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Séminaires

Formation and growth of labyrinthine drying patterns in 2-D porous model media

Pascal Panizza Matière Molle, Institut de Physique de Rennes

Jeudi 8 juin - 14h00 - Amphi Boreau

 

Drying of colloidal solutions in porous media is relevant to many fields such as soil physics, and civil engineering. For all these applications it is essential to understand and to predict the kinetics and topology of the crystal deposits that form during the drying process.
This problem is important for civil engineering owing to salt weathering which damages buildings and other engineering structures and in material science to engineer smart
nanoscale and microscale materials using evaporation as a novel route of controlled selfassembly. Despite the importance of all these applications, this issue still remains elusive notably because of complex couplings between the formation of crystal deposits and the various transport phenomena involved in the drying process which are difficult to unravel.
To address this issue, we investigate drying of colloidal suspensions or surfactant solutions confined in a 2D porous model medium which permits to overcome optical limitation inherent to bulk systems. We observe the formation of thin solid films that interconnect to form a maze structure. Our experiments show that the dynamics of formation of these unusual out of equilibrium patterns is triggered by a regular succession of Haines jumps. We identify the elementary mechanisms leading to the formation of the crystal deposits at the pore scale and then establish a pore-network model that well captures the topology of the labyrinthine structures observed in our experiments as well as their dynamics. Using simple arguments, we predict the topological changes in the drying pattern at the scale of an individual pore that occur when the geometrical parameters of the problem such as the sample thickness and the contact angle that the liquid makes with the substrate are varied. Our experiments confirm our theoretical predictions.

Fluorescent microscopy image of the maze structure observed after the drying of a colloidal suspensions in a 2 D model porous medium made of cylindrical posts whose height and diameter are respectively 30 and 100 micrometers. The interdistance between posts is 175 micrometers

Mixing in sheared particulate suspensions

Bloen Metzger IUSTI-CNRS UMR 7343, Aix-Marseille University

Jeudi 15 juin - 14h00 - Amphi Schützenberger

 

The question addressed is how the presence of rigid particles in a viscous shear flow accelerates mixing. High-resolution PIV measurements of the fluid phase were performed to reconstruct the stretching histories of material lines of the interstitial fluid. We found that the nature of the stretching law changes drastically from linear, in absence of particles, to exponential in the presence of particles. The mean and the standard-deviation of the material line elongations are found to grow exponentially in time and the distribution of elongations converges to a log-normal. A multiplicative stretching model, based on the distribution of local shear-rates and on their persistence time, is derived. This model which quantitatively captures the experimental stretching laws provides a complete description of the flow kinematics. Predictions of the mixing times are then inferred showing that the presence of particles accelerates mixing at large Péclet numbers. The wide distribution of stretching rates results in heterogeneous mixing and hence, broadly distributed mixing times.


Figure 1 : Mixing of a drop of dye in a sheared particulate suspension.

Collaborations : M. Souzy, H. Lhuissier, E. Villermaux
Ref : Stretching and mixing in sheared particulate suspensions (2017) JFM 812, 611-635.

Functional polymeric materials with various porosity scales : From design to application

Daniel Grande Institut de Chimie des Materiaux, Univ. Paris Est

Jeudi 22 juin 2017 - 14h00 - Amphi Langevin

 

Over the last decade, the generation of organic porous materials with tunable pore sizes and desired functionalities has been the subject of increasing attention in materials science. Interest in such porous frameworks originates from the large variety of applications in which they are involved, e.g. size/shape-selective nanoreactors, monoliths for advanced chromatographic techniques, nanofiltration membranes, high specific area catalytic supports, as well as 3-D scaffolds for tissue engineering.
This lecture examines the scope and limitations of three different approaches to porous polymers with controlled porosity and functionality at different length scales. The first approach relies on the synthesis of polystyrene-block-poly(D,L-lactide) diblock copolymers with functional groups at the junction between both blocks (e.g., COOH, SO3H, SH, CHO), followed by their macroscopic orientation, and the subsequent selective removal of the polyester block. The second strategy entails the preparation of biocompatible doubly porous crosslinked polymer materials through the use two distinct types of porogen templates, namely a macroporogen in combination with a nanoporogen. To generate the macroporosity, either CaCO3 or NaCl particles or fused PMMA beads are used, while the second porosity is obtained by using either hydroxyapatite nanoparticles or a porogenic solvent. Finally, 3-D macroporous scaffolds based on biodegradable polyesters have been engineered by electrospinning to generate nanofibrous biomaterials that mimic the extracellular matrix. The potentialities afforded by these approaches will be addressed, and some typical applications of the resulting porous materials will be illustrated.


Surface Stresses in Solids and its Effect on the Mechanical behavior of Soft Material

Chung-Yuen Hui Field of Theoretical and Applied Mechanics, Dept. of Mechanical and Aerospace Engineering, Cornell University

Mercredi 24 mai 2017

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How mechanics is involved in plant root growth in soils

Evelyne Kolb PMMH, UMR 7636, ESPCI

Jeudi 18 mai 2017

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Assembly of optically resonating colloids for the fabrication of metamaterials

Virgine Ponsinet Centre de Recherche Paul Pascal CNRS-Université de Bordeaux

Jeudi 11 mai 2017

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