Slow kinetics of capillary condensation in confined geometry: experiment and theory

When two solid surfaces are brought in contact, water vapor present in the ambient air may condense in the region of the contact to form a liquid bridge connecting the two surfaces: this is the so-called capillary condensation. This phenomenon has drastic consequences on the contact between solids, modifying the macroscopic adhesion and friction properties. In this paper, we present a survey of the work we have performed both experimentally and theoretically to understand the microscopic foundations of the kinetics of capillary condensation. From the theoretical point of view, we have computed the free energy barrier associated with the condensation of the liquid from the gas in a confined system. These calculations allow understanding of the existence of very large hysteresis, which is often associated with capillary condensation. These results are compatible with experimental results obtained with a surface forces apparatus in a vapor atmosphere, showing a large hysteresis of the surface energy of two parallel planes as a function of their distance. In the second part, we present some experiments on the influence of humidity on the avalanche angle of granular media. We show that the aging in time of this avalanche angle can be explained by the slow kinetics of capillary condensation in a random confined geometry.