HI,

I understood that the hydrophillicity is not permanent, may be after oxygen
plasma.  The surface of the PDMS forms Si-OH, hydrophobicity recovery effect
due to diffusion of cyclic oligomeric dimethylsiloxanes to the surface so
it becomes hydrophobic after sometime. Coat it with some reacting gas after
oxygen plasma.  If you make it more SiO2 maybe it remains
hydrophillic.  Check it out.

All the best.
RIGHEIRA Carnegie