Experiments on wetting on the scale of nanometers: Influence of the surface energy

Via ellipsometry, the time-dependent profiles of liquid droplets spreading completely on plane solid substrates are measured in the range 0--200 {}. Two different surfaces are used, a ``high-energy'' surface (wettable by water) and a ``low-energy'' surface (Langmuir-Blodgett layer of a double-bond-terminated fatty acid), respectively. We have observed that the shape of spreading droplets of the same fluid (polydimethylsiloxane) are quite different on these two surfaces: On the low-energy surface, the spreading occurs by the macroscopic lateral spreading of essentially one monolayer, away from the central part of the drop. On the contrary, for the high-energy surface, spreading occurs by the macroscopic lateral spreading of several monolayers in a kind of hierarchical process. Nevertheless, in both cases there is a signature of molecular structuring in the fluid flow at the center of the drop.