SOME SPECIFIC FEATURES OF SPREADING AT THE MOLECULAR SCALE - WETTING TRANSITION AND MOLECULAR SELF-ASSEMBLY

Profiles of microdroplets spreading on solid surfaces are studied in t he molecular range of thickness using spatially resolved ellipsometry. Surface induced layering leads to stepped profiles for nonvolatile li quids, the thickness of the successive molecular layers giving informa tion on the configuration of the molecules at the solid surface. For e xample, the worm-like molecules of polydimethylsiloxane (PDMS) lay fla t on the substrate, while the bulk configuration is a coil. On the sam e way, surfactant molecules behave differently on hydrophobic and hydr ophilic surfaces. After a general discussion of drop behaviour on fini te or infinite substrates, we present two examples illustrating the sp ecificity of the molecule-surface interactions. The first case could b e referred to as a ''wetting transition'': A hydrophobic layer is graf ted on a high energy surface, and the liquids are a series of PDMS. Th e lighter oils wet the surface, the heavier do not. The approach to tr ansition is clearly observed in the droplets shape: The thickness of t he film which grows at the bottom of the drop increases when transitio n is approached and diverges at transition, where the liquid becomes n onwetting. This is the ''pancake' first predicted by de Gennes. The se cond case concerns nonionic trisiloxane surfactants, which do not wet the substrates, but build an extended autophobic film on it. The profi le of the film strongly depends on the surface energy: it looks like a ''sand pile'' on high-energy surfaces, but the molecules self-assembl e into a perfect bilayer on low-energy ones. These phenomena are under stood by considering the chemical structure of the molecules, and the dynamics of the films is related to the ''superspreading'' properties of these surfactants.