Anomalous drying dynamics of a polymer solution on a substrate

When a polymer solution with volatile solvent is exposed to open air, an elastic layer (called a skin) is often formed at the surface of the solution due to evaporation of the solvent. After such a skin is formed, further extraction of the solvent from the solution caused by evaporation has generally been considered to reduce the pressure in the solution. We have found that, in PMMA/acetone droplet placed on a substrate, the liquid below the skin layer is pushed out as the solvent evaporates further. These phenomena indicate that the pressure in the solution increases by solvent evaporation. It is considered to be caused by the shrinkage and other structural changes taking place in the skin layer.