Solutal Marangoni effects on pattern and skin formations on the rapidly evaporating surface of polymer solution layer

The mechanism of the development of solutal Marangoni instability in a thin layer of polymer solution, by rapid evaporation of volatile solvent, is studied numerically. By considering the conservation of mass, momentum, and concentration across the evaporating surface, physically reliable kinematic and boundary conditions are derived and implemented in numerical simulations. To simulate the drying of a polymer solution more realistically up to the point where 80% of solvent was evaporated, the concentration-dependent evaporation rate, viscosity, and diffusivity and the movement of the interface are taken into account. Numerical simulations demonstrate that the generation and merging of convective cell motions in a layer during drying lead to surface patterns as the drying process continues. The drying of a polymer film and the development of the surface topography including thickness deviation depend on various physical phenomena such as Marangoni stress, surface tension, vapor recoil pressure, evaporation rate, initial concentration of polymer, and variation of viscosity and diffusivity with concentration. Meanwhile, the vapor recoil force plays little role in the onset of instability motion and the irregularity of the evaporation surface. Furthermore, both the diffusivity reduction and the viscosity thickening due to evaporative concentration play a critical role in the formation of the skin layer, because they suppress the Marangoni instability motion and therefore impede the convective transport of concentrated polymeric solute.