Defining hydrophilicity and hydrophobicity through advancing contact angles and wetting transitions
Journal
Journal of Colloid and Interface Science
Journal Volume
708
Start Page
139801
ISSN
00219797
Date Issued
2026-04-15
Author(s)
Abstract
Hypothesis: 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. Experiments: 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. Findings: The intrinsic advancing contact angles of the flat polydopamine-coated substrates (θAf) 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 θAf. Two transition-capability regimes were identified: when θAf > 80°, roughening enables a hydrophobic Wenzel-to-Cassie transition; while θAf < 55°, increasing roughness induces a hydrophilic Wenzel-to-hemiwicking (penetrating) transition. These results demonstrate that the characteristic θAf 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.
Subjects
advancing contact angle
hemiwicking state
hydrophilic
hydrophobic
microstructured surface roughness
polydopamine coating
Publisher
Academic Press Inc.
Type
journal article