Can anyone help me with Transport of Diluted Species?

Hi colleagues,
I am facing the problem with my project, and I would greatly appreciate if you can help me.
In short, I want to plot concentration distribution in a microfluidic channel, and for that purpose
I use the following physics;
- electric current to solve Laplace equation. Besides an inlet and outlet where the potential is applied, all walls are insulating.
- creeping flow (spf) to solve Navier-Stoke's equation
- transport of diluted species (chds) in which the flux equation R= ∂c/∂t + del.(-Ddelc)+ delc.u is solved to get the concentration distribution. (c=concentration, u=velocity)

So here is the issue. In our case, the total velocity (u) is a sum of electroosmotic velocity taken from the creeping flow and dielectrophoretic velocity which we have to manually type in. Our problem is that comsol stops finding the solution giving out "maximum number of iteration reached" only when the total velocity <0.
We suspected that it happens because it is not a steady state problem anymore (∂c/∂t is non-zero) so moved to the time-dependent solver, however it still does not solve and give me the same error message.
I know the problem comes from transport of diluted species but do not know what I can do to solve the problem. Maybe use different boundary conditions?

I will appreciate greatly if you can help me. My model is attached.