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Title: Novel pillar formation in evaporating poly(ethylene oxide) droplets
Author: Baldwin, K. A.
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2012
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Unlike the familiar “ring-stain" formed when spilt coffee drops are left to dry, liquids containing high molecular weight polymer molecules leave a range of other deposit pat terns. In this thesis I observe that aqueous solutions of the polymer poly(ethylene oxide) (PEO) dries to form either the common coffee-ring stain, flat uniform “pancakes", or tall central “pillars". To investigate this phenomenon, I varied experimental factors including: atmospheric temperature, humidity and pressure; polymer molecular weight and concentration; water-ethanol solvent ratios; droplet volume, contact angle and inclination. These factors indicate a region in parameter-space in which central pillars form, favouring fast drying, low temperature, high contact angle, high concentration, high or low (but not intermediate) water-ethanol ratio, and intermediate molecular weight. I identify four stages in the pillar forming drying process, including a pseudo-dewetting liquid stage which appears to be driven by the formation of a contracting spherulite collar around the droplet's 3-phase contact line. If the liquid base radius recedes quickly enough compared with the height reducing effects of the evaporation, the growing solid deposit eventually lifts the droplet from the surface, resulting in the final central pillars. This is characterised by a minimum droplet volume when precipitation begins, above which the receding radius vanishes before the volume is lost to evaporation, resulting in tall central structures. Conversely, if the volume at the precipitation time is below this value, the height will reach zero during the pseudo-dewetting stage and the common coffee-ring stain is the result. I show that the dimensionless Péclet number Pe, which compares the relative effects of evaporation and diffusion on the polymer motion, successfully predicts the precipitation time and thus the final deposit shape. To incorporate the effect of molecular weight into our understanding, a further parameter of liquid phase resistance to the contracting collar at high viscosities is introduced.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available