Drying of a 75μL droplet of 20% poly(ethylene oxide) solution in water, viewed from beneath using an inverted microscope (Nikon Eclipse TE2000-S) with 2x objective lens. The image measure 5mm across and the frame rate is increased by a factor of 300. The bright specks in the droplet are small clusters of polymer that would not dissolve, and help to visualise the flow within the droplet. Pinned drying (stage 1) occurs for the first 5 seconds, during which there is clear evidence for recirculation flow at the contact line, with the liquid near the base flowing radially outwards and moving inwards above. As the flow at the edge ceases, a bright region of solid deposit appears behind the retreating liquid droplet (stage 2). There is no longer evidence of recirculation flow within the droplet. At 14 seconds, the deposit has reached its maximum height at the end of stage 3 and final drying begins. A wide solid ring has been deposited with a liquid region in the centre. From around 18 seconds, darker lines appear in the bright deposit, indicating completely dry areas and at 24 seconds, the central liquid region begins to dry (darken) and by 40 seconds, the structure is hollow.
For more information see: Phys.Chem.Chem.Phys, 2010, DOI: 10.1039/b922727j or http://arxiv.org/abs/1003.5150
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