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

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.

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.

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.