Capillary Condensation between High-Energy Surfaces. An Experimental Study with a Surface Force Apparatus

We study the capillary condensation of an undersaturated vapour between high-energy surfaces with a surface force apparatus. The adhesion force shows a large hysteresis cycle when the surfaces are quasi-statistically brought to contact and moved apart, revealing an asymmetry between the condensation and evaporation processes. The condensation of a liquid bridge between the surfaces is associated with a force discontinuity, and occurs at distances which increase with the vapour pressure. The adhesion force is consistent with the classical theory of capillarity, and reveals the presence of wetting films. A mechanism for condensation is proposed based on the instability of the wetting films under their mutual attraction. We predict a condensation distance which grows linearly with the wetting film thickness, in good agreement with the experimental observation.