Instability and droplet formation in evaporating thin films of a binary solution

Abstract

We consider an instability phenomenon in a bilayer structure resulting from phase separation in a thin film of mixed solutions located on a water surface. The top layer consists of a hexane/hexadecylamine solution with thickness d(2), the lower one of an amyl acetate/cellulose solution with thickness d(1). During evaporation of the solvents from both layers, their thickness, surface tension, and viscosity change continuously with time. The thickness d(2) decreases significantly faster than the thickness d(1), because the evaporation rate of hexane is much larger than that of amyl acetate. Eventually, the top layer decomposes into droplets when its thickness d(2) was only a few nm, while the thickness d(1) was still some 100 nm. In addition to the experiments, we present calculations based on energetic arguments which are in good agreement with experimentally determined geometrical parameters of the droplet pattern, such as droplet diameter, droplet height, interdroplet distance, and number of droplets per unit area.