Pattern Formation in Evaporative Drying of a Polymer Solution Droplet over a Soft Swellable Substrate.

We report morphological evolution and pattern formation during evaporative drying of a droplet of polymethylmethacrylate (PMMA) dissolved in tetrahydrofuran over a soft, swellable cross-linked Sylgard 184 substrate. In contrast to the well-known coffee ring formation due to the evaporation of a polymer solution droplet over a rigid substrate, we show that the situation becomes far more complicated over a Sylgard 184 substrate due to solvent penetration and associated swelling. The combined effect of evaporation and diffusive penetration leads to significantly faster solvent loss and results in the formation of an in situ thin polymer shell over the free surface of the evaporating droplet due to the attainment of local glass-transition concentration. The diffusive penetration of the solvent also leads to the spreading of the three-phase contact line (TPCL) of the droplet after dispensing. The vertical component of surface tension acting at the TPCL results in the formation of peripheral creases along the boundary of the droplet after the TPCL pins. With the progressive solvent loss, the shell eventually collapses, resulting in a buckled morphology with a central depression. We show that the evolution pathway and the final deposit morphology depend strongly on the initial PMMA concentration ( C i ) in the droplet as it undergoes a transformation from a central depression surrounded by peripheral folds at lower C i to a central depression along with radial wrinkles at higher C i . During the late stage of the evolution process, the substrate undergoes de-swelling, which leads to flattening/rearrangement of the radial wrinkles, the extent of which again depends on C i . We explored how the deposition pathway and patterns vary over a topographically patterned substrate and found out that the presence of topographic patterns leads to even faster solvent consumption due to enhanced diffusive penetration at the corrugated liquid─substrate interface, eventually resulting in deposition with a smaller footprint and partially aligned radial wrinkles. The results significantly enhance our understanding of droplet evaporation over a substrate into which the solvent can penetrate and unravel the complex physics, which is significantly dominated by swelling rather than evaporation only, which is common over a rigid, non-interacting substrate.