It all started many years ago: I was in the lab planning a droplet evaporation experiment and I added a tiny amount of salt to a particle solution to balance the liquid/particle density ratio. When I left the droplet to evaporate I could not believe it… the flow inside the droplet went exactly in the opposite direction as it should go, and I mean exactly in the three dimensions and three components!. Years after, my colleagues from the University of Granada showed me some funky ring-shaped stains that they observed in salty droplets, and they also had a good hypothesis for the flow inversion.
It has been several years working on this project with colleagues from 3 different countries and 5 different institutions. A few recent collaborations have been crucial: on the one hand, the simulations from both Stefan Karpitschka (Max-Planck for Dynamics & Self-organization, Göttingen) and Christian Diddens (TU Eindhoven)
While writing the paper, I met the artist Maurice Mikkers, the man behind the imaginarium of tears, and the beautiful micrographs of dried tears here shown below. Everything made sense then, the outer rim that you can see in his micrographs also follows the mechanism we propose!.
The paper will be published soon in Physical Review Fluids, you can find an arXiv preprint here.