ABSTRACT
The adhesion between particles in a powder bed is increased as a result of the capillary condensation of water from a humid atmosphere. The adhesion of particles to surfaces is also affected by the same mechanism. The importance of this capillary condensation to processes such as dust suppression or the contamination of 'clean' surfaces is obvious [1]. More recently, the role of capillary condensation on the stick-slip motion of a particulate assembly has been reported [2]. It was postulated that the critical parameter of interest in controlling the motion of such an assembly were the forces operating between particles in stress chains throughout the powder bed. Motion of the powder can only be induced when the stress chains weaken sufficiently, as a result of dilation, under the action of a shear stress. Since dilation of the stress chains is a critical stage in the control of powder flow, the forces between particles in these stress chains that resist this dilation are clearly of interest. For elastic particles, the adhesion between particles in a dry atmosphere is controlled by the action of surface forces, primarily the intermolecular dispersion or van der Waals forces [3]. In a humid atmosphere, any capillary condensation between particles will result in an increased attractive interaction. Depending on the geometry of interaction, the relative humidity and the strength of the surface forces, the interaction pull-off force between the two surfaces can be predicted (see below).
