Evaporation of strong coffee drops

Coffee stains remain when coffee drops evaporate. Commonly, dilute coffee exhibits ring-like stains after evaporation governed by self-pinning and vapor diffusion. Modern printing technology requires dense-ink printing, so that it is useful to understand the evaporation dynamics of dense inks. Here, we experimentally explore how strong coffee drops evaporate and how coffee stains are generated from strong coffee that can give an insight into dense inks. We find that evaporation rates of strong coffee drops become slower as coffee concentrations become higher. The evaporation dynamics of strong coffee deviate greatly from dilute coffee, revealing nonlinear evaporation dynamics, attributed to the concentration-dependent fluid property. The nonlinear evaporation of strong coffee can be important in understanding the drying-mediated printing of dense nanoinks.