Hydrophobicity Does Not Affect Water Slip: Insights from Slip Length Mapping.

While slip at solid-water interfaces has attracted broad interest, slip lengths reported in experiments─especially on hydrophobic surfaces (i.e., contact angles >90°)─vary widely due to surface topographical and chemical heterogeneity, hindering quantitative understanding of the true slip length. Using highly sensitive frequency-modulation atomic force microscopy, we achieved simultaneous nanoscale mapping of slip length and surface topography, with a 159-fold improvement in slip length detection sensitivity. True slip lengths measured on flat regions of various substrates were found to be almost zero, following a scaling rule predicted by molecular dynamics simulations. As the only exception, a large slip length of 43.2 ± 5.8 nm was measured on graphite in deionized water, but it vanished upon immersion in electrolyte solutions. This unique behavior was rationalized by graphite's atomic-scale smoothness, chemical homogeneity, and ion adsorption. Our method experimentally advances a unified picture of fluid slip.