Furthermore, it has been estimated that the interaction between surface-adhered nanoparticles and silica surface is
small (Fig. 6). Since the van der Waals forces between the particles depend on the surface curvature at the contact point,
the nanoparticles aggregated between the silica particles. The adhesion force between the surface-adhered nanoparticles and the surface of the porous silica particles appeared to be small as compared to the smooth silica surface. Therefore, the increase in the CDF value was slow due to the removal of surface-adhered nanoparticles on nanoporous silica. On the basis of these two reasons, the difference in the CDF values for fused silica before and after ultrasonic cleaning is greater than that for porous silica.