How water meets a very hydrophobic surface

TL;DR

This study uses X-ray reflectivity to investigate the water-hydrophobic surface interface, revealing that the depletion region size correlates with contact angle and exceeds measurement resolution, but fluctuations are too small to detect.

Contribution

It provides definitive measurements of the water depletion layer at highly hydrophobic surfaces, clarifying previous inconsistent results.

Findings

Depletion region width increases with contact angle.

Depletion region exceeds measurement resolution.

Interface fluctuations are too small to measure.

Abstract

Is there a low-density region ('gap') between water and a hydrophobic surface? Previous X-ray/neutron reflectivity results have been inconsistent because the effect (if any) is sub-resolution for the surfaces studied. We have used X-ray reflectivity to probe the interface between water and more hydrophobic smooth surfaces. The depleted region width increases with contact angle and becomes larger than the resolution, allowing definitive measurements. Large fluctuations are predicted at this interface; however, we find that their contribution to the interface roughness is too small to measure.