Adhesive forces generated at a contact point between spherical and plate specimens made of the same material due to water vapor adsorption on them were measured for four samples by using an electrobalance. The samples used here are commercial soda-lime glass bead, granulated urea and potassium halides, i.e. KCl and KBr. The values of the adhesive forces were collated with the amounts of adsorbed estimated from adsorption isotherms of water vapor on the above particle samples.
The adhesive forces appeared at relative water vapor pressures of 0.25, 0.30, 0.30 and 0.17 for the glass bead, the granular urea, KCl and KBr, respectively. At this time, the thicknesses of adsorbed water layers were less than two layers for all the samples. Concerning the above results, this adhesive force can not be a binding-force due to a liquid bridge formed near the contact point. Therefore, the force which occurred in the early stage of a caking process is considered to be generated by the hydrogen bond induced between the two samples being attached with water layers. On the other hand, the force which increases considerably in high water vapor pressure range will be due to a liquid bridge between the two specimens. Moreover, it is concluded that the adsorbed water molecules less than 2 or 3 layers are influenced by surface properties of the solid, whereas the adsorbed more than 4 or 5 layers is no longer influenced from the solid surface. Thus, multi-layers of water molecules are thought to acquire an inherent property as the liquid water.