DRYING DROPLETS

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.

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.