Central spot formed in dried coffee-water-mixture droplets: Inverse coffee-ring effect

We have investigated the formation of central spot when coffee-water-mixture droplets are dried, which is a reverse effect of the famous coffee-ring effect (CRE). Typically, the central spot is unnoticeable or ignored even though it is significant. The ring-shaped stain is due to the capillary flow (CF). The CF is caused by the difference in the evaporation rates between the edge and the center of the droplet owing to a temperature gradient. The same temperature gradient, however, causes the so-called Marangoni flow (MF). The MF is routinely ignored, particularly when the solvent is water. However, from experiments, we found that, in addition to the ring-shaped stain formed by the CRE, central spot is formed. When the volume of a droplet of coffee-water mixture is ∼15 μL, the strongest central spot is formed We employed an improved enhanced heat-transfer model, in which the shape of the droplet was assumed to be spheroidal, to understand the effects of the MF on the central-spot formation. According to the model, the formation of central spot can be explained, at least, semi-quantitatively. As a result of the existence of a stagnation point, the direction of flow is changed. Furthermore, the dependence of its location on the initial geometrical parameters of the droplet is very important. In addition, the effect of gravity was investigated for pendant droplets. The central-spot formation is more prominent in pendant droplets than in sessile droplets because the gravity enhances the MF in the pendant droplets, while it hinders the MF in the sessile droplets.