Rain 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.
The 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.
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)
The wetting and spreading of a liquid on a solid surface has received much interest over the last few decades. The present state-of-the-art can be summarized as follows: experimental data is qualitatively well fitted by various models, but parameters obtained from the fits are often non-physical and/or cannot be related to the substrate properties. One important reason is that few studies, experimental or theoretical, have explicitly accounted for nano- or meso-scale heterogeneity. We believe that disorder is an exceedingly important ingredient and that in order to build a consistent picture of wetting dynamics one must first understand the wetting on real surfaces. We propose to use a novel material system to obtain model reference surfaces with very low hysteresis, then to add perfectly known defects and examine their effect on the various dynamical regimes, over a large range in contact line velocities. Our goal is to relate wetting dynamics to measurable properties of the surface.
Madame Kristina Davitt (Laboratoire de Physique Statistique UMR 8550)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
ENS CNRS UMR 8550 Laboratoire de Physique Statistique UMR 8550
Help of the ANR 218,129 euros
Beginning and duration of the scientific project: September 2014 - 36 Months