Thin film evaporation of n-octane on silicon: Experiments and theory

Microscope-based reflectometry was used to measure the thickness profiles of thin films of n-octane on silicon wafer substrates. Thin films, created in an axisymmetric capillary feeder, were subjected to heat or a nitrogen flow to promote evaporation into a saturated gas phase. Steady state film profiles were established, with a non-evaporating adsorbed film transitioning to the intrinsic meniscus. The reflectometer was coupled with micro-positioning motorized stages to provide two-dimensional profiles of the film thickness. A numerical model was formulated to include lubrication theory of the liquid flow within the film, heat conduction across the film from the heated wall to the liquid-vapor interface, kinetic theory of evaporation from the interface to the vapor phase, and disjoining pressure based on a retarded van der Waals interaction. When combined, the governing equations form a fourth-order, nonlinear differential equation for the film thickness vs. distance, which is solved numerically and compared to the experimental data. Useful results of the model include the liquid-vapor interface temperature, the Hamaker coefficient, and the evaporative mass flux profile.