Unveiling droplet morphologies: real-time viscosity mapping reveals the physics of drying polymer solutions.

The drying of polymer solution droplets produces a fascinating array of morphologies, driven by the formation of a gel-like 'skin' at the vapor/liquid interface. While this skin plays a crucial role in shaping the final droplet form, its precise influence has remained elusive. We present a study that combines innovative fluorescence techniques with classical fluid dynamics to provide insights into the effect of skin formation on the macroscopic final structure of the droplet. Using viscosity-sensitive molecular rotors, we achieve unprecedented real-time, spatially-resolved measurements of local viscosity during the drying process. This novel approach allows us to directly observe and quantify skin formation and growth with micrometer-scale precision. Our experiments reveal that the average thickness and spatial non-uniformity of the skin are the key determinants of the final droplet shape. Droplets were investigated under identical ambient conditions and pinned contact lines, varying only the initial contact angle. This approach yields three distinct morphologies: coffee-rings, 'Mexican hats', and snap-through buckled shapes. For initial contact angle around 30°, skin formation initiates at the contact line rather than the apex, explaining the classic coffee-ring effect. For initial contact angle around 55°, a thicker skin forms near the contact line compared to the apex, resulting in a weaker central region. As evaporation proceeds, this non-uniform skin deforms into the characteristic Mexican hat shape. In contrast, surfaces with initial contact angle around 110° produce a thin, uniform skin that undergoes a dramatic snap-through buckling instability. Crucially, we demonstrate that the timing of morphological changes is directly linked to abrupt variations in skin thickness. Our results not only provide a comprehensive understanding of these complex phenomena but also align with and extend recent theoretical predictions by Head. This work bridges the gap between microscopic skin dynamics and macroscopic droplet behavior, offering a new paradigm for controlling deposition patterns in applications ranging from inkjet printing to biomedical assays.