Impact of Sub-Nanoscale Surface Topography on Contact Line Profile: Insights from Coherence Scanning Interferometry

Despite the importance of the effect of subnanoscale roughness on contact line behavior, it is difficult to directly observe the local behavior of contact lines at the micro- and nanoscale, leaving significant gaps in our current understanding. In this research, we investigate contact line motions and their relationship with nanoscale surface topography using coherence scanning interferometry. Our experiments were conducted on the substrates with different wettability without changing nanoscale surface topography. Titanium dioxide was used as a substrate, the wettability of which was varied under UV-light irradiation. A ridge-like structure with a height of approximately 1 nm was observed to cause contact line deformation toward the droplet side, regardless of the direction of the contact line motion. This was explained in terms of an imbalance in the local capillary pressure at the nanoscale contact line. We also found that the deformation becomes larger on the more hydrophilic surface, which was rationalized by theoretical prediction based on analysis of the work done by the force acting on the contact line and the change in surface free energy associated with the deformation of the liquid/gas interface. Furthermore, it was revealed by contact angle measurements that the maximum pinning forces on a hydrophilic surface were less than half of those on a hydrophobic surface. We attributed the weak pinning force on the hydrophilic surface to cascading depinning, where the initial depinning event triggers a chain reaction of subsequent depinning events, driven by the conversion of excess surface energy to kinetic energy. Our experimental works provide new insights of the relationship between the subnanoscale surface roughness and macroscopic contact line motion.