Pressure drop and dynamic contact angle of triple-line motion in a hydrophobic microchannel

Surface wettability influences flow configuration in many cases, such as pool boiling, flow boiling, and adiabatic flow regime. In mini and microchannels, the flow regime of a two-phase flow is also influenced by surface wettability. In particular, slug flow in a two-phase flow regime varies according to surface wettability. In hydrophilic microchannels, slugs are generated with liquid films, but in hydrophobic microchannels, they are generated with triple-lines instead of liquid films. Moreover, a slug with a triple-line exhibits a higher pressure drop than a slug with a liquid film, due to the motion of the triple line. Previous researches have observed that the pressure drop of a triple-line is affected by the dynamic contact angle, channel diameter, and fluid properties. Furthermore, the dynamic contact angle is affected by velocity, static contact angle, and fluid properties. To understand the pressure drop of a triple-line, we measured the pressure drops of slugs with triple-lines for various diameters (0.546, 0.763, 1.018, 1555 and 2.075 mm), several fluids (DI water, DI water-1%, 5%, 10% ethanol), and a range of velocities (0.01–0.4 m/s). Dynamic contact angles were calculated from an equation describing the pressure drop of a triple-line. Over the given experimental range, previous correlations have underestimated the dynamic contact angles in comparison with the experimental values. We suggest that the reason for this underestimation is that previous correlations were developed solely in a dynamic contact angle regime in which inertial forces were not considered. Accordingly, we propose a new dynamic contact angle correlation for a regime in which inertial forces are considered.