Directional wetting anisotropy induced by surface roughness on polished copper surfaces

Understanding how surface roughness influences directional wettability is essential for designing functional metallic surfaces in applications ranging from heat transfer to microfluidic droplet transport. Previous studies reported the effects of polishing grit size on contact angles but were limited to a few roughness levels and did not quantify directional sensitivity or identify transition thresholds between anisotropic and isotropic wetting. This study systematically investigates wetting anisotropy on mechanically polished copper surfaces using fifteen grit levels (P100-P8000), producing a wide range of surface roughness (Ra = 0.05–1.62 µm). Apparent contact angle (CA), sliding angle (SA), and contact angle hysteresis (CAH) were measured along directions parallel and perpendicular to polishing grooves to capture anisotropic effects. Results reveal an exponential decay of CA with decreasing Ra (R2 > 0.96) and a clear threshold near Ra ≈ 0.5 µm, below which directional differences (ΔCA) drop from ∼30 to <2°, indicating a transition toward isotropic spreading. A sensitivity parameter (ΔCA/ΔRa) is introduced, enabling quantitative prediction of wettability changes with surface refinement. To complement this geometric analysis, directional work of adhesion (Wad) was computed from the receding angle using the Young-Dupré relation. Wad decreases as Ra decreases, and the directional difference becomes negligible on smooth surfaces; a regression-based sensitivity |ΔWad/ΔRa| peaks in the intermediate roughness band (0.5–1.0 µm). These findings establish both geometric and energetic scaling relationships linking roughness and directional wetting. The proposed sensitivity metrics (ΔCA/ΔRa and ΔWad/ΔRa) offer predictive tools for optimizing surface polishing in droplet mobility, water harvesting, and functional coatings.