When still drops evaporate from a surface, they do so in several phases, as illustrated in the video above. Initially, the drop forms a spherical cap. At this point the velocity within the droplet is so small that it is difficult to resolve, but particles within the drop move outward toward the contact line. As the drop evaporates, they form a circle of sediment – the familiar coffee ring. As the drop flattens, radial velocity increases, drawing more and more particles to the coffee ring. Eventually the drop pulls away from the ring, leaving surface tension and evaporation to compete in driving the internal flow. During this phase, some parts of the contact line try to re-establish the flow pattern that made the first ring; this leaves behind circular segments broken up by the increasing instabilities in the contact line. In the final stage, surface tension smooths some of the irregularities and drives an inward velocity that leaves behind radial sediment segments. (Video credit: G. Hernandez-Cruz et al.)
Evaporating Drops
