As shown in the photo above, when a drop of coffee is put on a substrate, after the drying process the drop exhibits a ring pattern. This phenomenon has been of course well-known in our daily life, and has been discussed in the soft-matter community. The pioneering work for this issue is the paper by Deegan. He explained the phenomena by the evaporation-induced flow in the droplet. The evaporation rate of the liquid is much faster at the edge of the droplet than that at the center. This difference in evaporation rate generates a liquid flow from the center to the edge. Consequently, the particles are pushed by this flow and concentrate at the edge.
The new study coming out has revealed that the coffee ring phenomenon mentioned above exhibits a variety of pattern formations in relation to the liquid crystalline phases and anisotropic frictions. The study would be of industrial importance for painting processes, for example.