Surface wettability of quartz and calcite: Impact of cleavage, exposure time, and scale of measurement

Accurately characterizing the wettability of solid surfaces is essential for various applications. Numerous factors, such as operating conditions and the features of the sample surface, affect wettability. In subsurface applications, precise wettability characterization of rock/fluid systems is particularly important because it determines fluid distribution and multiphase flow within porous media. Despite operating under similar conditions, significant variations are observed in the reported wettability of specific rock/fluid systems. Factors contributing to these inconsistencies include surface roughness and surface cleavage, which are often overlooked in contact angle measurements. Therefore, this study examines the impact of surface roughness and cleavage planes on freshly cleaved surfaces in comparison to surfaces exposed to the atmosphere. Pure calcite and clean quartz surfaces were selected in this study. For the surface cleavage effect, both macro and micro contact angles have been utilized, coupled with advanced image analysis to visualize the wettability changes as a function of scale. Furthermore, Fourier Transform Infrared (FTIR) spectroscopy is utilized to determine surface functional groups responsible for wettability variation due to atmospheric contaminants. Findings suggest that freshly cleaved surfaces exhibit greater hydrophilicity than their exposed counterparts, underscoring the necessity for caution when dealing with calcite and quartz due to the pivotal role of exposure time in determining wettability. Surface roughness measurements have been conducted to examine the impact of exposure time on surface topography and the results confirm that the change in surface roughness was negligible. The findings from this study enhance comprehension of the mechanisms at the nano-to milli-metre scale responsible for wettability variations. Also, a scientific understanding of the mechanisms responsible for wetting characteristics is established, which can be beneficial in addressing the discrepancies in the observed wetting behaviour.