While often a metaphor for boredom, watching paint dry can be highly intellectually engaging (to the right observer). For example, a long-standing problem faced by the ink-jet printing industry is one you will have encountered in everyday life - the coffee-ring effect - where droplets of coffee evolve from a uniform, watery hemisphere into a non-uniform, dry, ring stain. This effect isn't limited to coffee - red wine, ink, blood, sweat & tears all display this effect to various degrees, which is what makes it so troublesome if your aim is to print nicely uniform dots of ink.
The research highlighted here concerns the drying of polymer solution droplets, which first seemingly undergo this universal coffee-ring process, causing a build-up of polymer at the droplet edge, but upon further drying, this evolves into something remarkably non-ring-shaped - tall central pillars. Over a few publications, we explore the physical mechanism behind this pillar growth, and the precise conditions in which pillar formation is favourable.
- Drying and deposition of poly(ethylene oxide) droplets determined by Péclet number, Soft Matter, 2011.
- Monolith formation and ring-stain suppression in low-pressure evaporation of poly(ethylene oxide) droplets, Journal of Fluid Mechanics, 2012.
- Growth of solid conical structures during multistage drying of sessile poly(ethylene oxide) droplets, Physical Chemistry Chemical Physics, 2010.
- The effects of molecular weight, evaporation rate and polymer concentration on pillar formation in drying poly(ethylene oxide) droplets, Colloids and Surfaces A, 2012.
The results of this research were then naturally expanded into other droplet systems, such as blood, which will indeed also undergo this central growth stage when pressure is lowered. Along the way, we also developed a novel method for observing flows inside a drying droplet, in situ, using Optical Coherence Tomography, a technique which warranted its own publication.
- Classifying Dynamic Contact Lines in Drying Drops. Soft Matter, 2015.
- Imaging internal flows in a drying sessile polymer dispersion drop using Spectral Radar Optical Coherence Tomography (SR-OCT), Journal of Colloid and Interface Science, 2013.