F. Heslot

Thin wetting films

Abstract Describing the growth of a thin film which spreads spontaneously on a solid surface is a problem which is not totally solved yet. Theories have been proposed by de Gennes in the case of moderately thin films, which can be treated by hydrodynamics. However, ultrathin films, a few molecular layers thick, are often observed experimentally. They exibit peculiar, possibly step-like profiles which ask for a different theoretical approach. A simple model is proposed, which qualitatively accounts for the observed features.

Hydrodynamic fingering instability of driven wetting films: hindrance by diffusion

Recent experimental and theoretical efforts have revealed the existence of a fingering instability at the moving front of thin liquid films forced to spread under gravitational, rotational or surface shear stresses, as for example by using the Marangoni effect. The authors describe how the presence of a precursor film in front of the spreading macroscopic film, whether it is by prewetting the substrate or by surface diffusion or multilayer absorption, can prevent the development of the instability.

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

Wetting in nonhomogeneous situations: Capillary rise experiments

Abstract Exploratory experiments on wetting in nonideal situations are reported. The capillary rise of a completely wetting, nonvolatile liquid has been investigated in the presence of nonwetting defects, surface contamination or thermal gradients. A qualitative analysis of the results is proposed in each case. The aim of these studies is to provide information for forthcoming theoretical analyses.

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.

Droplet spreading in presence of contamination

Abstract The spreading of wetting fluids is a very slow process, and is thus sensitive to any external perturbation like roughness or contamination. We have studied the influence of surface and atmospheric contamination on the dynamics of spreading of silicone oil drops on glass surfaces. Atmospheric contamination gives rise to smooth surface tension gradients, while surface contamination produces localized ones. The profiles and dynamics of spreading drops have been studied in both cases.