its known that red blood cell membrane have a negative charge, and the charge of the silica surface is negative .
If you are using X1 PBS then the ionic strength is probably high enough to make the diffuse part of the electrical double layers surrounding the cells and the silica surface reduce in size sufficiently to permit irreversible attraction.
It's not just the presence of charge that is important but how far from the surface that charge can be "sensed" - i.e., the thickness of the diffuse part of the double layer.
Refer to any classic text on colloid chemistry and a discussion of electrostatic repulsion and DLVO theory.
The reason for RBC adhesion to glass is the charge on the surface of the cells. I recommend reading the following publications.
(6) Red Blood Cell Interaction with a Glass Surface | Request PDF (researchgate.net)
Chapter Red Blood Cell Interaction with a Glass Surface
If you want to reduce the adhesion of cells to glass, you need to cover the glass with albumin.
