The adhesion force between nonporous and porous silica particles was measured by AFM as a function of
relative humidity. Various porous samples were prepared by hydrothermal treatments in different conditions.
Geometric structure was evaluated by gas adsorption, and the mechanism of rehydroxylation that occurred in
hydrothermal treatments was investigated by silanol density, IR spectra, and immersional heat. The adhesion
force was measured by AFM, and the result was compared with the geometric structure and the rehydroxylation
process. As a result, we proved that capillary condensation occurred in small spaces between two particles
contacting each other after water filled the pores and capillary force appeared to contribute to a steep increase
of adhesion force. The adhesion force under low relative pressure was attributed to hydrogen bonding force
and reflected both the contact area and hydrophilicity of the surface.