Mouillage sur des surfaces réelles : Dynamique et nanorugosité – REALWET

This project relies on the realisation of two technical tasks: (i) producing a reference surface and (ii) producing surfaces with controlled heterogeneity. We use two types of reference surfaces: (a) a simple polymer pseudo-brush which can be nearly hysteresis-free (< 0.1 degrees) or (b) a self-assembled monolayer of silanes giving only a few degrees of hysteresis. By adding sintered silica nanospheres we add defects of controlled size, shape and density to the reference surface. A carefully designed dip-coating apparatus allows us to measure the contact line dynamics from 1 nm/s up to – and beyond! – the coating transition. We can now measure 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.

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 understandable as the pseudo-brush swells and is liquid-like under these conditions and therefore acts to smooth the 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. Despite energy barriers that can reach as low at 10 kBT, we do not see any emergence of thermal activation on the added defects, perhaps hidden by the large viscoelastic dissipation in the PDMS pseudo-brush.

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. The extent to which the polymer is swollen by the liquid drop or by another non-miscible liquid is thought to be an essential ingredient in understanding why the hysteresis is so exceptionally low.

2 articles published (PRL, Nature Communications)

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.