Transport Kinetics of Wetting Layers

The transport kinetics of wetting layers of ethanol and propane on silver substrates is investigated by monitoring the temporal decay of pulsed-laser-induced spatial thickness modulations. Our method allows to distinguish between different transport mechanisms, such as direct exchange with the vapour phase, viscous flow within the wetting layer, or surface diffusion. In either case, the activation energy found for the lateral transport points to viscous flow as the dominant mechanism in the investigated temperature range. For very thin films, the flow proceeds considerably slower than expected from bulk viscosity data. This suggests that the proximity of the substrate influences the morphology of the liquid, giving rise to an enhanced effective viscosity.