Visualization study for dynamic and Static contact angle hystreresis of a water/ethanol droplet in hydrophobic capillary tubes via synchrotron X-ray imaging*

Contact angle hysteresis have investigated by numerous researchers for understanding interfacial behavior at moving contact lines However, due to limitation of visualization techniques, previous experiments for dynamic contact angle hysteresis have conducted limitedly in hydrophilic capillary tubes based on visible ray. Recently, there is continuous need for research on dynamic contact angle hysteresis in hydrophobic capillary tubes on various research and industrial fields. Therefore, in this study, we have visualized the dynamic contact angles of water-ethanol mixture slug in hydrophobic microtubes using synchrotron X-ray imaging with high temporal (~1000 fps)/spatial (~1.1 μm / a pixel) resolutions. For validating effects of fluid properties and interfacial sizes, we have measured the dynamic static contact angle hysteresis of various aqueous fluids (distilled water, water–ethanol 5% mixture, and water–ethanol 10% mixture) in hydrophobic capillary tubes (D =1.580, 1.023 and 0.796 mm). Based on the visualized data, we verified the previous experimental correlations for dynamic contact angles. In additions, we measured pressure drops at moving contact lines of a liquid slugs in hydrophobic capillary tubes. We have verified the relations between dynamic contact hysteresis and pressure drops at moving contact lines.