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

Abstract

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 Ci to a central depression along with radial wrinkles at higher Ci . 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 Ci . 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.

Pattern Formation in Evaporating Polymer Solutions─Interplay between Dewetting and Decomposition

“…Evaporation is inherently a nonequilibrium process. The fraction of solutes in an initially dilute suspension progressively increases, resulting in variations in their flow behavior, which, in turn, could affect the wetting/dewetting rate and diffusion of solutes. ,,, Evaporation may create local concentration fluctuations, resulting in phase transitions like crystallization ,, and glass transition. ,− In addition, the interaction of the suspension with the substrate dictates the contact line dynamics and thus reflects on the resulting patterns. , To understand the development of evaporation-induced patterns, several theoretical approaches involving hydrodynamics and contact line dynamics, − gradient dynamics with lubrication approximation, − and moving boundary models were developed. − However, despite such progress, a complete understanding of the mechanisms underlying the pattern formation of evaporating solutions remains elusive.…”

Joint Experimental and Theoretical Study on Deposition Morphologies in Polymer Sessile Droplets

“…Sessile droplets may create films with complex morphologies after evaporation of volatile solvent. − In recent years, the related droplet processing techniques have been extensively investigated in fields of medical diagnosis, , inkjet printing, − spray cooling, spray coating, , and the preparation of functional materials. − For example, colloidal polystyrene droplets prepared by inkjet printing may have different lattice packing on microscopic scale and, in turn, affect the opal-like optical properties for holography on macroscopic scale; controlling the anisotropic orientation in deposited organic polymer semiconductors could improve carrier transport for microelectronic devices; , predefined deposition morphologies for biometric identification can assist the drug screening and delivery. , The sessile droplet techniques are able to prepare polymer films with milli- or micron-meter sizes, which complements conventional methods for large-area polymer films (e.g., dip coating, blade coating, and spin coating). − The biggest advantage is to deposit polymer films under the natural process of evaporation, which does not need additional energy input. The broad application of this technique requires the precise regulation of deposited morphologies during solvent evaporation.…”

“…For example, in a previous work, we revealed that the high-molecular-weight poly­(ethylene oxide) (PEO) chains were aligned along the radial direction of a coffee ring film by internal flow, while the low-molecular-weight PEO chains were isotropic. In addition to the molecular weight, it is also possible to generate many new deposition morphologies by varying polymer concentration and solvent evaporation rate. ,,, We believe that the bridge can be established between different deposition morphologies (such as multiring, coffee ring) and specific properties of polymer solutions. As far as we know, systematic experimental studies and theoretical considerations are still lacking to shed insight into the fundamental mechanism of the evaporation of polymer sessile droplets from a dynamic perspective.…”