Influence of roughness on capillary forces between hydrophilic surfaces

TL;DR

This study investigates how surface roughness and material properties influence capillary forces between hydrophilic surfaces, revealing persistent water layers and different force scaling behaviors across materials.

Contribution

It provides new insights into the impact of roughness and material hydrophilicity on capillary forces, with detailed measurements across various materials and conditions.

Findings

Large capillary forces persist even at 2% humidity.

Force scaling with roughness varies by material.

Nanometer-thick water layers act as bridges.

Abstract

Capillary forces have been measured by Atomic Force Microscopy in the plate-sphere setup between gold, borosilicate glass, GeSbTe, titanium, and UV irradiated amorphous titaniumdioxide surfaces. The force measurements were performed as a function contact time and surface roughness in the range 0.2 - 15 nm rms, and relative humidity ranging between 2 and 40 %. It is found that even for the lowest attainable relative humidity 2 % very large capillary forces are still present. The latter suggests the persistence of a nanometers thick adsorbed water layer that acts as a capillary bridge between contacting surfaces. Moreover, we found a significantly different scaling behavior of the force with rms roughness for materials with different hydrophilicity as compared to gold-gold surfaces.