Defining hydrophilicity and hydrophobicity through advancing contact angles and wetting transitions

The wetting behavior of a liquid drop on surfaces with varying hydrophilicity and hydrophobicity is crucial for fundamental research and practical applications. The conventional 90° contact angle threshold for distinguishing hydrophilic and hydrophobic surfaces has been debated.

This study systematically investigates advancing contact angles and wetting transitions on polydopamine-coated NOA81 substrates patterned with regular arrays of square micro-pillars. By controlling both the intrinsic surface wettability and microstructured roughness, we explore how these factors influence wetting behavior.

The intrinsic advancing contact angles of the flat polydopamine-coated substrates (θ_A^f) was tuned from 36.1° to 80.7°. Introducing square-pillar roughness substantially broadened the advancing contact angle (22° to 158°), revealing distinct roughness-dependent wetting behaviors for each θ_A^f. Two transition-capability regimes were identified: when θ_A^f > 80°, roughening enables a hydrophobic Wenzel-to-Cassie transition; while θ_A^f < 55°, increasing roughness induces a hydrophilic Wenzel-to-hemiwicking (penetrating) transition. These results demonstrate that the characteristic θ_A^f governs whether roughening activates hydrophobic or hydrophilic wetting pathways on square-pillar microstructured surfaces. The framework established here refines our understanding of roughness-mediated wetting transitions and provides practical guidance for designing microstructured surfaces with tailored wetting properties.