Surfactant-Mediated Crystallization Patterns in Evaporating Saline Droplets: A Segment Anything Model 2-Based Quantitative Study

Controlling deposition patterns from evaporating droplets is key for applications like inkjet printing and functional coatings. This study deciphered the coupled regulation of salt and surfactants on evaporation-driven crystallization of sessile saline droplets. Using the Segment Anything Model 2 (SAM 2) for quantitative dynamics analysis, we systematically investigated four surfactant-salt systems [polyoxyethylene sorbitan monooleate (Tween 80)-sodium chloride (NaCl), cocoamidopropyl betaine (CAPB)-NaCl, hexadecyltrimethylammonium bromide (CTAB)-NaCl, and sodium cocoyl glycinate (SCG)-NaCl] across a full concentration matrix. A crystal area radial distribution coefficient ( M ) was introduced, delineating three distinct patterns: ring-like ( M ≤ 0.4), uniform discrete (0.4 < M < 0.6), and central aggregation ( M ≥ 0.6). These patterns are governed respectively by outward capillary flow, inward Marangoni compensatory flow, and dynamic equilibrium between the two antagonistic flows. Crystallization patterns show strong salt-surfactant concentration coupling. Notably, pattern tunability differs: Tween, CAPB, and CTAB systems enable pattern evolution from ring-like to central aggregation via concentration adjustment, while the SCG system exhibits concentration robustness, exclusively forming stable ring-like deposits. Surfactants also diversify evaporation pathways, inducing a multistage “spreading–constant contact radius (CCR)–mixed” mode in most systems, while the SCG system enforces a CCR mode. Moreover, surfactants enhance nucleation density and refine crystal size. Ionic surfactants stabilize growth kinetics and strongly suppress crystal migration, promoting edge immobilization. This study provides quantitative analytical methods and mechanistic insights for synergistic regulation of crystallization patterns via salt–surfactant combinations.