On the lifetime of a coffee drop

Abstract

As we know, a coffee drop leaves after evaporation a circular stain whose diameter is generally that of the drop. This effect has been studied in depth for about 25 years, but it has been rarely emphasized that the stain dramatically amplifies the contact angle hysteresis. Accordingly, the liquid can remain pinned from the beginning to the end of the evaporation, which explains the size of the stain. Here we discuss a consequence of this strong pinning. It is found to make the kinetics of evaporation much faster than that of water, by about 30%, since a very flat drop evaporates quicker than a bulging one. This implies that any system blocking the contact line will accelerate evaporation, which we show by etching the substrate with a groove. We finally question the generality of the effect by considering various examples of evaporating complex liquids.