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

We report morphological evolution and pattern formationduringevaporative drying of a droplet of polymethylmethacrylate (PMMA) dissolvedin tetrahydrofuran over a soft, swellable cross-linked Sylgard 184substrate. In contrast to the well-known coffee ring formation dueto the evaporation of a polymer solution droplet over a rigid substrate,we show that the situation becomes far more complicated over a Sylgard184 substrate due to solvent penetration and associated swelling.The combined effect of evaporation and diffusive penetration leadsto significantly faster solvent loss and results in the formationof an in situ thin polymer shell over the free surface of the evaporatingdroplet due to the attainment of local glass-transition concentration.The diffusive penetration of the solvent also leads to the spreadingof the three-phase contact line (TPCL) of the droplet after dispensing.The vertical component of surface tension acting at the TPCL resultsin the formation of peripheral creases along the boundary of the dropletafter the TPCL pins. With the progressive solvent loss, the shelleventually collapses, resulting in a buckled morphology with a centraldepression. We show that the evolution pathway and the final depositmorphology depend strongly on the initial PMMA concentration ( C (i)) in the droplet as it undergoesa transformation from a central depression surrounded by peripheralfolds at lower C-i to a centraldepression 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/rearrangementof the radial wrinkles, the extent of which again depends on C-i . We explored how the depositionpathway and patterns vary over a topographically patterned substrateand found out that the presence of topographic patterns leads to evenfaster solvent consumption due to enhanced diffusive penetration atthe corrugated liquid substrate interface, eventually resultingin deposition with a smaller footprint and partially aligned radialwrinkles. The results significantly enhance our understanding of dropletevaporation over a substrate into which the solvent can penetrateand unravel the complex physics, which is significantly dominatedby swelling rather than evaporation only, which is common over a rigid,non-interacting substrate.

Automated summary

We investigated the morphological evolution and pattern formation during the evaporative drying of a droplet of PMMA dissolved in tetrahydrofuran on a soft, swellable Sylgard 184 substrate. Unlike the well-known coffee ring formation on a rigid substrate, the presence of solvent penetration and swelling on the Sylgard 184 substrate led to a more complex behavior. The combination of evaporation and diffusive penetration resulted in faster solvent loss, the formation of a polymer shell, and the spreading of the three-phase contact line.