Pattern formation by the drying of saline droplets on pillars

The application of saline solutions for surface coating is pertinent across multiple biomedical fields, various technological sectors, and industries, including agriculture. The drying of salt solution droplets is key to understanding and controlling morphological structures on surfaces. In this paper, we report the study of pattern formation from the evaporation of saline drops (NaCl, CsCl, and KCl) placed on pillars. Our findings indicate that regardless of saline concentration and type, the drying process can be categorized into three modes: stable, metastable, and unstable. In both the stable and metastable modes, the droplet fixes to the pillar during the drying process and ensuing pattern formation; however, the crucial distinction lies in the metastable mode, where the droplet additionally undergoes mass loss through fluid drainage from the walls of the pillars. Conversely, in the unstable drying mode, the droplet undergoes rapid collapse, leading to a substantial loss of mass. The distinct drying modes dictate the resulting patterns on the pillars. We employ measurements of configurational entropy and fractal dimension as quantitative metrics to assess the complexity, reproducibility, and similarity of patterns. The texture analysis reveals that, at low concentrations of both CsCl and KCl, significant differences emerge among the patterns produced by the different drying modes, especially highlighting differences in patterns of the stable drying mode. Finally, we analyze fertilizer deposition patterns to prove that all three drying modes can occur in complex fluids.