Self-organized target and spiral patterns through the "coffee ring" effect.

We studied the precipitation pattern of fullerene C60 nanocrystals generated through the evaporation of a confined liquid bridge. In contrast to the usual "coffee ring" pattern, both target and spiral patterns were observed. The characteristics of the pattern critically depended on the concentration of the solution, the temperature, and the level of vacuum. In addition, the morphology of the microscopic precipitates varied greatly as a function of these experimental parameters. This pattern formation can be interpreted as a two-step rhythmic nucleation/precipitation of fullerene crystals during receding motion of the contact line. Symmetric motion of the contact line produces a target pattern, and the propagation of distortion of the liquid interface caused by a disturbance generates a spiral pattern.