ESPCI web site
Clean oxidized silicon wafers are high energy surfaces : most liquids
spontaneously spread on them. Ahead the contact line of sessile droplets, a nanometer-thin film called "precursor film" spreads around the droplet. The spreading dynamics and morphology of these precursor films have long been thought as a way to quantify interactions of liquids at solid interfaces. Most past studies dealt with systems for which long range interactions were repulsive and total wetting conditions were met.
However, liquid/surface and liquid/liquid interactions were found intricate and could hardly be separated in these systems. The liquids that we consider in the present study are polymer melts (polybutadiene, polyisoprene, polystyrene) exhibiting pseudo-partial wetting condition on oxidized silicon wafers : a sessile droplet at rest coexists with a film. By taking advantage of ellipsometric microscopy, we study the morphology and dynamics of such films, and quantitatively probe the interactions at stake between the monomers and the surface. Remarkably, the outermost part of the film consists of a quasi-2D gas of semi-dilute flat chains. In addition to providing robust measurements of molecular interactions at solid interfaces, our work highlights the need to re-think the theoretical
existing framework for dense precursor films.