When a solid evaporation mask is placed above a drying thin film of polymer solution, spatial heterogeneities of the evaporation flux are generated. This leads to deformations of the liquid film. Once the film deformed, the liquid starts flowing because of gravity and capillarity. These flows transport polymer from the masked region to the unmasked region, which is why the dried polymer film is thinner in the masked region than in the unmasked region. Similar effects have been described qualitatively in literature and numerical simulations were used to predict the shape of the dried deposit. However, the role of gravity remains elusive and the dependence of the features of the formed pattern on the different parameters has not  been quantitatively established.

In this study, we show that the shape of the dried deposit can be predicted using an analytic approach. Our predictions are in quantitative agreement with the results we obtained experimentally by drying films of polystyrene/toluene solutions. We also demonstrate that gravity can play a role in the deformations of the films, even if their initial thicknesses are 1 order of magnitude smaller than the capillary length.

This work has been published in Langmuir journal https://doi.org/10.1021/acs.langmuir.2c02917

3D profile of a polymer layer obtained by drying a film of polystyrene/toluene solution under an evaporation mask