Doctoral thesis viva voce : Antonin Marchand

Static and Dynamic Wetting: Geometrical Effects at Molecular Scales

Antonin Marchand, doctorant. Crédits : ESPCI ParisTechThis thesis highlights different geometric effects involved in wetting phenomena. The first part is dedicated to the development of a model of interactions in liquids to determine, from the geometry of the interface, the distribution of capillary forces at a molecular level. In this context we propose an new microscopic interpretation of Young’s force balance that leads to the equilibrium contact angle in a corner of fluid. In the second part, this model is applied to phenomena as line tension, electrowetting and elastocapillary interactions — these all involve capillary forces at molecular scales. The effects of line tension are studied by molecular dynamics simulations of nanodrops, and we present a geometrical interpretation of the phenomenon. The existence of a pre-wetting film is predicted during the saturation of the phenomenon of electrowetting. The microscopic capillary model also shows a peculiar distribution of forces in the solid in the vicinity of the contact line, which can be observed when the substrate is deformable. In particular, the model predicts an additional Laplace pressure when a solid is immersed in a liquid — this is confirmed experimentally. We also study the influence of the wettability of liquids on the bending and buckling of an elastic plate under the influence of capillary forces. Finally, the dynamic transition of air entrainment is examined, and we highlight the crucial role of dissipation in the air flow when it is driven and confined by the liquid.