic" gels, a class of materials that are sensitive to ultrasonic vibrations. By using a combination of rheological and structural characterization, we evidence and quantify a strong softening in three widely different colloidal gels submitted to ultrasonic vibrations (with submicron amplitude and frequency 20-500 kHz). This softening is attributed to micron-sized cracks within the gel network that may or may not fully heal once vibrations are turned off depending on the acoustic intensity. Ultrasonic vibrations are further shown to dramatically decrease the gel yield stress and accelerate shear-induced fluidization. Ultrasound-assisted fluidization dynamics appear to be governed by an effective temperature that depends on the acoustic intensity. Our work opens the way to a full control of elastic and flow properties by ultrasonic vibrations as well as to future theoretical and numerical modeling of such rheo-acoustic gels.Comment: 21 pages, 14 figure

Effet des ultrasons basse fréquence sur l’hydrodynamique d’un réacteur annulaire continu : approche expérimentale en Distribution des Temps de Séjour (DTS)

Ultrasound (US) are particularly interesting for their mechanical effects enabling transfers activation, in particular by generating mixing. However, this phenomenon has not yet been quantified in a continuous reactor, which is nevertheless a key point for the intensification of such processes. For this purpose, this work characterized the hydrodynamics within a continuous annular reactor under low frequency ultrasound via a Residence Time Distribution experimental approach (RTD). Reliable and reproducible experimental protocol and data processing method were developed. The experiments under silent conditions showed that,due to its geometry, the studied reactor had dead zones that are not negligible. The comparison of these results with those obtained under US had clearly demonstrated the action of US in the flow rate range investigated (laminar flow). The comparison of the RTD curves, as well as the average residence time values obtained, confirmed the US effect on the mixing within the reactor. By creating micro-mixing, ultrasound also reduced dead zones. The study of US power influence showed a threshold beyond which its contribution on hydrodynamics is less marked. This point is encouraging for the scale up of reactors under ultrasound

Transitions sol-gel de colloïdes anisotropes sous champs de cisaillement, pression et ondes ultrasonores, caractérisées par diffusion de rayons x aux petits angles in-situ

L'objectif de ce travail est de caractériser aux échelles mésoscopiques, l'effet combiné des champs de pression, hydrodynamiques et ultrasonores sur les mécanismes de transition sol-gel de colloïdes anisotropes d'argiles lors de l'ultrafiltration tangentielle. Pour cela, des cellules de filtration ont été développées en intégrant une lame vibrante sollicitée à 20kHz par un générateur ultrasonore. Ces cellules de filtration permettent l'observation in-situ aux échelles nanométriques par diffusion de rayons X aux petits angles (SAXS). Différentes suspensions aqueuses d'argiles ont été étudiées : des argiles naturelles de montmorillonite Wyoming-Na et des argiles synthétiques de Laponite en présence ou non d'un peptisant le tetrasodium diphosphate (Na4P2O7). Par ailleurs l'effet des ultrasons sur le comportement rhéologique de suspensions a aussi été étudié.  L'effet du pré-cisaillement induit par la pompe du circuit de filtration et l'effet des ultrasons, sur les contraintes de cisaillement des suspensions de Laponite ont été mises en évidence. Les deux sollicitations réduisent les niveaux de contrainte et l'effet est plus marqué sur les suspensions avec peptisant (à interaction répulsive dominante) que sur les suspensions sans peptisant (à interaction attractive dominante). Les évolutions temporelles de la structure et de la concentration en colloïdes en fonction de la distance à la membrane ont ainsi été caractérisées sous différentes conditions de filtration et de sollicitations ultrasonores. Deux mécanismes principaux ont été mis en évidence lors de l'application des ultrasons : soit un mécanisme de fracturation ou d'intensification locale de l'écoulement lorsque les colloïdes forment un réseau dense très anisotrope (cas des suspensions de Montmorillonite et de Laponite sans peptisant), soit un mécanisme d'érosion des couches concentrées pour les colloïdes assemblés en structures ouvertes (cas des suspensions de Laponite avec peptisant)

Mapping rootable depth and root zone plant-available water holding capacity of the soil of sub-Saharan Africa

In rainfed crop production, root zone plant-available water holding capacity (RZ-PAWHC) of the soil has a large influence on crop growth and the yield response to management inputs such as improved seeds and fertilisers. However, data are lacking for this parameter in sub-Saharan Africa (SSA). This study produced the first spatially explicit, coherent and complete maps of the rootable depth and RZ-PAWHC of soil in SSA. We compiled georeferenced data from 28,000 soil profiles from SSA, which were used as input for digital soil mapping (DSM) techniques to produce soil property maps of SSA. Based on these soil properties, we developed and parameterised (pedotransfer) functions, rules and criteria to evaluate soil water retention at field capacity and wilting point, the soil fine earth fraction from coarse fragments content and, for maize, the soil rootability (relative to threshold values) and rootable depth. Maps of these secondary soil properties were derived using the primary soil property maps as input for the evaluation rules and the results were aggregated over the rootable depth to obtain a map of RZ-PAWHC, with a spatial resolution of 1 km2. The mean RZ-PAWHC for SSA is 74mm and the associated average root zone depth is 96 cm. Pearson correlation between the two is 0.95. RZ-PAWHC proves most limited by the rootable depth but is also highly sensitive to the definition of field capacity. The total soil volume of SSA potentially rootable by maize is reduced by one third (over 10,500 km3) due to soil conditions restricting root zone depth. Of these, 4800 km3 are due to limited depth of aeration, which is the factor most severely limiting in terms of extent (km2), and 2500 km3 due to sodicity which is most severely limiting in terms of degree (depth in cm). Depth of soil to bedrock reduces the rootable soil volume by 2500 km3, aluminium toxicity by 600 km3, porosity by 120 km3 and alkalinity by 20 km3. The accuracy of the map of rootable depth and thus of RZ-PAWHC could not be validated quantitatively due to absent data on rootability and rootable depth but is limited by the accuracy of the primary soil property maps. The methodological framework is robust and has been operationalised such that the maps can easily be updated as additional data become available

Mapping rootable depth and root zone plant-available water holding capacity of the soil of sub-Saharan Africa

In rainfed crop production, root zone plant-available water holding capacity (RZ-PAWHC) of the soil has a large influence on crop growth and the yield response to management inputs such as improved seeds and fertilisers. However, data are lacking for this parameter in sub-Saharan Africa (SSA). This study produced the first spatially explicit, coherent and complete maps of the rootable depth and RZ-PAWHC of soil in SSA. We compiled georeferenced data from 28,000 soil profiles from SSA, which were used as input for digital soil mapping (DSM) techniques to produce soil property maps of SSA. Based on these soil properties, we developed and parameterised (pedotransfer) functions, rules and criteria to evaluate soil water retention at field capacity and wilting point, the soil fine earth fraction from coarse fragments content and, for maize, the soil rootability (relative to threshold values) and rootable depth. Maps of these secondary soil properties were derived using the primary soil property maps as input for the evaluation rules and the results were aggregated over the rootable depth to obtain a map of RZ-PAWHC, with a spatial resolution of 1 km2. The mean RZ-PAWHC for SSA is 74mm and the associated average root zone depth is 96 cm. Pearson correlation between the two is 0.95. RZ-PAWHC proves most limited by the rootable depth but is also highly sensitive to the definition of field capacity. The total soil volume of SSA potentially rootable by maize is reduced by one third (over 10,500 km3) due to soil conditions restricting root zone depth. Of these, 4800 km3 are due to limited depth of aeration, which is the factor most severely limiting in terms of extent (km2), and 2500 km3 due to sodicity which is most severely limiting in terms of degree (depth in cm). Depth of soil to bedrock reduces the rootable soil volume by 2500 km3, aluminium toxicity by 600 km3, porosity by 120 km3 and alkalinity by 20 km3. The accuracy of the map of rootable depth and thus of RZ-PAWHC could not be validated quantitatively due to absent data on rootability and rootable depth but is limited by the accuracy of the primary soil property maps. The methodological framework is robust and has been operationalised such that the maps can easily be updated as additional data become available

Conception et étude d'un évaporateur membranaire à membrane métallique

L'Evaporation Membranaire est un nouveau type de procédé utilisant un contacteur à membrane liquide/gaz pour la concentration de produits thermosensibles à haute valeur ajoutée. Ce procédé permet de mettre en contact par l'intermédiaire d'une membrane macroporeuse métallique hydrophobe, une solution aqueuse à concentrer et une phase extractante à savoir de l'air sec. La force motrice du transfert est la différence de pression partielle de vapeur d'eau existant de part et d'autre de la membrane. Les propriétés de conduction thermique et électrique des membranes métalliques permettent un apport d'énergie au niveau des pores de la membrane pour compenser le phénomène de polarisation de température entraîné par l'évaporation du solvant. Ce travail est divisé en trois parties. La première concerne la caractérisation des membranes et la détermination de l'influence du traitement de surface, permettant de les rendre hydrophobes, sur leurs propriétés structurelles. La deuxième partie est consacrée à l'étude expérimentale du procédé d'évaporation membranaire. L'influence de nombreux paramètres opératoires sur les performances du procédé a ainsi été déterminée et des flux évaporatoires comparables à ceux obtenus en évaporation osmotique ont été obtenus démontrant ainsi la faisabilité du procédé. Un modèle théorique tenant compte des transferts de masse et de chaleur a été développé dans une troisième partie. Le modèle a été validé et a permis de prouver que dans les conditions opératoires étudiées, la résistance de la couche limite en phase gaz limite les performances du procédé. Par ailleurs, des simulations ont permis de mettre en évidence l'effet de polarisation de température se produisant à la surface de la membrane lors de la vaporisation de l'eauMembrane Evaporation is a new type of process using a liquid/gas membrane contactor for the concentration of thermo sensitive products with high added value. This process makes it possible to put in contact by means of a hydrophobic metal macroporous membrane, an aqueous solution to be concentrated with a flow of dry air which is the extracting fluid. The driving force of the transfer is the difference of water vapour partial pressure between the both sides of the membrane. Thermal and electric conduction properties of metallic membranes allow supplying, at pores level, the necessary energy to compensate the phenomenon of polarization of temperature caused by the solvent evaporation. This work is divided into three parts. The first relates to the characterization of the membranes and the determination of the influence of the surface treatment, making it possible to make them hydrophobic, on their structural properties. The second part is devoted to the experimental study of the membrane evaporation process. The influence of different operational parameters on the process performances was thus given. Evaporating fluxes reached were comparable with those of osmotic evaporation process thus proving the feasibility of the membrane evaporation process. A theoretical model taking into account mass and heat transfer was developed in a third part. The model was validated and made it possible to demonstrate that under the studied operating conditions, the resistance of the boundary layer in gas phase limits the performances of the process. In addition, simulations allowed highlighting the effect of polarization of temperature occurring at the surface of the membrane during the vaporization of waterMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Influence of membrane resistance on swelling and removal of colloidal filter cake after filtration pressure release

The influence of filtration membrane resistance on colloidal filter cake swelling after the filtration pressurerelease is discussed in frame of filtration-consolidation theory. It is argued that in the case of reversiblycompressible filter cake, the solid pressure relaxation can result in filtrate inflow across the membrane with thefollowing absorption by the filter cake part adjacent to the membrane surface. According to the model results, thefiltrate inflow and the filter cake swelling rates increase with decreasing of the membrane hydraulic resistance.At low membrane resistance, the swelling can result in the non-monotonous bell-like solid concentration distribution in the filter cake (solid concentration on the cake-membrane surface is lower than the maximal solidconcentration). In contrast, the filter cake swelling in the absence of filter inflow across the membrane (or thedeposit swelling on the impermeable surface) results in the monotonous decrease of the solid concentration. Thenon-monotonous solid concentration distribution can facilitate the membrane rinsing and cleaning and lead tothe filter cake removal before its complete swelling

In-situ local flow fields and shear stress near membrane surface of colloidal suspensions concentrated during cross-flow ultrafiltration probed by SAXS and micro-PIV.

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Propriétés d’écoulement et de structure des couches de polarisation de concentration lors de l’ultrafiltration tangentielle de colloïdes, sondés par SAXS et micro PIV in-situ.

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