Influence of evaporation on the morphology of a thin film of a partially miscible binary mixture

Abstract

In this paper, we develop numerical, theoretical and experimental analyses of the different morphologies that can be created by the phase separation phenomena that are induced by solvent evaporation in a thin film of a partially miscible binary mixture. Disregarding hydrodynamic effects, the Cahn–Hilliard–Cook and temperature equations are used to describe the thermodynamics of non-isothermal phase separation in a 2D thin film. Numerical simulations are performed to investigate the interplay between evaporation and phase separation and we examine the effect on the morphology of the film of several parameters such as the initial thickness of the layer, or the initial temperature and concentration of the mixture. Interestingly, the competition between evaporation and phase separation is shown to be the main determinant of the choice between a lamellar or a lateral pattern in the beginning of phase separation. For moderate evaporation rate, the spinodal instability takes place close to the evaporating interface and a lateral structure is formed. For stronger evaporation, the spinodal instability does not occur and a lamellar structure is created. In addition, the mid- or long- term evolution of the system is also considered. The thickness of the film is an important parameter in this analysis and possible modifications of the pattern over time are emphasized. Detailed physical and theoretical interpretations are proposed for the results and experiments in a Hele-Shaw cell that nicely confirm our predictions are presented.