Dynamic contact angles and pressure drop at moving contact lines of water/ethanol mixture slug in hydrophobic capillary tubes via synchrotron x-ray imaging

Abstract

For decades, numerous researchers have evaluated dynamic contact angles to understand the interfacial behaviors of moving contact lines. However, owing to the limitations of visualization techniques based on light rays, experiments for dynamic contact angles have typically been conducted in high solid–liquid interaction cases (e.g., hydrophilic surfaces with oil fluids). Currently, in research and industrial fields, there is a continuous need for research on dynamic contact angles in low solid–liquid interaction cases (e.g., hydrophobic surfaces with aqueous fluids); therefore, in this study, we used synchrotron x-ray imaging with high temporal (∼1000 fps)/spatial (∼1.1  μm/pixel) resolutions to precisely measure the dynamic contact angles of various aqueous fluids (distilled water, water–ethanol 5% mixture, and water–ethanol 10% mixture) in hydrophobic minitubes (D = 1.580, 1.023 mm) and microtubes (D = 0.796 mm). Based on the visualization data, we analyzed the dynamic/static contact angles and pressure drops at the moving contact lines according to the fluid properties and tube diameters. This study proposes that the contact angle hysteresis, dynamic contact angles, and pressure drop at the moving contact lines in hydrophobic capillary tubes can be estimated using modified equations and correlations.