Impact of Sub-Nanoscale\nSurface Topography on Contact\nLine Profile: Insights from Coherence Scanning Interferometry

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