P. Levinson

Dynamics of wetting: local contact angles

We discuss the motion of a triple line for a fluid spreading on a flat solid surface in conditions of partial wetting: the equilibrium contact angle θ e is assumed to be finite but small: 0 e [Lt ] 1. We distinguish four regions: (1) a molecular domain of size a (≈ a few Angstroms) very near the triple line, where the continuum description breaks down; (2) a proximal region (of length a /θ 2 e and height a /θ e ) where the long-range Van der Waals forces dominate; (3) a central region, where capillary forces and Poiseuille friction are the only important ingredients; (4) a distal region where macroscopic features (related to the size of the droplet, or to gravitational forces) come into play. In regions (2, 3, 4) the flow may be described in a lubrication approximation, and with a linearized form of the capillary forces. We restrict our attention to low capillary numbers Ca and expand the profiles to first order in Ca near the static solution. The main results are: ( a ) the logarithmic singularity which would have occurred in a simple wedge picture is truncated by the long-range forces, at a fluid thickness a /θ e . This effect is more important, at small θ e , than the effects of slippage which have often been proposed to remove the singularity, and which would lead to a truncation thickness comparable with the molecular size a ; ( b ) in the central region, the local slope θ( x ) grows logarithmically with the distance x from the triple line; ( c ) one can match explicitly the solutions in the central and distal region: we do this for one specific example: a plate plunging into a fluid with an incidence angle exactly equal to θ e . In this case we show that, far inside the distal region, the perturbation of the slope decays like 1/ x 2 .

An ellipsometric study of adsorption isotherms

Adsorption isotherms of alkanes or silicone oil oligomers on silicon wafers covered with natural oxide, or bearing hydrophobic grafted layers, have been studied at room temperature. At large thicknesses, information on the long-range interactions is extracted, and plausible values of the Hamaker constants, i.e. of the strength of the van der Waals interaction, are obtained for alkanes. At molecular thicknesses, the amount adsorbed depends on the specific short-range interactions with the surface layer. The presence of an adsorbed layer of water on the bare silica surfaces must be taken into account in the interpretation of data

Some specific aspects of the behaviour of ultrathin films

Abstract The thickness profiles of microdroplets of non-volatile liquids spreading on solid surfaces have been recorded by ellipsometry. In many cases the droplets take a stepped-pyramidal shape in the molecular range of thickness owing to surface-induced layering. A model has been proposed recently to describe the spreading of completely layered droplets. We check the ability of this model to account for experimental data, and determine its range of applicability.

Adsorption isotherms of alkanes on silica: The role of preadsorbed layers

Abstract Adsorption isotherms of alkanes on oxidized silicon wafers have been recorded by ellipsometry at room temperature. A first series of experiments has been performed on wafers heated for increasing times at 850°C. Heating under normal atmosphere increases the thickness of the silica layer and decreases the amount of silanol groups at the silica surface. The short-range interaction between alkanes and surface is controlled by the amount of silanols, while the long-range interaction depends on the silica thickness. A second series of experiments has been performed on wafers bearing an adsorbed hydrophobic layer. If the alkane does not penetrate this layer, a decrease in wettability is obtained, as expected. This is the case for layers of trimethylsilyl groups. Thicker layers are penetrated by alkanes, and the interpretation of adsorption results is not straightforward. An extreme situation is an adsorbed polymer layer swollen by the alkane. The aim of these experiments is to get a better understanding of wettability of model systems, and ultimately of reservoir rocks.