Dynamics of pseudo-partial wetting studied by neutron reflectometry
We have presented a new analysis technique for neutron reflectometry. It is based on a combination of different scattering models coexisting side by side within the same sample. It extends possible applications of neutron reflectometry to non-uniform samples.
We present here the first results of an application of this technique to study pseudo-partial wetting dynamics in the process of condensation/evaporation of an octane on the liquid (surfactant modified aqueous solution) and solid (polished 111 silicon crystal) substrates. We describe the pseudo-partial wetting--a wetting regime, characterized by the coexistence of a thin liquid film and its residual droplet. The origin of this phenomenon is in van der Waals interaction between the substrate and air through the wetting liquid agent. We review two different approaches to calculate these interactions and use Lifshitz theory to estimate Hamaker constant for the silicon-octane-air system. Negative sign of this constant is a necessary condition for the pseudo-partial wetting.
We describe the dynamics of structural changes of the oil layer on substrate surface in the process of oil condensation/evaporation, Our neutron reflectometry data (analyzed on the basis of the new approach) demonstrate the presence of pseudo-partial wetting in this process and confirm its predicted dynamics.