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