A positive surface charge has been shown to encourage clotting in a number of models.

The time field generates a surface charge that is not unlike static electricity.

If there's no surface charge on the interface, the normal component of D is continuous.

Putting this result back into the potential, and ignoring the surface charge for now:

Current rectification can occur when the surface charges at the wall are of the same sign.

Ions inside the fluid is no longer shielded from surface charge.

So the introduction of a surface charge is not unrealistic at all.

In some cases proteins are attracted to surfaces by an excessive surface charge.

Handling the cup while disassembled does not provide enough contact to remove all the surface charge.

Gouy-Chapman theory describes the effect of a static surface charge on a surface's potential.