Drying Drops of Paint Suspension: From Fried Eggs to Quasi-Homogeneous Patterns

Drying of multicomponent sessile drops is a complex phenomenon involving intricate mechanisms. Here, we study the evaporation of drops made of paint suspension and investigate the influence of the substrate temperature and suspension concentration on the resulting deposit patterns. At low concentrations and temperatures, the pigments appear highly concentrated in a narrow area at the center of the drop, a morphology we call fried eggs. Increasing the temperature or concentration leads to more homogeneous patterns. From a top-view camera used for monitoring the whole evaporative process, we identify three mechanisms responsible for the final pattern: inward/outward flows that convect the pigments, gelation of the paint suspension where pigments accumulate, and final drying of the drop that freezes the location of the pigments onto the substrate. The relative kinetics of these three mechanisms upon concentration and temperature govern the deposit growth and the morphology of the final pattern. These observations are quantitatively supported by rheological measurements highlighting a strong increase of the viscosity with concentration, consistent with the gelation mechanism. Finally, we show that the kinetics of drop drying is controlled by the substrate temperature.

Automated summary

The study investigates the drying of multicomponent sessile drops and its influencing factors, specifically the substrate temperature and suspension concentration. It is found that the deposit patterns are influenced by these factors, with low concentrations and temperatures resulting in highly concentrated pigments at the center of the drop. Increasing the temperature or concentration leads to more homogeneous patterns. Three mechanisms are identified to be responsible for the final pattern: inward/outward flows, gelation of the paint suspension, and final drying of the drop. Rheological measurements support these observations, indicating an increase in viscosity with concentration consistent with the gelation mechanism. Additionally, the study shows that the drying kinetics of the drop is controlled by the substrate temperature.