Direct measurements of the adhesion between a glass particle and a glass surface in a humid atmosphere

An atomic force microscope has been used to measure the adhesion between individual silica-glass particles and a glass substrate in the presence of water vapour. It was found that the adhesion between the surfaces was not significantly altered, when compared with the dry case, at relative water vapour pressures of less than 0.6. Above this level of water vapour, the magnitude of the adhesion between the surfaces increased by approximately an order of magnitude. The transition of behaviour at a relative water vapour pressure of 0.6 corresponds to the formation of a capillary annulus having a critical Kelvin radius of approximately 2 nm. These findings are in good agreement with previous research data for the interaction of mica surfaces in water vapour. Force-distance data recorded as the particle and surface are separated from one another indicate the presence of a capillary neck between the surfaces. The form of these force-distance data indicates that the separation occurs under non-equilibrium conditions that more closely resemble the expected interaction under constant volume conditions (for the capillary neck). The results of this study also indicate the important role of equilibration time for the surfaces when not in contact. Successive force scans with only short equilibration times when the surfaces are separated result in the development of larger than expected capillary interaction forces. The results are relevant to the interactions between particles in a powder bed under flow.