Dear Young ho Cho, SiO2 traitement by chamical etching (or by physical means) is a hydroxylation reaction on the surface (this is hydrophilic), after a while the --Si-OH groups re-condense again to make --Si-O-Si-- (which is hydrophobic). Please have a look at the attached file. My Regards

Possibly, there is an alternative explanation and/ or some additional effects to be considered. Silicon oxide has a high surface free energy (SFE). Therefore, it tends to adsorb substances from the surrounding atmosphere. Among others, hydrocarbons get adsorbed. You can see this with XPS or SIMS analysis. These hydrocarbons reduce the SFE dramatically; the surface becomes hydrophobic. If you remove the adsorbed layer the high SFE SiO2 surface appears as hydrophilic. In case of the piranha solution, the hydrocarbons get oxidized and dissolved before the silanol groups are formed. Alternatively, you can use electrical discharge plasma or ion sputtering to remove the adsorbed layer and you will see the hydrophilic surface. Of course, the exposure to atmosphere results in the immediate adsorption of water, which again might form silanol groups. The adsorption of hydrocarbons usually takes longer than the adsorption of water.

In the case of cleaning the surface with liquids it is important to consider non-volatile components in the liquid. Even ppm concentrations can leave residues which can be detected by wetting experiments with water ("analysis depth" < 1 nm). Select liquids with a low concentration of non-volatile components. Let the liquid run off the surface rather than evaporate it.