Understanding mineral and metal surface wettability heterogeneity through bubble contact angle distribution analysis.

HYPOTHESIS: Surface wettability is commonly characterized using single or average contact angle measurements. However, for chemically and physically heterogeneous solid-liquid interfaces, particularly under sub-monolayer adlayer conditions, average contact angles may not adequately represent spatial variations in adsorption-induced hydrophobicity. We hypothesize that measuring, mapping, and analyzing the variation of contact angles across a representative surface, reported as a distribution of contact angles, provides a more meaningful descriptor of adsorption-induced wettability heterogeneity, capturing its mesoscale manifestation even on high-purity mineral and metal surfaces. EXPERIMENTS: Captive bubble contact angle (BCA) measurements were performed on copper and chalcocite surfaces treated with amphiphilic sulfur donor-based ligands under controlled aqueous conditions. Instead of reducing data to a single or average value, contact angle data were analyzed using two distribution-based approaches: kernel density estimation and high-density region analysis. These provide a more realistic description of surface wettability and new insights into understanding adsorption-induced wetting behavior in complex aqueous interfacial systems relevant to surface modification processes. FINDINGS: Contact angle distributions capture spatial heterogeneity in surface hydrophobicity that is not represented by single or average values, revealing adsorption-induced wettability heterogeneity across sub-millimeter length scales. Distribution-based analysis provides a statistically robust description of interfacial wettability and enables direct visualization of the effects of non-uniform ligand adsorption on high-grade mineral or high-purity metal surfaces under sub-monolayer conditions. To our knowledge, this study provides the first systematic application of contact angle distribution analysis to quantify adsorption-induced wettability heterogeneity across mineral and metal surfaces under controlled conditions.