Peculiar surface profile of poly(ethylene oxide) film with ring-like nucleation distribution induced by Marangoni flow effect

The drying processes of poly(ethylene oxide) (PEO) droplets on isothermal and non-isothermal substrates were studied and interesting results were found. When PEO droplet was evaporated isothermally at 30 °C, common evaporation with usual ‘puddle’ structure was formed. However, at a heating rate of 20 °C/min, the drying process developed two stages. At lower temperatures, a pinned contact line drying behavior was observed. At elevated temperatures (e.g., >60 °C), the free liquid surface started to shrink. The resultant PEO film appeared as a ‘dual-ring’ profile with a ring-like nucleation. Further annealing and AFM results suggested that nucleation behavior was due to the PEO chain memory effect of flow field in the drying droplet. This memory effect provided an inner and visual mark of the flow field. The rheological properties and surface temperature were also investigated to estimate Marangoni number (Ma), which increased significantly with temperature. The following mechanism was therefore proposed. At relatively low temperatures, the capillary flow was dominant, resulting in the formation of ‘ring’ at the edge. When the temperature increased to 60 °C, high temperature caused a sharp decrease of viscous stress and the increase of surface stress gradient, which resulted in the receding of contact line. Consequently, Marangoni convection became dominant and ‘dual-ring’ morphology finally formed.