How to create different levels of hydrophobicity on glass beads surface?
Lets assume first that the surface $\ce{Si-O}$ bonds of teh glass are terminated by $\ce{Si-OH}$ groups. Hydrophilicity will depend greatly on the ration of $\ce{Si-OH}$ to $\ce{Si-R}$ groups on the surface of the glass. Chlorotrimethylsilane is great for a completely hydrophobic surface as it will cap all of the $\ce{Si-OH}$ groups to $\ce{Si-O-Si(CH3)3}$ groups allowing no polar interaction of the surface with the water. dichlorodimethylsilane is not as good leaving you with $\ce{-Si-O-(Si(CH3)2)-O-Si{-}}$ and $\ce{-Si-O-Si(CH3)2OH}$ groups at the surface which affords a bit more interaction.  Octadecyltrichlorosilane does provide even more polar groups than the last two, but because of the large octadecyl group this will likely make a more hydrophobic surface than the chlortrimethylsilane.  The tetraethyl orthosilicate or rather tetraethoxy silane (TEOS) will hydrolze to $\ce{Si(OH)4}$ in water which you can use to help make the surface more hydrophilic.

If I had to do this I would make various binary mixtures of TEOS, chlorotrimetyhlsilane, and dichlorodimethylsilane and treat the surface with them under constant conditions and see what spectrum of hydrophilicity you achieve. For the most hydrophobic surfaces I would use mixtures of octadecyltrichlorosilane and chlorotrimetyhlsilane.