Wetting mechanisms at the microscale: From classical models to topological pinning

Wetting phenomena at the microscale govern critical interfacial behaviors, spanning natural self-cleaning surfaces and engineered microfluidic devices. Classical thermodynamics, formulated in the Young, Wenzel, and Cassie-Baxter models, forms the basis of static wettability. This review examines the transition from global interfacial tension minimization to topological wetting. In this approach, localized energy barriers at the three-phase contact line (TCL) largely determine macroscopic behavior. We analyze the microscopic origins of contact angle hysteresis (CAH) and pinning, showing that surface topology can outweigh intrinsic chemistry in defining apparent macroscopic wetting states. We highlight how closed-loop micro/nano structures impose continuous pinning, enabling programmable control of droplet adhesion and anisotropic transport independent of material chemistry. Furthermore, we discuss practical applications of these principles in anti-icing, microdroplet reactors, and self-cleaning surfaces, emphasizing the role of structural optimization in overcoming the durability-functionality trade-off. Finally, we outline future challenges in the field. We propose that integrating artificial intelligence (AI)-driven inverse design and adaptive ``4D'' interfaces will advance wetting science from a descriptive discipline into a precise engineering toolkit based on dynamic contact line energy.