Poisson Nernst Planck and Navier Stokes equation

Hi everyone,

I am trying to simulate the electro-osmotic flow in a nano-channel using the Poisson-Nernst-Planck and Navier-Stokes equations coupling them from the Electrostatics,Transport of dilute Species and Laminar flow modules respectively using stationary solver in Comsol 4.0a. The nano-channel has surface charge on the walls and is filled with an electrolyte solution,and an external electric field is applied to bring about the flow.

After applying the boundary conditions,on computing I am getting the error message "No convergence even when using minimum damping factor. Returned solution has not converged"...

Now the boundary conditions for the Navier Stokes have normal flow,no pressure at the inlet and outlet. When I am increasing the pressure to some positive value,my solution converges,but the conditions for my simulation have to be zero pressure conditions. The boundary where i am having surface charge has been provided with finer mesh and the no of mesh elements seem to be enough.

So it would be very helpful,if somebody can point out what is the reason for getting such an error message and how can I go about getting a converged solution.

Regards,
Mitchel