Wetting dynamics of oil/water compound droplets on multilevel wettability-patterned surfaces

Oil/water compound droplets with core–shell structures are crucial in open-surface microfluidic devices, particularly for applications in droplet-based reactors, emulsification, and material synthesis. However, a comprehensive understanding of the wetting dynamics that govern the formation of core–shell compound droplets remains elusive, limiting the ability to predict and control compound droplet morphology, stability, and encapsulation efficiency. Here, a novel transient two-dimensional axisymmetric (2D) phase-field model was developed to accurately capture the wetting dynamics of oil/water compound droplets on multilevel wettability-patterned surfaces. This study systematically investigates the effects of key factors, including the Weber number (We), oil/water volume ratio, pattern size, multilevel wettability differences, and three-dimensional (3D) structure, on the dynamics of wetting characteristics of compound droplets. Results show that increasing the We accelerates droplet spreading, but wettability boundaries remain crucial in governing encapsulation, with larger oil droplets achieving full core–shell structures. Wettability pattern size regulates spreading range and equilibrium shape, multilevel wettability differences create stabilizing interfacial energy barriers, and concave 3D structures further enhance core–shell assembly. This work presents groundbreaking insights into the wetting dynamics of compound droplets, providing valuable theoretical guidance for designing functional interfaces and multiphase systems in droplet-based applications.