An overview of the role of capillary condensation in wet caking of powders

Abstract This work deals with the role of capillary condensation phenomenon on the caking behavior of powders. A theoretical basis is established to provide a quantitative description of the behavior of powders subjected to humidification. This approach is based on the integration of water condensation and liquid bridge formation within binary contacts in regular or random packing of particles. The existence of a minimum and a maximum limiting filling angle is demonstrated based on thermodynamic limitations. The role of the particle mean size, the wetting contact angle, the interparticle gap and the coordination number on limiting filling angles as well as the amount of condensed water at various water activities ( a w ) is investigated. The results show that for smooth and non soluble particles, capillary condensation plays a significant role only for sub-micronic powders for which the presence of water in liquid state (then liquid bridges) is expected at low water activities, sensibly less than the saturation activity ( a w  = 1).