S. Villette

Dynamics of Wetting: From Theory to Experiment

The main available theories for the dynamics of wetting are brieflysummarized and discussed in reference to experiments. In partial wetting,hydrodynamic and molecular theories are equivalently efficient, even if thephysical meaning of parameters is not so clear in the former ones. Incomplete wetting, hydrodynamic theories are the only ones valid at lowangles, but some care has to be taken in the interpretation of the“slip length” introduced to remove the divergence of thedissipation at the contact line. The situation is less favourable at themolecular scale, where the theoretical description is still at itsbeginning, due to the multiplicity of behaviours.

Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films.

We study experimentally the molecular weight

Ultrathin liquid films. Ellipsometric study and AFM preliminary investigations

M

Investigation of layered microdroplets using ellipsometric techniques

dependence of spreading rates of molecularly thin precursor films, growing at the bottom of droplets of polymer liquids. In accord with previous observations, we find that the radial extension R(t) of the film grows with time as R(t) = (D_{exp} t)^{1/2}. Our data substantiate the M-dependence of D_{exp}; we show that it follows D_{exp} \sim M^{-\gamma}, where the exponent \gamma is dependent on the chemical composition of the solid surface, determining its frictional properties with respect to the molecular transport. In the specific case of hydrophilic substrates, the frictional properties can be modified by the change of the relative humidity (RH). We find that \gamma \approx 1 at low RH and tends to zero when RH gets progressively increased. We propose simple theoretical arguments which explain the observed behavior in the limits of low and high RH.

Examples of wetting transition: the effect of long and short range interactions

Films of molecular thickness exhibit a large variability, both in their structural properties and in their spreading dynamics. The thickness profiles of nonvolatile liquid microdroplets spreading spontaneously on smooth solid surfaces provide information on short range liquid-liquid and liquid-solid interactions. These profiles can in principle be recorded by various techniques, including spatially resolved ellipsometry and atomic force microscopy (AFM). The main part of the paper deals with results obtained using ellipsometry. In particular, the influence of water adsorbed on the substrate on the spreading dynamics of various liquids is reported. Unexpected absorbed on the substrate on the spreading dynamics of various liquids is reported. Unexpected problems met with the AFM technique are discussed, further work is under way.

Spreading of molecularly thin wetting films on solid interfaces

Abstract Previously single wavelength ellipsometry has been successfully used to study the spreading of liquid polymer microdroplets on solid substrates. Extending that experimental approach, we present in this study the first experiments of spatially resolved spectroscopic ellipsometry applied to the spreading of layered microdroplets and show that it is possible to recover the experimental thickness profiles of polydimethylsiloxane and squalane on hydrophilic silicon wafers. To complement these molecular measurements, we also present new computer simulation results concerning the velocity field and the mass transfer during this spreading process. These results are in agreement with the predictions of the de Gennes–Cazabat model.

Spreading of polymeric liquids at a microscopic scale

The thickness profiles of microdroplets spreading spontaneously on solid surfaces have been studied using spatially resolved ellipsometry. Information on interaction and rheology can be extracted. We present experimental findings illustrating how long or short range interactions may induce a wetting transition in an isotropic liquid. The parameters modified to approach the transition are the silica thickness of a silicon substrate and the atmospheric relative humidity. The first one induces a change in long range interaction and the second one a change of conformation in liquid molecules in contact with the solid.

Wetting on the molecular scale and the role of water. A case study of wetting of hydrophilic silica surfaces

In this paper we study kinetics of spreading of thin liquid films on solid interfaces. We present an overview of current experimental picture and discuss available theoretical approaches and their limitations. We report some new experimental results on spreading of molecularly thin liquid films and propose an analytically solvable microscopic model, which reproduces experimentally observed behaviors and provides a seemingly plausible explanation of the underlying physical processes.

The mechanism of spreading : A microscopic description

The spreading of polymeric liquids on solid substrates has been a very lively subject of research both by experimentalists and theoreticians. It has been observed that some drops, which spread on solids may form a precursor film of molecular thickness with possible accompanying layers, with a radius growing as a square root of time in the early stages after deposition. These results have been obtained for several polymeric liquids on top of bare or grafted oxidized silicone wafers. The first theoretical model describing this phenomenon was developed by de Gennes and Cazabat. In this original paper the spreading is described as a competition between the driving force, which is due to the wall attraction and the friction between layers of liquid, and the solid. Assuming the essentially viscous nature of the friction forces, the authors were able to recover most of the experimental observations mentioned before. However, the friction parameters have to be introduced at a macroscopic scale and are questionable at a microscopic point of view. It is the aim of this paper to give a review of the experimental observations completed, case by case, by microscopic simulations, giving a more detailed understanding of the rich variety of these phenomena.