On the interpretation of contact angle for geomaterial wettability: Contact area versus three-phase contact line

Several techniques are available to characterise the wettability of porous rocks wherein the contact angle measurement technique has gained popularity, especially in tight rocks. Recent studies in surface science however raise an important issue on contact angle measurement and its theoretical analysis - whether the contact angle is controlled by the properties of the substrate area underneath the droplet or by the properties of the substrate at the circumference forming the three-phase contact line. This important issue has not been discussed in geomaterial contact angle characterisation despite its significant importance. We, therefore, conducted a series of experiments to study the role of contact area and three-phase contact line on dictating the contact angle. The experiments considered the material heterogeneity (i.e. clay, organic matter, and acrylic), surface roughness and geometry, and its patterns. We also used a new approach using interferometry to investigate the contact angle localisation. The results of this study revealed that the formation of the contact angle is only dictated by the three-phase contact line and not the contact area. No change in contact angle was observed when different materials or surface geometries were introduced under the droplet. The results also suggested that an increase in roughness can lead to erroneous contact angle measurements due to localisation of the contact angle and changes in the three-phase contact line point. The results further showed that the surface pattern plays a major role in contact angle measurement and must be considered if present. Finally, the combined understanding of the results indicated that the current contact angle measurements of geomaterials might carry significant error and that additional methodologies for determining their surface wettability should be used.