Nanoscale surface roughness enhances wicking through closely spaced and interconnected depressions

Hemi-wicking has been widely studied on engineered high-aspect-ratio and hierarchical micro/nanostructures, where well-defined geometries promote capillary transport. However, the mechanisms governing hemi-wicking on practical surfaces with weak, irregular nanoscale roughness remain poorly understood. Here, glass surfaces with nanoscale roughness were prepared by chemical etching, and wetting near an advancing water contact line was observed using Coherence Scanning Interferometry (CSI). The same area was measured before and during wetting. The wet-state measurements revealed a nanoscale-thick liquid film extending tens of micrometers ahead of the apparent contact line, even on a surface with a roughness factor of only 1.003, indicating an almost negligible increase in surface area. By aligning the wet and dry profiles, the film length was accurately quantified. Statistical analysis of the geometrical features extracted from the dry surface topography showed that longer films formed where depressions were more closely spaced and where deeper parts of the roughness were connected into continuous pathways extending farther ahead of the contact line. These geometries can enhance capillary driving pressure and effective permeability by shortening liquid-transfer distances and providing alternative in-plane pathways around local blockages. A Darcy-type interpretation indicates that film extension is governed by network-level liquid transport rather than isolated flow-path width alone. The liquid film also weakened structural pinning and reduced the local advancing contact angle. These findings show that weak nanoscale roughness, often regarded as a surface imperfection, can serve as a design parameter for controlling passive liquid transport and contact-line motion.