Three-Dimensional Observation of Three-Phase Contact Line with Precursor Film on Nanostructured Surfaces

In contrast to the thorough research on precursor films on homogeneous surfaces, their behavior on surface structures remains unclear. In this talk, we report the behavior of the three-phase contact line of silicone oil droplets on quartz surfaces with nanosized grooves observed by confocal scanning interferometry (CSI). CSI observations revealed that the contact angle of the silicon oil droplet was approximately 3.2°, and a precursor film with tens nanometer thickness existed. The contact line became pinned when it touched the top edge of the nano-groove. We found that, while the contact line remained pinned, the precursor film continued to penetrate into the groove and its thickness increased over time. It was also found that the contact line depinned with a smaller contact angle change (0.9°) than predicted by the groove's slope angle of 11.0°. These phenomena were not observed in the case of the partially-wetted glycerol droplet that lacked a precursor film. From these observations we conclude that the precursor film weakens the pinning effect, resulting in the easy spread of the liquid film.