Projet ANR-14-CE32-0007 – REALWET : Mouillage sur des surfaces réelles : Dynamique et nanorugosité
Programme : Défi de tous les savoirs (DS10)
Année de financement : 2014
Coordinateur du projet : Madame Kristina Davitt (Laboratoire de Physique Statistique UMR 8550)
Partenaires : Laboratoire public
Montant : 218129.0 euros
Date de début : 2014
Durée : 48 mois
Résumé : Mouillage sur des surfaces réelles : dynamique et nanorugositéRain running down a car windshield, butter sloshing around a Teflon pan… Despite the ubiquity of the phenomenon, we still do not fully understand how a liquid moves over a solid surface. Real surfaces are rough, chemically heterogeneous, coated with molecular layers, or even all of the above, complicating attempts to isolate fundamental physical principles. We believe that in order to build a consistent picture of wetting dynamics one must first understand the wetting on real surfaces.
ObjectifsThe fundamental objective of this project is to relate wetting behavior to measurable properties of a surface.  We produce model surfaces and then carefully measure the full range of contact line dynamics on these surfaces. First we strive to produce a “perfect” reference surface that exhibits no contact angle hysteresis and then we will introduce perfectly known defects. In addition to producing a zero-hysteresis reference surface, we aim to understand the contact line dynamics on such a surface. Then, once defects have been added, we aim to quantitatively understand their effect on the dynamics. Specifically, we aim to link the size, shape, and density of the defects to the amplitude of the hysteresis and to the different dynamical regimes.
MéthodesThis project relies on the realisation of two technical tasks: (i) producing a reference surface and (ii) producing surfaces with controlled heterogeneity. Using a simple polymer pseudo-brush we have obtained surfaces that are nearly free of hysteresis (< 0.1 degrees) and using sintered silica nanospheres we have been able to tune the size and shape of individual defects. We have built a dip-coating apparatus that is capable of measuring the contact line dynamics from 1 nm/s up to and beyond the coating transition. We can now produce a set of measurements of the full contact line dynamics on a series of surfaces with a series of well-chosen liquids. Interpretation of the data involves both modeling of individual defects (how the specific shape of a defect determines the force it exerts on the contact line) and modeling the influence of nano-scale defects on the dynamics at all scales.
Résumé de la soumission : Le mouvement d’un ménisque sur une surface solide est un sujet qui a suscité beaucoup d’intérêt depuis quelques décennies. Malgré cela, l’état de l’art peut se résumer comme suit : les résultats expérimentaux peuvent être bien ajustés par différents modèles, mais les paramètres microscopiques déduits des ajustements sont souvent irréalistes et/ou ne peuvent pas être reliées aux propriétés du substrat. Une des raisons en est que peu d’études, expérimentales ou théoriques, prennent explicitement en compte les hétérogénéités à l’échelle nano- ou méso-scopique. Nous pensons que le désordre est un ingrédient extrêmement important et que pour se faire une représentation cohérente de la dynamique de mouillage, il est d’abord nécessaire de comprendre le mouillage sur des surfaces réelles. Nous proposons d’utiliser un système novateur dans le domaine pour obtenir des surfaces de références modèles avec très peu d’hystérésis, et ensuite d’ajouter des défauts bien caractérisés. Nous étudierons ensuite leur effet sur  différents régimes dynamiques, et ce sur une grande gamme de vitesses de ligne de contact. Notre but est de lier la dynamique de mouillage à des propriétés mesurables de la surface.
Résultats : We have measured and modeled the dynamics on surfaces covered with short-chain polymer pseudo-brushes. We find that when using a liquid that is a good solvent for the polymer, the hysteresis is minimum for an intermediate N. Under optimum conditions, it can be less than one-tenth of a degree.  Qualitatively, this is understood since the pseudo-brush is liquid-like under these conditions and therefore hides any defects on the solid surface. We have proposed that the dynamics on such thin polymer coatings are dominated by viscoelastic dissipation in the brush layer and the predicted scaling accounts quantitatively for the dynamics at intermediate N.
Then we have studied the dependence of the hysteresis and dynamics on the size, shape and density of added nano-scale defects. We find that the hysteresis scales roughly as expected from a simple Joanny-de Gennes type of model. For example, the hysteresis scales linearly with the concentration up to very high surface coverages. We have determined that the defects used to date (6 nm to 80 nm in height) have energy barriers that are relatively large, which is consistent with the absence of any thermally-activated regime.
Perspectives : One surprise is to find that the addition of a molecularly-thin polymer layer on a solid surface can practically eliminate contact angle hysteresis. In one case, we find a hysteresis of less than 0.07°. This is in contrast to the “record-low values” of about 1° which have so far only been obtained by carefully controlling the self-assembly of monolayers of organic molecules such as silanes and thiols on smooth surfaces and only on small surface areas. It is also surprising that the dynamics on these polymer pseudo-brush layers, which are only a few nanometers thick, appears to be described by the same ingredients that describe the motion of drops on bulk deformable surfaces.
Publications et brevets : 2 articles under peer review