Pattern formation by droplet evaporation and imbibition in watercolor paintings
Watercolor or aquarelle is one of the oldest painting methods in which pigments suspended in an aqueous liquid are deposited over a substrate, typically an absorbing material such as paper. The physical processes which lead to pattern formation have not been studied in any depth, in spite of being closely related to flows in other contexts. Identifying and understanding these processes is of imperative importance to restore and preserve watercolor paintings. Here, we report an experimental study of the effect of the pigment concentration and paper humidity on the pattern formation derived from evaporation of droplets of watercolor paintings. Optical analysis reveals the formation of color gradients, stratifications, flat regions, borders, dendritic shapes, and radial tips. We found that droplet evaporation on dry paper forms ring-shaped stains which resemble to the \emph{coffee-stain effect} regardless of the nature of the pigment. The mean pixel intensity of such deposits follows an exponential function that saturates at high concentration, while the thickness of the coffee ring increase for watercolor inks containing colloidal particles and does not change for non-colloidal . Our experiments reveal that water distribution on the paper surface, and not the volumetric absorption of water on the paper, determine the structural characteristics of watercolor stains. We show evidence that the cornerstone in the creation of complex patterns in watercolor paintings is driven by the coffee ring effect and imbibition processes. Our findings aim to serve as framework for further investigations of the complex processes involved in this ancient art form and could guide restoration processes needed to preserve the heritage value of historical watercolor artworks.