Model and experiments of the transient evolution of a thin, evaporating liquid film

• The transient evolution of thin liquid films was measured using reflectometry.
• A numerical model was developed using lubrication theory with kinetic evaporation.
• The region of highest evaporation reacts rapidly to perturbations of the film.

Experimental thickness profiles of evaporating thin films of n-octane and n-heptane on silicon were recorded using reflectometry. These films ranged from approximately ten nanometers thick in the adsorbed region to several micrometers thick in the meniscus region. For steady-state measurements, the reflectometer was traversed along the film to give thickness versus horizontal distance. For transient measurements, the reflectometer was fixed at several points as thickness was measured over time. While film thickness and substrate temperature measurements are practical, those of the evaporative flux profile and total heat transfer are not. Hence, a numerical model was formulated based on transient lubrication theory and modified to include kinetic evaporation and disjoining pressure. The model uses an ordinary differential equation solver to advance the governing equations in time with staggered finite differences for the spatial derivatives. Good correlation of the transient film thickness was achieved between the simulations and experiments. Thus, we have confidence in the inferred evaporative flux profiles and liquid–vapor interface temperatures. With this information, instantaneous heat transfer coefficients were derived for the entirety of the thin film region.