Dynamics of Contact Angles and Hemiwicking on Rock Fracture Faces

The dynamics of contact angles and capillary wicking (hemiwicking) were investigated on rock fracture surfaces from a selection of low-porosity rocks with different mineralogy: Burlington Limestone, Crossville Sandstone, Mancos Shale, Sierra White Granite, Vermilion Bay Granite, and Westerly Granite. Wetting height data for rough fracture faces were acquired in a parallel view using dynamic neutron radiography at the Oak Ridge National Laboratory Neutron Imaging Facility. Hemiwicking rates on the rock fracture surfaces were determined using a high-speed optical setup with a perpendicular viewpoint. Wetting height versus time relationships for both methods were delineated through changepoint analysis. The contact angle of the fracture surface (𝜃_R) was then quantified based on the maximum wetting height. Statistical significance was assessed at the 95% confidence level. Analysis of variance indicated statistically significant differences in mean 𝜃_R values between rock types. Regression analyses between 𝜃_R and the contact angles of polished rock surfaces (𝜃_A) and the Wenzel Roughness Factor yielded statistically non-significant relationships. Linear regression showed that the median wetting height during hemiwicking behaved linearly with respect to the square root of time. Surface sorptivity was quantified by the proportionality constant between the height of capillary wetting and the square root of time. Analysis of variance indicated statistically significant differences between rock types in mean surface sorptivity values. A statistically significant negative relationship was observed between surface sorptivity and 𝜃_A, while non-significant relationships were observed between surface sorptivity and 𝜃_R, and the Wenzel Roughness Factor. An analysis of variance of the interquartile range (IQR) for wetting height revealed statistically significant dependencies on both rock type and time, with no interaction. Overall, the results point to differences in mineralogy, rather than roughness, as the main control of contact angle and hemiwicking dynamics on rock fracture faces.