Projet ANR-07-BLAN-0158 – DYNINSTAMOBI : Dynamique et instabilités de la ligne de contact mobile: théorie et expérience
Programme : Blanc (BLANC) 2007
Année de financement : 2007
Coordinateur du projet : CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR PARIS B (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR PARIS B)
Partenaires : Centre national de la recherche scientifique; Centre national de la recherche scientifique; Centre national de la recherche scientifique; Centre national de la recherche scientifique
Montant : 240000.0 euros
Date de début : 2007
Durée : 36 mois
Résumé : "	For simple cases, the physics of the moving contact line is well-understood from the physico-chemical and thermodynamical points of view: we know how to take into account the disjoining pressure to eliminate the viscous dissipation divergence which occurs at the macroscopic scale for quasi-equilibrium situations involving simple, Newtonian and non volatile fluids. -  -  Now, we want to consider the dynamics, i.e., the spontaneous spreading of fluids, or the advancing or receding motion of the moving contact line in more complex systems. We have in mind liquid crystals, surfactant solutions and thin films, and volatile liquids. This dynamics can be complemented with interfacial instabilities, sometimes but not always localized at the moving contact line. The goal of this project is to perform well controlled experiments together with theoretical treatments. They will be based on the lubrication approximation and its extensions. -  -  	During the last years, the experimental groups have discovered new features and new phenomena whose analysis requires a tight collaboration with specialized theorists. Such collaborations will justify new systematic studies. Today, we think that at least three topics may be at the origin of a fruitful collaboration. -  -  First, recent experiments on liquid crystal spreading in the nematic phase with antagonist anchoring at the interfaces show an anomalous spreading dynamics, together with a strong distortion of the profile in the micrometric range of thickness. On hydrophilic substrates, instabilities are observed, located or not at the contact line. Two questions arise at that point. First, the link between the dynamics and the elasticity of the nematic film must be established, and the exact role of the sharp transition between the nematic film and the molecular precursor present ahead on the substrate must be elucidated. Second, the theorists have to adapt the formalism of thin film instabilities without moving contact line to the case of the drops studied experimentally, and investigate the role of anchoring defects. -  -  	In the case of surfactant solutions, the very rich phenomenology which is observed experimentally is poorly described by the existing theories and a close contact between theory and experiment is necessary to improve the understanding. Moreover, for soluble surfactants, the super-spreading phenomena which are of great interest for the industrial applications are absolutely not understood. -  -  	More recently, we got interested into the dynamics of spreading-retraction of droplets made of a wetting, volatile liquid. The model available at the present time accounts for the dynamics of the smallest drops, but the description of the interface profile close to the contact line requires refinement. Transient contact line instabilities are observed which need a specific analysis. -  -  	The aim of the theorists is to account for the anomalous dynamics well characterised in experiments. They intend to establish the formalism of the instabilities which are understood actually in thin films to the case of films whose thickness varies rapidly as it is the case near the contact line. They want to understand the dynamics of defects which appears during spreading of liquid crystal droplets and clearly drive one type of instability. More precisely, they want to adapt the techniques of nonlinear physics and free-boundary problems to the stochastic dynamics when the defects of the substrate disturb the dynamics. -  -  The experimental groups may perform complementary measurements when needed, but will focus on two points. First, a very precise analysis of interfaces profiles for evaporating drops is currently under way and will be continued. Second, in the case of liquid crystals, the processes occurring at the sharp transition between nematic film and molecular precursor, and also at the edge of the molecular precursor itself, will be investigated by Brewster Angle Microscopy. Obtainin