Structure and dynamics of pseudo-partial wetting precursor films

Wetting of liquids on high energy solids where the spreading parameter is positive is expected to lead to nanometer-thick precursor films spreading out of the macroscopic liquid body. In this film, Van der Waals interactions induce either disjoining or conjoining Derjaguin pressure that respectively tend to separate the film interfaces or to bring them together.
We report for the first time systematic observations of droplets and their precursor films of high polarisability polymer melts on silica, which lead to a pseudo-partial wetting situation: at equilibrium, the droplet coexists with its film, in which the Derjaguin pressure is conjoining. By ellipsometry, we clearly evidence the unique structure and dynamics of these precursor films compared to total wetting films. Whereas in total wetting, the precursor film ends with a sharp step and consists of dense macromolecules, the profile of pseudo-partial wetting films is diffusive and the molecules form a quasi-two-dimensional gas with a diffusion coefficient specific to each polymer. These results were extended to a series of common organic polymers and rationalized within the framework of Van der Waals interactions, and short range surface interactions.