In situ wrinkling during evaporative drying of a polymer solution drop on a soft swellable substrate

A sessile liquid drop can deform the substrate on which it rests if the solid is sufficiently “soft” because of the capillary forces exerted by the drop. The drying of complex fluids such as polymer solution involves complex spatiotemporal evolution which involves solvent diffusion, glass transition etc. We have studied here evaporative drying of a polymethylmethacrylate (PMMA) in tetrahydrofuran (THF) solution drop on a soft PDMS substrate. We have observed two distinct type of wrinkles depending on the initial polymer concentration, such as radial lines and central depression with concentric rings A thin polymeric skin at air-polymer interface of the drop is formed due to fast evaporation as well as fast diffusion of THF into the substrate, which gets wrinkled with gradual decrease in the solution volume enclosed by it.
We have also investigated the effect of UV Ozone treatment of the substrate on the final droplet morphology. Different wrinkle morphologies for different UVO durations such as dimples, random lines and spherullite etc. are observed due to differential swelling tendency of the effective bilayer substrate consisting of the oxidized and pristine PDMS layer, which are frozen by polymer deposition along the contour of the wrinkles.