Dynamic contact angle measurement of hydrophilic open microchannels: The role of surface wettability

Wetting dynamics play a major role in many practical applications; however, many fundamental problems remain unresolved, especially in relation to the dynamic contact angle. The present study investigates the movement of a gas-liquid interface in an open microchannel with different materials and wettability. Using a high-speed microscope camera, the shape of the dynamic equilibrium of the gas-liquid interface in a flowing state was recorded, including the microscopic regions near the contact line. The results show that the effects of the surface wetting condition play a crucial role in altering the apparent dynamic contact angle. During the experiment, we observed an interesting phenomenon where the dynamic equilibrium contact angle under flow conditions is larger than the contact angle predicted by the Cox-Voinov law. This may be attributed to the existence of friction between the fluid and the wall surface under flow, which reduces the additional pressure at the gas-liquid interface, and the magnitude of friction is manifested through the hysteresis contact angle. The instability and periodic variation of the microscopic contact angle are caused by the surface heterogeneity near the contact line, namely, the spatial variation of solid-gas and solid-liquid interfacial tensions. Understanding the surface properties can help optimize the interface system's design and improve its efficiency for use.