Watercolor painting: Pigment pattern formation in drying aqueous droplets

Watercolor paintings have been used by humans throughout history to capture our perception of the natural world. From early men to modern artists, this painting technique has not changed significantly. It uses pigments, initially suspended in an aqueous solution, which is placed on a wet or dry surface, commonly paper. After the evaporation of the solvent, fascinating textures, which form part of the artwork, emerge. In this investigation, we used fluid mechanics to understand the complex mechanisms behind the watercolor paintings materialization. By conducting experiments, we scrutinize the effect of the pigment concentration, paper humidity, and ethanol content on the formation of the patterns derived from evaporation of droplets. We observe that the patterns evolve as pigment concentration increases, changing from coffee-ring-type to quasi-uniform stains. However, when the paper humidity and ethanol concentrations are also changed, the deposition process of pigments is altered, leading to a wide variety of patterns in the deposit. We discuss the complex flow mechanisms within the drying drop, such as percolation, aggregation and Marangoni processes, that are behind the pattern formation.