Coffee is a particle suspension or, more specifically, a colloidal dispersion. When the water in a coffee drop evaporates, a ring-shaped stain remains. The mechanism of the stain’s formation has been well described by Deegan et al.: The evaporation takes place from the edge of the stain. Because the amount of liquid at the edge gets restored with liquid from the centre of the drop a capillary flow is generated as the liquid evaporates. This flow drags more and more particles to the edge of the stain leaving the ring-shape behind.
This deposition mechanism can be applied to make colloidal crystals, but for a long time the mechanism of the order-disorder transition has been unclear, i. e. it has been unknown under which conditions the evaporation would result in a colloidal crystals. Detlef Lohse and co-workers have recently brought light to the basic mechanism of this order-disorder transition: At the beginning of the evaporation the liquid flow to the edges is low and the particles dragged to the edge have time to crystallize. At the end the speed of the flow is a lot higher and particles with high-speed are packed into a disordered phase.
Read more:
“Order-to-Disorder Transition in Ring-Shaped Colloidal Stains”, Phys. Rev. Lett. 107, 085502 (2011)
“Contact line deposits in an evaporating drop”, Phys. Rev. E 62, 756–765 (2000)