Effects of humidity and substrate hydrophilicity on nanoscale friction

The effect of water menisci at the asperity junction on the friction level and oscillation was investigated by using atomic force microscopy. The menisci formed through capillary action were controlled by changing the humidity of the experimental environment and the ZnO substrate's hydrophilicity. The hydrophilic ZnO substrate showed higher friction levels than the hydrophobic substrate, while this difference was indistinguishable at humidity levels above 40% relative humidity. Indentation experiments demonstrated that the friction level was determined by the stiffness of the water menisci that was changed by hydrophilicity of the substrate and humidity level. The stick-slip amplitude was also increased in the case of the hydrophilic substrate, supporting the hypothesis of repeated formation and deformation of the solid-like menisci during sliding.