From coffee rings to coffee eyes

We discuss how the stain left after evaporation of a suspension evolves with heating of the glass or plastic on which the liquid has been deposited. Upon increasing the substrate temperature, it is found that the stain gradually changes from the usually observed ring to an "eye" shape, that is, a combination of the thick central stain and the thin outer ring. Both the size and the relative volume of the central stain increase with the substrate temperature. The main mechanism for this phenomenon is proposed to be an enhanced Marangoni recirculation flow on hot substrates. These findings can be exploited to continuously tune the morphology of coffee stains, with potential applications in self-assembly and ink-jet printing.