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David Fairhurst: „Pillars from drying polymer droplets”
Nottingham Trent University, United Kingdom

Unlike the familiar "ring-stain" formed when droplets of spilt coffee are left to dry, liquids containing high molecular weight polymer molecules leave a range of other deposit patterns. In particular, we observer that aqueous solutions of the common polymer poly(ethylene oxide) (PEO) dry to form a tall central pillar. We identify four stages in the drying process, including a receding stage which appears to be driven by the formation of solid precipitate at the contact line. The growing solid deposit eventually lifts the droplet from the surface, resulting in the final central deposits. To investigate this phenomenon, we have varied experimental factors including: atmospheric temperature, humidity and pressure; molecular weight and concentration of the polymer solution; volume and contact angle of the initial droplet. We find a region in parameter-space for which the central pillars form, favouring fast drying conditions, high contact angle droplets, high concentration solutions and an intermediate range of molecular weights. We show that a dimensionless Péclet-like number Pe, which compares the relative effects of evaporation and diffusion successfully predicts the final deposit shape.